OCS Study 

MMS 96-0027 

Distribution and Abundance of 

Cetaceans in the North-Central and 

Western Gulf of Mexico, Final Report 

Volume II : Technical Report 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 96-0027 

Distribution and Abundance of 

Cetaceans in the North-Central and 

Western Gulf of Mexico, Final Report 

Volume II : Technical Report 

Editors 

Randall W. Davis 

Giulietta S. Fargion 

Prepared under MMS Contract 

14-35-0001-30619 and 

Interagency Agreement 16197 

by 

Texas Institute of Oceanography 

Texas A&M University at Galveston 

Galveston, Texas 77553 

and 

Southeast Fisheries Science Center 

National Marine Fisheries Service 

75 Virginia Beach Drive 

Miami, Florida 33149 

Published by 

U.S. Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region August 1996 
DISCLAIMER 

This report was prepared under contract between the Minerals Management 

Service (MMS), the Texas Institute of Oceanography, and the National Marine 

Fisheries Service. This report has been technically reviewed by the MMS and 

approved for publication. Approval does not signify that the contents 

necessarily reflect the views and policies of the Service, nor does mention of 

trade names or commercial products constitute endorsement or 

recommendation for use. It is, however, exempt from review and compliance 

with the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Unit 

(Mail Stop 5034) at the following address: 

U.S . Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Unit (MS 5034) 

1201 Flmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone: (504) 736-2519 or 

1 (800) 200-GULF 

CITATION 

Suggested citation : 

Davis, R.W., and G.S . Fargion, eds . 1996. Distribution and abundance of 

cetaceans in the north-central and western Gulf of Mexico: Final 

Report. Volume II : Technical Report. OCS Study MMS 96-0027 . 

Prepared by the Texas Institute of Oceanography and the National 

Marine Fisheries Service. U.S . Dept. of the Interior, Minerals Mgmt. 

Service, Gulf of Mexico OCS Region, New Orleans, LA. 357 pp. 

ABOUT THE COVER 

The cover art depicts a clymene dolphin and is the work of Mark Grace, an 

employee of the National Marine Fisheries Service Laboratory at Pascagoula, 

MS. 

iii 
ABSTRACT 

The purpose of this study (hereafter referred to as the GulfCet Program) was to 

determine the distribution and abundance of cetaceans (whales and dolphins) 

in areas potentially affected by future oil and gas activities along the 

continental slope in the north-central and western Gulf of Mexico. This 

3.75 year project commenced on 1 October 1991 and concluded on 15 July 1995. 

The study area was bounded by the Florida-Alabama border, the Texas-Mexico 

border, and the 100 m and 2,000 m isobaths. The distribution and abundance of 

cetaceans were determined from seasonal aerial and shipboard visual surveys 

and shipboard acoustic surveys. In addition, hydrographic data were collected 

in situ and by satellite remote sensing to characterize the habitats of cetaceans 

in the study area. Finally, tagging and tracking of sperm whales using satellite 

telemetry was attempted. 

Cetaceans were observed throughout the study area during all four seasons. 

Nineteen species were identified, including two species (melon-headed whales 

and Fraser's dolphins) that were previously thought to be rare in the Gulf. 

Pantropical spotted dolphins, bottlenose dolphins, clymene dolphins, striped 

dolphins, Atlantic spotted dolphins, and melon-headed whales were the most 

common small cetaceans. The most common large cetacean was the sperm 

whale. Only one species of baleen whale, the Bryde's whale, was sighted, and 

the estimated abundance of this species was very low . The mean annual 

abundance for all cetaceans was estimated to be 19,198 animals. 

The oceanography in the study area was complex and dynamic, with mesoscale 

features that showed large annual and interannual variability. Warm- and 

cold-core rings (eddies) and the fresh water effluent from the Mississippi 

River were the most distinctive hydrographic features observed in the study 

area. The marine habitat for this area can be characterized as tropical to 

subtropical with a mixed layer that is seasonally deepest in the winter. 

With the exception of bottom depth, there was no significant correlation of 

cetacean distribution with any of the hydrographic variables examined. 

Cetaceans could be divided into three groups relative to bottom depth. The first 

group, which occurred on the continental shelf or along the shelf break, 

consisted of Atlantic spotted dolphins and bottlenose dolphins. The second 

group consisted only of Risso's dolphin and occurred along the mid-to-upper 

slope. The third group included sperm whales, pygmy/dwarf sperm whales, 

pantropical spotted dolphins, striped dolphins, and Mesoplodon spp. This third 

or deep-water group typically occurred along the mid-to-lower slope in water 

over 1,000 m deep. There was some indication that sperm whales may be found 

in conjunction with the edge of warm-core rings, where upwelling events 

may enhance productivity and prey abundance. 

The potential effects of oil and gas exploration and production activity on 

cetaceans along the continental slope cannot be predicted with certainty. 

However, it can be anticipated that cetaceans will encounter construction 

activity, ship traffic, seismic exploration, and underwater noise as the oil and 

gas industry moves into yet deeper water. The GulfCet Program has 

demonstrated that any future monitoring 0 programs would need to be long-
term, with relatively intensive sampling effort in order to detect significant 

changes in the abundance and distribution of most cetaceans . 

vi 
TABLE OF CONTENTS 

CONTRIBiJTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . .xi 

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .xiii 

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .xxv 

LIST OF ABBREVIATIONS AND ACRONYMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxix 

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . .xxxi 

I . INTRODUC'I'ION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .---. . . . . .1 

R.W. Davis 

1.1 Background and Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 

1.2 Program Participants . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .1 

1.3 Report Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 

1.4 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . . .8 

I I . OVERVIEW OF THE GULF OF MEXICO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 9 

R.W. Davis, B. Wiirsig, G.S . Fargion, T.A. Jefferson, and C. Schroeder 

2.1 Geographic Overview. . .-------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

2.2 General Oceanography of the Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 12 

2.2 .1 Circulation Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 12 

2.2.2 Water Temperature and Salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 14 

2.2 .3 Mississippi River Influence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

2.2 .4 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 18 

2.3 Historical Overview of the Distribution of Cetaceans in the 

Offshore Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 21 

2.3 .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . 21 

2.3 .2 Data Acquisition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .------ . . . . . . . . . . . . . . . 23 

2.3 .3 Species Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

2.3 .4 Summary and Conclusions. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 

2.4. Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 43 

I I I . VISUAL SURVEYS ABOARD SHIPS AND AIRCRAFT . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 55 

L.J . Han sen, K.D. r4ullin, T.A. Jefferson, and G.P. Scott 

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SS 

3.2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

3.2 .1 Data Acquisition. . . . . . . . . . . . . . . . . . . .. . . .------ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

3.2 .2 Data Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 

3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-- .--- . . . 69 

3.3 .1 Cetacean Abundance from Ship Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

3.3 .2 Cetacean Abundance from Aerial Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 

3.3 .3 Cetacean Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

3.3 .4 Species Accounts . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . 87 

3.3 .5 Distribution, Density, and Abundance of Sea Turtles . . . . . . . . . . 114 

3.4 Discuss ion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 

3.4.1 Comparison of Ship and Aircraft Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 

3.4.2 Cetacean Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 121 

3.4.3 Cetacean Abundance and Group Size . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 1222 

vii 
3.4.4 Distribution and Abundance of Sea Turtles . . . . . . . . . . . . . . . . . . . . . . .. . . . 127 

3.5 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .128 

IV. ACOUSTIC SURVEYS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 133 

J.C. Norris, W.E. Evans, R. Benson, and T.D. Sparks 

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 133 

4.2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 134 

4.2 .1 Data Acquisition. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .------ . . . . . . . . 134 

4.2 .2 Data Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 141 

4.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 

4.3 .1 Acoustic Effort . . . . . . . . . . . . . . . . . . . .---- .-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . 150 

4.3 .2 Species Recorded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . 150 

4.3 .3 Distribution of Cetaceans by Species . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . 156 

4.3 .4 Dolphin Acoustics. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. .. 169 

4.3 .5 Sperm Whale Group Size Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 176 

4.4 Discussion. . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. 181 

4.4.1 Sperm Whale Distribution and Density Estimates . . . . . . . . . . .. .. .. 181 

4.4.2 Dolphin Density Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 181 

4.4.3 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 182 

4.5 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 183 

V. BEHAVIOR OF CETACEANS RELATIVE TO SURVEY VESSELS . . . . . . . . . . . . . . . . . . . . . .. .. .. 189 

B. Wiirsig, S.K. Lynn, and K.D. Mullin 

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 189 

5.2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . 190 

5.2.1 Shipboard Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- .. .. 190 

5.2 .2 Aerial Observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 191 

5.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. 192 

5.3 .1 Shipboard Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 

5.3 .2 Aerial Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 

5.4 Discussion. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .--- . . . . .. 199 

5.5 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . .------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . .. .. 204 

VI . OCEANOGRAPHIC SURVEYS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 

G.S. Fargion, N. May, T.D. Leming, and C. Schroeder 

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 

6.2 Hydrographic Data Collection and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 

6.2 .1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 

6.2 .2 Transect and Cruise Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 

6.2 .3 Shipboard Measurements and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 

6.3 Hydrographic Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .--- . . . . . . . . . 216 

6.3 .1 Cruise Summaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 

6.3 .2 Temperature-Salinity Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 

6.3 .3 20°C, 15'C, and 8°C Isotherm Depths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 

6.3 .4 Dynamic Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 

6.3 .5 1992-1994 Oceanographic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 

6.3 .6 Chlorophyll a Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 

6.3 .7 Mississippi River Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 

6.4 Discussion of the Major Oceanographic Features of the 

North-Central and Western Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 

6.4.1 Eddy Histories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 

6.4.2 Fresh water Influx. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . 263 

viii 
6.4 .3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .264 

6.5 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .265 

VII . BIRD SURVEYS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 

D.E. Peake 

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 

7.2 Methods. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-- .---- . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 

7.3 Results. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2n 

7.3 .1 Seabirds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .272 

7.3 .2 Non-seabirds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . .--- . . .--- . . . . . . . . . . . . . . . . . . . .298 

7.4 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .299 

7.4 .1 Species Accounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .299 

7.4 .2 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .302 

7.5 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303 

VIII . STUDIES OF SPERM WHALES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 

B.R. Mate, M. Duncan, D.W. Weller, A. Schiro, S.K. Lynn, J .C. Norris, B. 

Wiirsig, and W.E . Evans 

8.1 Satellite Tagging of Sperm Whales. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 

8.1 .1 Data Acquisition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 

8.1 .2 Tagging Cruise Chronology. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 

8.1 .3 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 

8.2 Sperm Whale Focal Cruise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . 316 

8.2 .1 Locating and Tracking Sperm Whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 

8.2 .2 Acoustic Localization of Vocalizing Sperm Whales . . . . . . . . . . . . . 318 

8.2 .3 Photo-identification of Sperm Whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 

8.2 .4 An Interaction Between Sperm Whales and Pilot 

Whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 

8.2 .5 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 

8.3 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 

IX. CETACEAN HABITAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 

R.W. Davis, G.S . Fargion, W.E . Evans, N. May, and T.D. Leming 

9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 

9.2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 

9.2 .1 Geographic Information System (GIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 

9.2 .2 Statistical Methods. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 

9.3 Environmental Profiles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 

9.4 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 

9.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 

9.6 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 

X. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE STUDIES . . . . . . . . . . . . . . . . . . . . 351 

R.W. Davis, W.E . Evans, B. Wiirsig, and G.S . Fargion 

10.1 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .355 

ix 
CONTRIBUTORS 

Robert H. Benson - Center for Bioacoustics, Mail Stop 3367, Texas A&M 

University, College Station, Texas 77843. 

Randall W. Davis - Department of Marine Biology, Texas A&M University, 

P.O. Box 1675, Galveston, Texas 77551 . 

Mike Duncan - Department of Oceanography, Texas A&M University, College 

Station, Texas 77843 . 

William E. Evans - Texas Institute of Oceanography, Texas A&M University, 

P.O. Box 1675, Galveston, Texas, 77551. 

Giulietta S. Fargion - GulfCet Program Office, Texas A&M University, 

5007 Avenue U, Galveston, Texas 77551. Current address: EOSDIS Core 

System, Science Office, 1616A McCormick Dr., Iandover, Maryland 20785. 

Larry J. Hansen - Southeast Fisheries Science Center, National Marine 

Fisheries Service, P.O. Box 12607, Charleston, South Carolina 29422. 

Thomas A. Jefferson - Marine Mammal Research Program, Texas A&M 

University, 4700 Avenue U, Bldg. 303, Galveston, Texas, 77551 . Current 

address: Ocean Park Conservation Foundation, Ocean Park Corporation, 

Aberdeen, Hong Kong. 

Thomas D. Leming - National Marine Fisheries Service, John C. Stennis Space 

Center, Bldg. 1103, Rm. 218, Stennis Space Center, Mississippi 39529. 

Spencer K. Lynn - GulfCet Program Office, Texas A&M University, 

5007 Avenue U, Galveston, Texas 77551 . 

Bruce R. Mate - Hatfield Marine Science Center, Oregon State University, 

Newport, Oregon 97365 . 

L Nelson May - National Marine Fisheries Service, John C. Stennis Space 

Center, Bldg. 1103, Rm. 218, Stennis Space Center, Mississippi 39529. 

Keith D. Mullin - Southeast Fisheries Science Center, National Marine 

Fisheries Service, P.O. Drawer 1207, Pascagoula, Mississippi 39568. 

Jeffrey C. Norris - Marine Acoustics Lab, Texas A&M University, 5007 Avenue 

U, Galveston, Texas 77551 . 

Dwight E. Peake - GulfCet Program Office, Texas A&M University, 

5007 Avenue U, Galveston, Texas 77551. Current address: 30 Lebrun Ct., 

Galveston, Texas 77551 . 

Andrew Schiro - Marine Mammal Research Program, Texas A&M University, 

4700 Avenue U, Bldg. 303, Galveston, Texas, 77551 . 

xi 
Gerald P. Scott - Southeast Fisheries Science Center, National Marine Fisheries 

Service, 75 Virginia Beach Drive, Miami, Florida 33149. 

Cheryl Schroeder - GulfCet Program Office, Texas A&M University, 

5007 Avenue U, Galveston, Texas 77551. Current address: University of 

Rhode Island, Graduate School of Oceanography, Narragansett Bay 

Campus, Marine Bldg. 9, Narragansett, RI 02882 

Troy D. Sparks - Marine Acoustics Lab, Texas A&M University, 5007 Avenue U, 

Galveston, Texas 77551 . 

David W. Weller - Marine Mammal Research Program, Texas A&M University, 

4700 Avenue U, Bldg. 303, Galveston, Texas, 77551. 

Bernd Wiirsig - Marine Mammal Research Program, Texas A&M University, 

4700 Avenue U, Bldg. 303, Galveston, Texas, 77551 . 

xii 
LIST OF FIGURES 

Figure 1.1 Study area between the 100 and 2,000 m isobaths, 

extending as far east as the Florida-Alabama border, and 

as far southwest as the Texas-Mexico border . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Figure 2.1 Bathymetry and major physiographic features of the 

Gulf of Mexico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 

Figure 2.2 Bottom depth profiles of the Gulf of Mexico and GulfCet 

study area tabulated from Herring's (1993) bathymetric 

data set showing the percentage of each region which is 

less than a given depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 

Figure 2.3 Schematic of the life-cycle of a Loop Current 

anticyclonic or warm-core eddy (W) as it is pinched off 

or separated from the Loop Current, and its possible paths 

through the western Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

Figure 2 .4 Comparison of 1993 total Mississippi River discharge 

volume to that of the minimum, maximum, and mean 

river discharge volumes (in cubic meters/second x 1,000) 

for 1930-1992 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 

Figure 2.5 Time series 1979-198G of Mississippi River flow and 

chlorophyll pigment data from CZCS (data in close 

proximity to Pelican/Longhorn station 12-125) . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Figure 2 .6 Time series 1979-1986 of Mississippi River flow & 

chlorophyll pigment data from CZCS (data in close 

proximity to Pelican/ Longhorn station 11-106) . . . . . . . . . . . . . . . . . . . . . . . . .20 

Figure 2 .7 Map of the study area showing locations of survey blocks 

from previous studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 

Figure 2 .8 Records of sperm whales in the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Figure 2 .9 Records of Risso's dolphins in the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . 35 

Figure 2.10 Records of Atlantic spotted dolphins in the Gulf of 

Mexico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .- .--- . . . .36 

Figure 2.11 Records of pantropical spotted dolphins in the Gulf of 

Mexico. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .------ .-- . . . . . . . . . . . . . . . . . . . . . . . . . .37 

Figure 2.12 Records of striped dolphins in the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . 39 

Figure 2.13 Records of clymene dolphins in the Gulf of Mexico . . . . . . . . . . . . . . . . . . . 40 

Figure 3 .1 R/V Longhorn and RN Pelican marine mammal visual 

and acoustic track-lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56 

xiii 
Figure 3.2 Representative NOAA Ship Oregon II marine mammal 

cruise track. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58 

Figure 3 .3 NOAH Ship Oregon II standard ichthyoplankton/marine 

mammal cruise track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . .58 

Figure 3 .4 Diagram of perpendicular sighting distance (PSD) and 

other sighting parameters for shipboard (A) and aerial 

(B) surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 

Figure 3 .5 GulfCet aerial survey track-lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .61 

Figure 3 .6 The locations of all cetacean groups sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . .83 

Figure 3 .7 The locations of all cetacean groups sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84 

Figure 3 .8 The locations of all cetacean groups sighted during 

GulfCet summer aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85 

Figure 3 .9 The locations of all cetacean groups sighted during 

GulfCet fall aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .85 

Figure 3 .10 The locations of all cetacean groups sighted during 

GulfCet winter aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .86 

Figure 3 .11 The locations of all cetacean groups sighted during 

GulfCet spring aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .86 

Figure 3 .12 The location of the Bryde's/sei whale sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .91 

Figure 3 .13 The locations of the Bryde's/sei whale sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91 

Figure 3 .14 The locations of all sperm whales sighted during GulfCet 

ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .92 

Figure 3 .15 The locations of all sperm whales sighted during GulfCet 

aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .92 

Figure 3 .16 The locations of all pygmy sperm whales sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .94 

Figure 3 .17 The locations of all dwarf sperm whales sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .94 

Figure 3 .18 The locations of all pygmy/dwarf sperm whales sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95 

xiv 
Figure 3.19 The locations of all pygmy/dwarf sperm whales sighted 

during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . .95 

Figure 3.20 The locations of all Cuvier's beaked whales sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .96 

Figure 3.21 The locations of all Cuvier's beaked whales sighted 

during GulfCet aerial surveys .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96 

Figure 3.22 The locations of all unidentified Ziphiids sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . .---- . . . . . . . . . . . . . . . . . . . . . .98 

Figure 3.23 The locations of all unidentified Ziphiids sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .98 

Figure 3.24 The locations of all melon-headed whales sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . .99 

Figure 3.25 The locations of all melon-headed whales sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99 

Figure 3.26 The locations of all pygmy killer whales sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100 

Figure 3.27 The locations of all pygmy killer/melon-headed whales 

sighted during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 

Figure 3.28 The locations of all false killer whales sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102 

Figure 3.29 The locations of all false killer whales sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102 

Figure 3.30 The locations of all killer whales sighted during GulfCet 

ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103 

Figure 3.31 The locations of all short-finned pilot whales sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104 

Figure 3.32 The locations of all short-finned pilot whales sighted 

during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104 

Figure 3.33 The locations of all rough-toothed dolphins sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106 

Figure 3.34 The locations of all rough-toothed dolphins sighted 

during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106 

Figure 3.35 The locations of all Fraser's dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . .-- .-- . . . . . .107 

xv 
Figure 3.36 The locations of all Fraser's dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107 

Figure 3.37 The locations of all bottlenose dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .108 

Figure 3.38 The locations of all bottlenose dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108 

Figure 3.39 The locations of all Risso's dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109 

Figure 3 .40 The locations of all Risso's dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109 

Figure 3.41 The locations of all Atlantic spotted dolphins sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111 

Figure 3 .42 The locations of all Atlantic spotted dolphins sighted 

during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111 

Figure 3 .43 The locations of all pantropical spotted dolphins sighted 

during GulfCet ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112 

Figure 3.44 The locations of all pantropical spotted dolphins sighted 

during GulfCet aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112 

Figure 3 .45 The locations of all striped dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..113 

Figure 3 .46 The locations of all striped dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113 

Figure 3 .47 The locations of all spinner dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115 

Figure 3 .48 The locations of all spinner dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115 

Figure 3 .49 The locations of all clymene dolphins sighted during 

GulfCet ship surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116 

Figure 3 .50 The locations of all clymene dolphins sighted during 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116 

Figure 3 .51 The locations of leatherback sea turtles sighted during all 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119 

Figure 3 .52 The locations of Kemp's ridley, loggerhead, and 

unidentified chelonid sea turtles sighted during all 

GulfCet aerial surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119 

xvi 
Figure 4.1 Diagram of (a) the 525 m linear acoustic array 

illustrating its various components and (b) a detailed 

diagram of the active section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135 

Figure 4.2 Illustration of the high frequency section of the 

hydrophone array.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137 

Figure 4.3 The directivity pattern of the hydrophone array . . . . . . . . . . . . . . . . . . . . . .138 

Figure 4.4 The configuration of the on-board electronics, array 

handling system, and the array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139 

Figure 4.5 The detection function for sperm whales using the 

acoustic array. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..146 

Figure 4.6 Distribution of sperm whale contacts on R/V Longhorn 

Cruise 1 (first spring cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .151 

Figure 4.7 Distribution of sperm whale contacts on R/V Pelican 

Cruise 5 (second spring cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .151 

Figure 4.8 Distribution of sperm whale contacts on R/V Pelican 

Cruise 2 (first summer cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 

Figure 4.9 Distribution of sperm whale contacts on R/V Pelican 

Cruise 6 (second summer cruise). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 

Figure 4.10 Distribution of sperm whale contacts on R/V Pelican 

Cruise 3 (first fall cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153 

Figure 4.11 Distribution of sperm whale contacts on R/V Pelican 

Cruise 7 (second fall cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153 

Figure 4.12 Distribution of sperm whale contacts on R/V Pelican 

Cruise 4 (winter cruise) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154 

Figure 4.13 Hours of effort by time of day for Pelican/Longhorn 

Cruises 1-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .155 

Figure 4.14 Kilometers of sperm whale contacts for 

Pelican/Longhorn Cruises 1-7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 

Figure 4.15 The daily pattern of hours of sperm whale contacts per 

hours of effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .162 

Figure 4.16 Duration of contacts by hour of the day during which 

they started relative to sunrise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .164 

Figure 4.17 Autocorrelation of duration of sperm whale contacts by 

time of day. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165 

xvii 
Figure 4.18 Sperm whale contacts divided into four water depth 

categories and distribution of water depth in the study 

area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .167 

Figure 4.19 Number of acoustic contacts for all species except sperm 

whales for Pelican/Longhorn Cruises 1-7, including 

expected number of contacts based on effort . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .168 

Figure 4.20 Longhorn Cruise 1 acoustic contacts, except sperm 

whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. . . . . . . . . . . . .170 

Figure 4.21 Pelican Cruise 2 acoustic contacts, except sperm whales . . . . . . . . . .170 

Figure 4.22 Pelican Cruise 3 acoustic contacts, except sperm whales . . . . . . . . . .171 

Figure 4.23 Pelican Cruise 4 acoustic contacts, except sperm whales . . . . . . . . . .171 

Figure 4.24 Pelican Cruise S acoustic contacts, except sperm whales . . . . . . . . . .172 

Figure 4.25 Pelican Cruise 6 acoustic contacts, except sperm whales . . . . . . . . . .172 

Figure 4.26 Pelican Cruise 7 acoustic contacts, except sperm whales . . . . . . . . . .173 

Figure 4.27 Number of acoustic contacts of all cetaceans, except 

sperm whales, by season . . . . . . . . . . . . . . . . . . . . . . .--- . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .174 

Figure 4.28 Plot of the first two canonical variates as computed from 

the linear combination of the measured whistle variables 

for four species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .175 

Figure 4.29 Sperm whale group density by track-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178 

Figure 4.30 Density of dolphin contacts, per 1,000 square km, by 

track-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .180 

Figure 5 .1 Distance from ship of cetaceans when initially sighted . . . . . . . . . . .193 

Figure 5 .2 Initial sighting distances from ship for species 

categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .194 

Figure 5 .3 Initial sighting distances by individual species and six 

pooled groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .195 

Figure 5 .4 Percentage of sightings during which cetaceans (species 

with <_ 10 sightings) exhibited a change in behavior 

relative to aerial surveys . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .199 

Figure 6.1 Representative Oregon II hydrographic survey station 

plan for leg 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .209 

Figure 6.2 Representative Oregon II hydrographic survey station 

plan for leg 2. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .209 

xviii 
Figure 6.3 Representative Oregon II hydrographic survey station 

plan for leg 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .210 

Figure 6.4 Longhorn and Pelican hydrographic survey station 

plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .212 

Figure 6 .5 Temperature versus salinity relationship for all CTD 

stations from Pelican and Longhorn Cruises 1-7 . . . . . . . . . . . . . . . . . . . . . . . . 219 

Figure 6 .6 Temperature versus salinity relationship for all CTD 

stations from Oregon II Cruises 199-209. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 220 

Figure 6 .7 Pelican fall Cruise 3 temperature versus salinity 

relationship for CTD stations within Eddy V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .222 

Figure 6.8 T-7 XBT temperature data for Pelican and Longhorn 

Cruises 1-7 (every sixth data point was plotted) . . . . . . . . . . . . . . . . . . . . . . . . . . 223 

Figure 6.9 T-7 XBT temperature data for Oregon II Cruises 199-209 

(every sixth data point was plotted) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 224 

Figure 6.10 Topography of the 20 °C isotherm based on all XBT and 

CTD data from winter Pelican Cruise 4, 12-27 February 

1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . .. . . .. .. . . . . .. . . . . . . . .226 

Figure 6.11 Topography of the 15 °C isotherm based on all XBT and 

CTD data from winter Pelican Cruise 4, 12-27 February 

1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .227 

Figure 6.12 Topography of the 8 °C isotherm based on all XBT and CTD 

data from winter Pelican Cruise 4, 12-27 February 1993 . . . . . . . . . . . 228 

Figure 6.13 NOAH-AVHRR SST (°C) analysis of the western Gulf of 

Mexico for 12 February 1993, coinciding with the 

beginning of Pelican winter cruise 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .229 

Figure 6.14. Pelican summer Cruise 2 surface dynamic topography 

(dyn cm) with respect to 800 m, with LATEX C drifter 

#247 track from August 1992 superimposed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 

Figure 6.15 . Pelican fall Cruise 3 surface dynamic topography (dyn 

cm) with respect to 800 m, with LATEX C drifter #2447 

track from November 1992 superimposed . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .233 

Figure 6.16 Pelican winter Cruise 4 surface dynamic topography 

(dyn cm) with respect to 800 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .234 

Figure 6.17 Pelican spring Cruise S surface dynamic topography 

(dyn cm) with respect to 500 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .235 

Figure 6.18 Pelican summer Cruise 6 surface dynamic topography 

(dyn cm) with respect to 800 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .237 

xi?: 
Figure 6.19 Pelican fall Cruise 7 surface dynamic topography (dyn 

cm) with respect to 800 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .238 

Figure 6.20 Vertical profile of chlorophyll a concentrations (mg/m3) 

from inshore (100 m CTD) Pelican stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 

Figure 6.21 Vertical profile of chlorophyll a_ concentrations (mg/m3) 

from offshore (all CTD stations except 100 m) Pelican 

stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .241 

Figure 6.22 Vertical profile of chlorophyll a_ concentrations (mg/m3) 

from Pelican Mississippi River plume stations (11-106, 

12-119, 12-125, 13-126, and 13-133) . . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 242 

Figure 6.23 Integrals (mg/m'-) of chlorophyll g- concentrations 

versus depth (up to 100 m) for Pelican fall Cruise 3. . . . . . . . . . . . . . . . . . .243 

Figure 6.24 Integrals (mg/m'-) of chlorophyll a_ concentrations 

versus depth (up to 100 m) for Pelican winter Cruise 4. . . . . . . . . . . . .24 

Figure 6.25 Time series diagrams showing the Mississippi River 

plume and associated primary SST fronts on (a) 4 October 

1992, (b) 11 October 1992, and (c) 13 October 1992 . . . . . . . . . . . . . . . . . . . . . ..246 

Figure 6.26 Pelican fall 1992 Cruise 3 MIDAS wind data plotted to show 

the northeasterly to easterly wind forcing in area of 

fresh water extrusion near 27°N, 91°30'W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. ..247 

Figure 6.27 NOAA-AVHRR reflectance analysis of the centralwestern Gulf of Mexico for 10 August 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..248 

Figure 6.28 Pelican summer Cruise 6 MIDAS wind data plot to show 

the northwesterly to westerly wind forcing in the 

Mississippi River plume region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 

Figure 6.29 Fresh water fraction (%) for 0-3 m depths during the 

August 1992 Pelican summer Cruise 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .251 

Figure 6.30 Fresh water fraction (%) for 0-3 m depths during the 

August 1993 Pelican summer Cruise 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .252 

Figure 6.31 NOAH-AVHRR SST (°C) analysis of the western Gulf of 

Mexico for 12 October 1992, showing Eddy- V (northwest) 

and Eddy U (southeast). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255 

Figure 6.32 Smoothed tracks of LATEX C drifter buoys 02447, 02449, 

and 07837, indicating Eddies U and V separately and after 

their coalescing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .257 

Figure 6.33 NOAH-AVHRR SST (°C) analysis in the western Gulf of 

Mexico for 18 April 1993, showing Eddy U and Eddy V 

interacting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .258 

xx 
Figure 6.34 NOAA-AVHRR SST (°C) analysis of the western Gulf of 

Mexico for 28 November 1993, indicating Eddy X (northcentral) and Eddy W (northwest) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .260 

Figure 6.35 NOAH-AVHRR SST (°C) analysis of the western Gulf of 

Mexico, showing the Loop Current (LC.) and Eddies X and 

W merging together to form one large eddy in the SW 

corner of the Gulf. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .261 

Figure 6.36 NOAA-AVHRR SST (°C) analysis of the western Gulf of 

Mexico for 7 May 1994, showing the Loop Current (L.C.) 

with no eddies visible in the central or NW Gulf . . . . . . . . . . . . . . . . . . . . . . . .262 

Figure 7.1 Audubon's shearwater sightings for Pelican Cruises 3-7 

and Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .279 

Figure 7.2 Wilson's Storm-Petrel sightings for Pelican Cruises 3-7 

and Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .279 

Figure 7.3 Band-rumped Storm-Petrel sightings for Pelican Cruises 

3-7 and Oregon II Cruises 199, 203, 204, and 209 . . . . . . . . . . . . . . . . . . . . . . . . . .280 

Figure 7.4 Unidentified Storm-Petrel sightings for Pelican Cruises 

3-7 and Oregon II Cruises 199, 203, 204, and ?09 . . . . . . . . . . . . . . . . . . . . . . . . . .280 

Figure 7.5 Magnificent Frigatebird sightings for Pelican Cruises 3-7 

and Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .282 

Figure 7.6 Unidentified frigatebird sightings for Pelican Cruises 3-7 

and Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .282 

Figure 7.7 Northern Gannett sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .283 

Figure 7.8 Masked Booby sightings for Pelican Cruises 3-7 and 

Oregon 11 Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .283 

Figure 7.9 Pomarine Jaeger sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284 

Figure 7.10 Unidentified jaeger sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284 

Figure 7.11 Laughing Gull sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .285 

Figure 7.12 Ring-billed Gull sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . .285 

Figure 7.13 Herring Gull sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287 

xai 
Figure 7.14 Royal Tern sightings for Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287 

Figure 7.15 Sandwich Tern sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. . . . . . . . . .. . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .288 

Figure 7.16 Bridled Tern sightings for Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . .288 

Figure 7.17 Sooty Tern sightings for Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .289 

Figure 7.18 Black Tern sightings for Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..289 

Figure 7.19 Unidentified tern sightings for Pelican Cruises 3-7 and 

Oregon li Cruises 199, 203, 204, and 209. . . . . . . . . . . . . . . . . . . .. .. . . .. . . . . . . . . . . .. . .290 

Figure 7.20 Spring sightings. (a) Mean number of combined bird 

sightings per day for Oregon II Cruises 199, 204, 209, and 

Pelican Cruise 5. (b) Number of sightings and individuals 

observed per hour during Pelican Cruise S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .291 

Figure 7.21 Summer sightings. (a) Mean number of bird sightings 

per day for Pelican Cruise 6. (b) Number of sightings and 

individuals observed per hour during Pelican Cruise 6. . . . . . . . . . ..292 

Figure 7.22 Fall sightings. (a) Mean number of combined bird 

sightings per day for Pelican Cruises 3 and 7. (b) Number 

of sightings and individuals observed per hour during 

Pelican Cruise 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..293 

Figure 7.23 Winter sightings . (a) Mean number of combined bird 

sightings per day for Oregon II Cruise 203 and Pelican 

Cruise 4. (b) Number of sightings and individuals 

observed per hour during Pelican Cruise 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .294 

Figure 7.24 All seasons. (a) Mean number of combined bird sightings 

per day for Oregon II Cruises 199, 203 204, 209 and Pelican 

Cruises 3-7 . (b) Number of sightings and individuals 

observed per hour during Pelican Cruise 3-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 

Figure 8.1 Diagram of the satellite telemeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 

Figure 8.2 Sperm whale sightings, 6 to 29 June 1993, aboard R/V 

Acadiana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .309 

Figure 8.3 R/V Acadiana cruise track and sperm whale sightings 6 

June to 22 June 1993, prior to seismic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 

xxii 
Figure 8.4 R/V Acadiana cruise track and sperm whale sightings 23 

June to 24 June 1993, with seismic activity beginning . . . . . . . . . . .. . . 313 

Figure 8.5 R/V Acadiana cruise track and sperm whale sightings 25 

June to 29 June 1993, with seismic vessel activity . . . . . . . . . . . . . . . . . . . . . .314 

Figure 8.6 Cruise track of R/V Pelican, 20-28 August 1994 (TIO Cruise 

8), and sperm whale sightings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .317 

Figure 8.7 Illustration of the use of the horizontal and vertical 

hydrophone arrays to locate sperm whales in 3-

dimensional space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .319 

Figure 8.8 The results of localizing vocalizing sperm whales using 

vertical and horizontal arrays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .320 

Figure 8.9 Sighting dates of identified sperm whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .321 

Figure 9.1 Boa plots of the sea surface temperature associated with 

sightings of eight cetacean species or species categories . . . . . . . .336 

Figure 9.2 Boa plots of the sea surface temperature gradient 

associated with sightings of eight cetacean species or 

species categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .337 

Figure 9.3 Box plots of the bottom depth associated with sightings of 

eight cetacean species or species categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .339 

Figure 9.4 Box plots of the bottom depth gradient associated with 

sightings of eight cetacean species or species categories . . . . . . . .341 

xxiii 
LIST OF TABLES 

Table 1.1 Cetaceans of the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 

Table 1.2 Types of data collected by season and survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 

Table 1.3 GulfCet management structure, principal investigators, 

and their affiliations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 

Table 3.1 Pooling categories for estimating f(O) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65 

Table 3.2 The estimated value of f(0) and associated statistics . . . . . . . . . . . . . . . . . . . 66 

Table 3.3 Categories of cetaceans with similar sighting 

characteristics for which sightings were pooled to 

estimate the parameter f(0) for aerial surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 

Table 3.4 Estimate of f(0) for each category of cetaceans with 

similar sighting characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68 

Table 3.5 Shipboard visual survey effort and cetacean groups 

sighted by vessel and season. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 

Table 3.6 Overall sighting and group-size statistics used for 

estimating cetacean abundances from shipboard 

surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70 

Table 3.7 Summer sighting and group-size statistics used for 

estimating cetacean abundances from shipboard 

surveys . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71 

Table 3.8 Fall sighting and group-size statistics used for estimating 

cetacean abundances from shipboard surveys . . . . . . . . . . . . . . . . . . . . . . . . . . .72 

Table 3.9 Winter sighting and group-size statistics used for 

estimating cetacean abundances from shipboard 

surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 

Table 3.10 Spring sighting and group-size statistics used for 

estimating cetacean abundances from shipboard 

surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

Table 3.11 Summary of the on-effort results of each GulfCet aerial 

survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 

Table 3.12 Summary of on-effort seasonal results of GulfCet aerial 

surveys . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

Table 3 .13 Number of on-effort sightings of cetacean species during 

aerial surveys by temporal strata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 

xxv 
Table 3.14 Estimates of the parameters used to estimate the 

abundance of all cetaceans combined in the aerial survey 

study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . .79 

Table 3.15 Density and abundance estimates of all cetacean species 

combined in the aerial survey study area . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 80 

Table 3.16 Estimates of the parameters used to estimate the 

abundance of cetaceans in the aerial survey study area 

for the entire study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .81 

Table 3.17 Overall density and abundance estimates of cetacean 

species in the aerial survey study area for all seasons 

combined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . .82 

Table 3.18 Estimates of the parameters used to estimate the 

abundance of all cetaceans sighted 20 or more times in 

the aerial survey study area by year and season . . . . . . . . . . . . . . . . . . . . . . . . 88 

Table 3.19 Density and abundance estimates of cetacean species 

sighted 20 or more times by year and season in the aerial 

survey study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . .90 

Table 3.20 Abundance estimates and related statistics of sea turtle 

species in the aerial survey study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 

Table 3.21 Comparisons of overall density estimates from aerial and 

ship surveys, by species. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120 

Table 3.22 Ranked abundances and mean group sizes of cetaceans 

from all ship surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . .123 

Table 3.23 Comparison of mean group size estimates from ship and 

aerial surveys, by species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . .124 

Table 4.1 Number of on-effort acoustic contacts by cruise and 

species. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . .. .156 

Table 4.2 Analysis of sperm whale contacts by cruise . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 157 

Table 4.3 Matrix of pair-wise comparisons of the lengths of all 

sperm whale contacts by cruise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .158 

Table 4.4 Number and length of sperm whale contacts by seasons . . . . . . . . . . 160 

Table 4.5 Matrix of pair-wise comparisons of the lengths of all 

sperm whale contacts by season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 

Table 4.6 Summary statistics of number of sperm whales per visual 

sighting for Pelican/ Longhorn and Oregon II visual 

teams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .177 

xxvi 
Table 5.1 Behavioral reactions of cetaceans relative to the survey 

ship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .196 

Table 5.2 Paired comparisons (Fisher's LSD) of initial sighting 

distances for the 6 species categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 

Table 5.3 Sensitivity of cetaceans and initial behaviors to 

disturbance by survey aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .200 

Table 5.4 Responses of cetaceans, grouped by species categories, to 

the survey aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201 

Table 6.1 TIO and SEFSC hydrographic cruise chronology . . . . . . . . . . . . . . . . . . . . . . . . . 208 

Table 6.2 The type and number of stations and samples taken on 

the Oregon II and Pelican/Longhorn hydrographic 

cruises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .217 

Table 6.3 Oceanographic features located during 1992-1994 and 

their properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .231 

Table 6.4 Chlorophyll a concentrations (mg/m3) and integral 

values (mg/m'-) by quarter for all seven Pelican cruises 

(100 m integration level) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .245 

Table 6.5 Oceanographic features located during GulfCet, LATEX, 

and SOOP programs during 1992-1993, and their 

properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .254 

Table 7.1 Orders, families, and species of seabirds identified on the 

GulfCet cruises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .273 

Table 7.2 Seabird sighting results for Pelican Cruises 3-7 . . . . . . . . . .. . . . . . . . . . . . . . . 274 

Table 7.3 Summary of seabird sightings by percentage of total 

seabird sightings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .275 

Table 7.4 Mean bird sightings per day by season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .276 

Table 7.5 Summary of bird sightings per effort-hour for Pelican 

Cruises 3, 4, 5, and 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .277 

Table 7.6 Summary of the number of bird individuals seen per 

effort-hour on Pelican Cruises 3, 4, 5, and 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .278 

Table 7.7 Summary of seasonal species diversity and total bird 

sightings per day. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .--- . . . . . . . . . . . . . . . . . . . . . . . . .296 

Table 7.8 Comparison of selected species pairs by depth using the 

Two-Sample Wilcoxin Rank-Sum Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 

xxvii 
Table 7.9 Non-seabird sightings by season for Pelican Cruises 3-7 

and all four Oregon II cruises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .298 

Table 8.1 Sperm whale sightings, 7-29 June 1993, RN Acadiana . . . . . . . . . . . . . 310 

Table 8.2 Cetaceans other than sperm whales seen on a sperm 

whale tagging cruise 7-29 June 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .315 

Table 9.1 Cetacean species or species categories that had 10 or more 

sightings for most of the seven environmental variables 

used in the statistical comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 

Table 9.2 Environmental profiles for the cetacean species or 

species categories that had 10 or more sightings for most 

of the variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .333 

Table 10.1 Cetaceans ranked by abundance with mean body mass, 

species-total biomass, and minimum food requirements . . . . . . . . . . . 352 

xxviii 
LIST OF ABBREVIATIONS AND ACRONYMS 

The following acronyms and abbreviations are used throughout this report: 

AAIW Antarctic Intermediate Water 

AGIS Advanced Geographical Information System 

AVHRR Advanced Very High Resolution Radiometer 

CEI'AP Cetacean and Turtle Assessment Program 

CI Confidence Interval 

CID Conductivity, Temperature, and Depth Profiler 

CV Coefficient of Variation 

CZCS Coastal Zone Color Scanner 

DB Decibars 

DBDBS Digital Bathymetric Database-5 Minute Resolution 

dB rel gP Decibels Relative to 1 micro Pascal 

DDS Delta Data Systems, Inc. 

DMA Defense Mapping Agency 

Dyn Ht Dynamic Height 

ESA Endangered Species Act 

FSW Effective Strip Width 

ETP Eastern Tropical Pacific 

GCW Gulf Common Water 

GIS Geographic Information System 

GPS Global Positioning System 

GMMI Gulf of Mexico Master Image 

GulfCet r4MS North-central and Western Gulf of Mexico Cetacean 

Study (this study) 

HPLC High Pressure Liquid Chromatography 

HMSC Hatfield Marine Science Center, OSU 

IMLS Institute of Marine Life Sciences, TAMUG 

IO Independent Observer 

IPS Inches Per Second 

K-S Kolmogorov-Smirnov Two-Sample Statistical Test 

LATEX-A Louisiana and Texas Shelf Circulation and Transport 

Process Study 

LATEX-B Louisiana and Texas Mississippi River Plume Study 

LATEX-C Louisiana and Texas Eddy Circulation Study 

LiJMCON Louisiana Universities Marine Consortium 

MIDAS Multiple Interface Data Acquisition System 

MMPA Marine Mammal Protection Act 

MMRP Marine Mammal Research Program, TAMUG 

MMS Minerals Management Service 

NADW North Atlantic Deep Water 

NAVOCFANO U.S . Naval Oceanographic Office 

NESDIS National Environmental Satellite, Data, and Information 

Service 

NMFS National Marine Fisheries Service 

NOAH National Oceanic and Atmospheric Administration 

r10DC National Oceanographic Data Center 

NOS National Ocean Survey 

xxix 
LIST OF ABBREVIATIONS AND ACRONYMS (continued) 

OCS Outer Continental Shelf 

ONR Office of Naval Research 

06U Oregon State University 

PC Personal Computer 

PSS 78 The Practical Salinity Scale 1978 

PSD Perpendicular Sighting Distance 

PSU Practical Salinity Units 

RTS Real Time Spectrograms 

PTT Platform Transmitter Terminals 

SAS Statistical Analysis System 

SBE Sea-Bird Electronics, Inc. 

SD Standard Deviation 

SE Standard Error 

SEAMAP Southeast Area Monitoring and Assessment Program 

SEFSC Southeast Fisheries Science Center, NMFS 

SGI Silicon Graphics, Inc. 

SIO Scripps Institution of Oceanography 

SLDR Satellite-linked Depth Recorder 

SPL Sound Pressure Level 

SRB Scientific Review Board 

SSC Stennis Space Center 

SST Sea Surface Temperature 

STI Sea Tech, Inc. 

SiJW Subtropical Underwater 

SVA Specific Volume Anomaly 

TAMU Teas A&M University (College Station) 

TAMUG Teas A&M University at Galveston 

TAMUS Texas A&M University System 

'no Texas Institute of Oceanography 

TL Transmission Loss 

T-S Temperature-salinity Relationship 

USGS United States Geological Survey 

VIM Vibration Isolating Mechanisms 

WMO World Meteorological Organization 

XBT Expendable Bathythermograph 

xxx 
ACKNOWLEDGMENTS 

Many persons contributed to this study, including the crews and graduate 

students who worked on the research vessels and aircraft, laboratory 

technicians, and the data management staff. The contributors to this report 

would like to recognize the efforts of T. Adams, M. Armstrong, A. Baldridge, 

N.B. Barros, M.F. Baumgartner, L. Bellaponni, S. Benson, S. Brager, E. Brandon, 

P. Brown, I . Burke, T. Byrne, D.K. and M.C. Caldwell, R. Carroll, G. Childress, 

R. Clark, N. Clauss, V.G. Cockcroft, S. Collier, J. Cotton, P. Crotwell, L. Delos 

Santos, K. Dudzinski, T. Engelhardt, D. Fertl, A. Forest, T. Frohoff, C. Graham, 

A. Grundt, M.J . Hafner, E. Haubold, H.J . Herring, L. Higgins, W. Hoggard, 

M. Howley, D. Johnson, S. Kanatous, A. Landry, Jr., S. Leatherwood, 

D. Ljungblad, B. Mase, M.L. Mate, L. May, M.B. McDuff, J.G. Mead, L. MontanaLuna, P. Mumford, S . Nieukirk, D.K. Odell, S . O'Sullivan, W.F. Perrin, 

W.L Perryman, J. Peterson, R. Pitman, K. Rademacher, L Respess, F. Risinger, 

C. Roden, C . Rogers, D.J . Schmidly, L. Sharp, L.K.M. Shoda, K. Stafford, 

W.E. Stevens III, H. Svehlak, R.J . Tarpley, T. Tenbrink, Wang Ding, L. West, 

L Wilkins, B. Wilson, and G.A.J . Worthy. M. Armstong, P. Brown, N. Clauss, and 

H. Whitehead contributed significantly to Section 8. Special thanks to the 

officers and crew of NOAH Ship Oregon II and the NOAH Aircraft Operations 

Center and pilots B. Taggart, S. Nokutis, T. O'Mara, S. Pape, M. Pickett, M. White. 

We thank Aspen Helicopters, Inc. and their pilot J. Drust. Special thanks also 

go to LUMCON's S. Rabalais, R/V Pelican's captain, C. Riffe and his crew, and 

the crew of University of Texas' R/V Longhorn . The direction and advice of 

R. Avent (Contracting Officer's Technical Representative) was of great service 

throughout this project. Thanks also go to the Scientific Review Board (SRB) 

members for their thoughtful review and comments on the goals and results of 

this project. The GulfCet Data Management Office wishes to express its 

appreciation to all those who served as reviewers of this document. D. Brandon 

served as data analyst and technical illustrator. 

We gratefully acknowledge the Texas Institute of Oceanography and the 

National Marine Fisheries Service for cost-sharing the use of all research 

vessels used for the surveys and sperm whale tagging cruises on this project, 

and the Institute of Marine Life Sciences (TAMUG) for cost-sharing one set of 

"Big Eye" binoculars and various other research items. 

This study was funded by the U.S Department of the Interior, Minerals 

Management Service, under contract C 1435-0001-30619, and Interagency 

Agreement 16197 with the National Marine Fisheries Service. 

xxxi 
I . INTRODUCTION 

R.W. Davis 

1.1 Background and Objectives 

The Minerals Management Service (MMS) is responsible for assuring that the 

exploration and production of oil and gas reserves located more than three 

miles offshore and within the U.S . Exclusive Economic Zone are conducted in a 

manner that reduces risks to the marine environment. To meet their 

responsibilities under the Marine Mammal Protection Act of 1972 and the 

Endangered Species Act of 1973, the MMS must understand the effects of oil and 

gas operations on marine mammals. As the oil and gas industry moves into 

deeper water along the continental slope in their continuing search for 

extractable reserves, information is needed on the distribution, abundance, 

behavior, and habitat of cetaceans, especially large and deep-water species in 

the Gulf of Mexico (Table 1 .1). This study, hereafter called the GulfCet Program, 

was designed to help the MMS assess the potential effects of deepwater oil and 

gas exploration and production on marine mammals in the Gulf of Mexico. 

The purpose of this study was to determine the distribution and abundance of 

cetaceans along the continental slope in the north-central and western Gulf of 

Mexico. The study was restricted to an area bounded by the Florida-Alabama 

border, the Texas-Mexico border, and the 100 m and 2,000 m isobaths 

(Figure 1.1) . This 3 .75 year project commenced on 1 October 1991 and concluded 

on 15 July 1995 . In addition to conducting aerial visual, shipboard visual, and 

shipboard acoustic marine mammal surveys, the GulfCet Program collected 

hydrographic data in situ and by remote sensing to characterize the marine 

habitat of cetaceans in the study area (Table 1 .2) . An attempt was also made to 

tag sperm whales and track their movements using satellite telemetry. 

1.2 Program Participants 

The GulfCet Program was administered by the Texas Institute of Oceanography 

(TIO), which is part of the Texas A&M University System (TAMUS). Researchers 

at Texas A&M University Campuses at Galveston (TAMUG) and College Station 

(TAMU) provided expertise in marine mammal biology, bioacoustics, and 

oceanography. Expertise in aerial and shipboard surveys of marine mammals, 

satellite remote sensing, and Geographical Information Systems (GIS) was 

provided by the National Marine Fisheries Service (NMFS) at the Southeast 

Fisheries Science Centers (SEFSC). The SEFSC that participated in this study 

were the Miami Laboratory and the Mississippi Laboratories, with facilities at 

Pascagoula and Stennis Space Center. This part of the project was contracted 

under a separate Interagency Agreement between the MMS and the NMFS. 

Finally, the program included scientists from the Hatfield Marine Science 

Center at Oregon State University, who have developed techniques to tag and 

track whales using satellite telemetry. A list of the program's participants is 

shown in Table 1 .3 . 
Table 1.1 . Cetaceans of the Gulf of Mexico 

Balaenidae 

Northern right whale 

Balaenopteridae 

Blue whale 

Fin whale 

Sei whale 

Bryde's whale 

Minke whale 

Humpback whale 

Physeteridae 

Sperm whale 

Pygmy sperm whale 

Dwarf sperm whale 

Ziphiidae 

Cuvier's beaked whale 

Blainville's beaked whale 

Sowerby's beaked whale 

Gervais' beaked whale 

Delphinidae 

Melon-headed whale 

Pygmy killer whale 

False killer whale 

Killer whale 

Short-finned pilot whale 

Rough-toothed dolphin 

Fraser's dolphin 

Bottlenose dolphin 

Risso's dolphin 

Atlantic spotted dolphin 

Pantropical spotted dolphin 

Striped dolphin 

Spinner dolphin 

Clymene dolphin 

Eubalaena glacialis 

Balaenoptera musculus 

Balaenoptera physalus 

Balaenoptera borealis 

Balaenoptera edeni 

Balaenoptera acutorostr-ata 

Megaptera novaeangliae 

Physeter macrocephalus 

Kogia breviceps 

Kogia sim us 

Ziphius cavirostris 

Mesoplodon densirostris 

Mesoplodon bidens 

Mesoplodon europaeus 

Peponocephala electr-a 

Feresa a tten ua to 

Pseudorca crassidens 

Orcinus orca 

Globicephala macrorhynchus 

Steno bredanensis 

Lagenodelphis hosei 

Tursiops truncatus 

Grampus griseus 

Stenella frontalis 

Stenella attenuata 

Stenella coeruleoalba 

Stenella longirostris 

Stenella clymene 

Adapted from Mullin et al. 1991 . 

2 
W 

30°N 

25° 

95° 90° 85° 80°W 

Figure 1.1 . Study area between the 100 and 2,000 m isobaths, extending as far east as the Florida-Alabama border, 

and as far southwest as the Texas-Mexico border. 
Table 1 .2. Types of data collected by season and survey. 

Marine Mammal Hydrographic 

Surveys Surveys 

Fv CID 

" 

A 

y O L 

O ~ 

n 

~ 

Z 

L 

Survey Dates J Q CJ X V c 

Spring 1992 

RN Longhorn Cruise 1 15 Apr-1 May 1992 r r r r r r r 

NOAH Ship Oregon 17 Cruise 199 

Leg 1 17 Apr-4 May 1992 r r r r r r 

Leg 2 6-25 Niay 1992 r r r r r r 

Leg 3 26 May-8 Jun 1992 r r r r r 

Summer 1992 

R/V Pelican Cruise 2 10-24 Aug 1992 r r r r r r r 

Aerial 1 10 Aug-19 Sep 1992 r r 

Fall 1992 

Aerial 2 3 Nov-16 Dec 1992 r r 

R/V Pelican Cruise 3 8-22 Nov 1992 r r r r r r r 

Winter 1993 

NOAH Ship Oregon II Cruise 203 

Leg 1 5-17 Jan 1993 r r r r r 

Leg 2 18-30 Jan 1993 r r r r r 

Leg 3 1-14 Feb 1993 r r V r r 

Aerial 3 1 Feb-22 Mar 1993 r 

R/V Pelican Cruise 4 12-27 Feb 1993 r r r r r r 

Spring 1993 

Aerial 4 25 Apr-1 Jun 1993 r 

NOAH Ship Oregon 11 Cruise 204 

Leg 1 3-17 May 1993 r r r r r 

Leg 2 18 N1av-2 Jun 1993 r r r r r 

Leg 3 4-15 Jun 1993 r r r r r 

R/V Pelican Cruise s ?3 May-5 Jun 1993 r r r r r r 

r 

r 

r 

r 

r 

r 

r 

r 

r 

r 

4 
Table 1 .2. Types of data collected by season and survey. (continued) 

Marine Mammal I Hydrographic 

Surveys Surveys 

y 

_^. 

y 

O L U J 

E ,0 ~.. .,' 

r V Fm ~ c'~ ~ c~ 

Survey Dates J < U X U cn Z x 

Summer 1993 

Aerial 5 1-21 Aug 1993 r r 

R/V Pelican Cruise 6 28 Aug-5 Sep 1993 r r r r r r r 

Fall 1993 

Aerial 6 31 Oct-16 Dec 1993 r r 

R/V Pelican Cruise ? 3-14 Dec 1993 r r r r r r r 

Winter 1994 

Aerial 7 

Spring 1994 

NOAH Ship Oregon II Cruise 209 

Leg 1 

Leg 2 

Leg 3 

Leg 

Aerial 8 

31 Jan-15 Mar 1994 r r 

15-24 Apr 199 r r r r r 

?7 Apr-18 May 199 r r r r r 

20-29 May 1994 r r r r 

30 May-10 Jun 199- r r r r r 

2 May-2 Jun 199 r r 

Summer 1994 

RN Pelican Cruise 8 20-28 Aug 1993 r r r 

5 
Table 1 .3 . GulfCet management structure, principal investigators, and their affiliations . 

Randall Davis 

Bernd Wiirsig 

Gerald Scott 

William I?vans 

C;iulielta Fargion 

Robert Benson 

Larry flansen 

Thomas l.eming 

Bruce Mate 

Nelson May 

Keith Mullin 

Program Manager, Principal Investigator 

Deputy Program Manager, Principal Investigator 

Program Manager for SFFSC 

Principal Investigator, TIC) President 

Data Manager, Principal Investigator 

Principal Investigator 

Principal Investigator 

Principal Investigator 

Principal Investigator 

Principal Investigator 

Principal Investigator 

'1'IO, TAMUG 

'I'IO, '1'AMUG 

NMPS, SEFSC, Miami Laboratory 

1'IO, `I'AMiIG 

TIO, TANIi1G 

TIO, "I'AMLI 

NMPS, Sl:l'S( :, Miami Laboratory 

NMFS, SEFSC, Stennis Space Cancer 

OSU, EIMSC 

NMFS, SI:FSC, SCennis Space Center 

NMFS, SEFSC, Pascagoula Laboratory 

'1'I0= "Texas Institute of Oceanography SLFSC= Southeast Fisheries Science Center 

°` '1'nMi1G= "Texas A&M University, Galveston OSU= Oregon State University 

'('AMU= Texas n&M University, College Station FIMSC= Hatfield Marine Science Center 

NMFS- National Marine Fisheries Service 
The GulfCet Program had a Scientific Review Board (SRB) composed of five 

scientists who reviewed and commented on the project's goals, methodologies, 

results, analyses, and conclusions. The SRB members were: 

J. Thomas, Ph.D. 

Office of Aquatic Studies 

Western Illinois University 

Macomb, IL 61455 

H. Whitehead, Ph.D. 

Department of Biology 

Dalhousie University 

Halifax, Nova Scotia, 

Canada B3 H 4J1 

S. Reilly, Ph. D. 

NMFS-Southwest Fisheries Center 

8604 La Jolla Shores Dr. 

La Jolla, CA 92038 

J. Cochrane, Ph.D. 

Dept. of Oceanography 

Texas A&M University 

College Station, TX 77843 

K. Norris, Ph.D. 

1985 Smith Grade 

Santa Cruz, CA 95060 

Dr. N. Bray of the Scripps Institution of Oceanography, La Jolla, California, was 

a previous SRB member who was replaced by Dr. J. Cochrane in September 

1993. 

1 .3 Report Organization 

This report is organized like a book, with an introduction and separate 

chapters covering the major parts of the study. Based on their respective areas 

of expertise, each author contributed to their individual chapters and to 

Chapters 9 (Habitat) and 10 (Conclusions and Recommendations) . 

The Introduction, Chapter l, describes the purpose of this study, the program 

objectives, and the program participants . In Chapter 2, Davis and May provide 

a geographical overview of the Gulf of Mexico. Fargion and Leming then 

describe the general oceanography of the region. A historical overview of the 

abundance and distribution of cetaceans is provided by Jefferson. In Chapter 

3, Hansen and Mullin present the results of the aerial and shipboard visual 

surveys of cetaceans and sea turtles. This was one of the largest parts of the 

GulfCet program, and this chapter forms the core of the species abundance 

estimates, seasonal distribution, and estimates of group sizes in the study area. 

Evans, Benson, Norris, and Sparks present the results of the ship-board 

acoustic surveys in Chapter 4. The use of towed acoustic hydrophone arrays is 

a relatively new technique for censusing marine mammals, and it proved very 

7 
useful for detecting many species of cetaceans and, in the case of sperm 

whales, estimating abundance. The behavioral reaction of cetaceans to the 

aerial and shipboard survey platforms is presented by Wursig, Lynn, and 

Mullin in Chapter S . Their results show how cetacean behavior towards a 

survey platform may influence abundance and distribution estimates. Fargion 

presents the results of the oceanographic surveys in Chapter 6. The GulfCet 

program conducted an extensive survey of the marine environment 

concurrently with the visual and acoustic surveys of cetaceans. These results 

were then used in an analysis of habitat. Although not part of the original 

scope of work, an ornithological survey was conducted by Peake and members 

of the National Marine Fisheries Service during cetacean surveys. The results 

are presented in Chapter 7. Chapter 8 presents data collected during the sperm 

whale focal cruise (TIO Cruise 8), and, although no data was obtained, Mate 

describes the attempts to attach satellite telemeters to sperm whales during 

dedicated cruises in order to track their movements at sea and record their 

diving behavior. He also discusses the technical problems that were 

experienced . There has been increasing interest in habitat partitioning 

among cetaceans, although acquiring the simultaneous data on cetacean 

distribution and environmental characteristics has been difficult. In Chapter 

9, Davis, May, Fargion, and Evans analyze the data from Chapters 3 and 6 to 

develop an environmental profile for cetaceans living in the study area. Some 

evidence is provided for habitat partitioning. In the final Chapter, Davis, 

Mullin, Fargion, May, and Evans draw the final conclusions and make 

recommendations for future research. 

1 .4 Literature Cited 

Mullin, K., W. Hoggard, C. Roden, R. Lohoefener, and C. Rogers, and B. Taggart. 

1991 . Cetaceans on the upper continental slope in the north-central 

Gulf of Mexico. OCS Study MMS 91-0027. U.S . Dept. of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Regional Office, New 

Orleans, LA. 108 pp. 

8 
II . OVERVIEW OF THE GULF OF MEXICO 

R.W. Davis, B. Wiirsig, G.S . Fargion, T.A. Jefferson, and C. Schroeder 

This section of the report will acquaint readers with the general geography, 

climatology and oceanography of the northern Gulf of Mexico. In addition, we 

review historical data on the distribution and abundance of cetaceans in 

offshore waters of the Gulf. We hope that this brief introduction will assist the 

reader in understanding the results and conclusions in the chapters that 

follow . 

2.1 Geographic Overview 

The region encompassing the Gulf of Mexico and Caribbean Sea have been 

termed the "American Mediterranean" since both are isolated and semienclosed basins (Sverdrup et al . 1949) . The Gulf basin encompasses an area of 

about 1 .5 million km2 and is bounded by the United States, Mexico, and Cuba. 

The basin consists of sialic basement materials, and in the east and southeast, 

the carbonate structures of the Florida-Bahama Platform and CampecheYucatan Bank, respectively (Brooks 1973). The Gulf is connected to the 

Caribbean Sea via the Yucatan Straits, a relatively deep (2,000 m) channel, and 

to the Atlantic Ocean through the Florida Straits, a Billed channel with a depth 

of about 860 m (Jones 1973). Based on tabulations from Herring's (1993) 

bathymetric data, continental shelf waters less than 180 m deep cover about 

35 .4% of the total area of the Gulf. The continental shelf varies greatly in 

width. Along the Florida west coast, the southern coast of Texas, and the 

northern coast of the Yucatan Peninsula, the continental shelf is 160-240 lm 

wide. In contrast, it is only 32-48 lm wide at the mouth of the Mississippi River 

and along certain coastal areas of the Bay of Campeche, Mexico. The 

continental slope, defined as bottom depths between 180 and 3,000 m, covers 

about 39 .2% of the total area and contains steep escarpments and numerous 

submarine canyons. The areas located in depths greater than 3,000 m (i .e ., 

Sigsbee Plain and sections of the Lower Mississippi Fan) make up the 

remaining 25.4% of the total area. At its deepest point, on the Sigsbee Plain, the 

Gulf is 3,700 m deep. The bathymetry and principal physiographic features are 

shown in Figure 2.1 . Whereas the continental shelf is a smooth, gently sloping 

plain, the upper continental slope in the north-central and western Gulf is 

characterized by complex hill and basin topography. The average gradient in 

the study area is less steep than the average gradient for the entire Gulf of 

Mexico (Figure 2.2) . 

The formation of the Gulf basin apparently began during the late Paleozoic to 

early Mesozoic eras (Kennett 1982) . The evolutionary history of the Gulf has 

been characterized by significant physiographic changes in the region due to 

global climate changes, sea level oscillations, sediment deposition, erosion, and 

subsidence. The veneer of sediments which covers the region can be classified 

into two categories: those of terrigenous origin, consisting of quartz sand, 

clay, and silts eroded from the continental land masses; and calcareous 

sediments originating from marine flora and fauna. The sediment map of the 

9 
O 

-98 -96 -94 -92 -90 -88 -86 -84 -82 -80 

3 ~ ~.. . ~~.... 0 

, ~ ` o -- 

Tensa -Louislana Shelf 00 

28 8 - ~ -. 

~ ~ 

~ 

2 - 3°°u ~ 6 

Slpbee Abyssal Plain ' ~ o~~locida 

0 1 . 24 

o 

CBmpeChe ~ r 

Bank 2 

,~ D° 

Ba of o w ~ 0 

Pe`~f 0 - P ° 

0 

4^~ ( /> I I I I 

-98 -96 -94 -92 -90 -88 -86 -84 -82 -80 

Figure 2 .1 . Bathymetry and major physiographic features of the Gulf of Mexico. 
u 

a 

Relative Cumulative Frequency (%) 

Figure 2 .2 . Bottom depth profiles of the Gulf of Mexico and GulfCet study area tabulated from Herring's (1993) 

bathymetric data set showing the percentage of each region which is less than a given depth. 

10 20 30 40 50 60 70 80 90 100 
Gulf from Uchupi and Emery (1968) indicates that the two sediment classes 

vary both spatially and in composition across the region . Terrigenous 

sediments are present in bands adjacent to land along the continental shelf, 

with the exception of the Yucutan-Campeche Shelf, and in the northern and 

western Gulf along the continental slope. Calcareous sediments are evident in 

the deeper areas of the Sigsbee Plain and Lower Mississippi Fan, along the 

Yucutan-Campeche Shelf, and the outer continental shelf off the Florida west 

coast. 

The climatology of the central and western Gulf, which includes the GulfCet 

study area, is influenced by two seasonal weather patterns with well-defined 

transitional periods (Florida A&M University 1988) . During the summer and 

early fall months (May to October), moist maritime tropical air dominates, with 

a mean air temperature of approximately 26° C. During the winter and early 

spring months (December to March), the mid-latitude polar jet stream pushes 

southward and displaces the maritime tropical air with cold, dry continental 

air. The mean air temperature during the two transitional months of April and 

November is about 20° C. Winds are typically influenced by tropical air masses 

arriving from the south and southeast in spring and summer, and by cold air 

fronts moving southward in the autumn and winter. Mean winter air 

temperature is around 13° C. Severe hurricanes frequently enter the Gulf 

from the mid-Atlantic Ocean, especially during the summer months. Wind and 

fresh water discharge from the Miss issippi/Atchafalaya River system are the 

dominant factors controlling hydrographic conditions and variability on the 

inner portions of the Texas-Louisiana shelf (Kelly 1988). The 

Mississippi/Atchafalaya River system contributes about 73g'o of the fresh water 

entering the northern Gulf. About 20% is contributed by precipitation, and the 

remaining 7% comes from smaller rivers and streams (Darnell and Phillips 

1988). 

Tides in the Gulf are usually diurnal (one high and one low tide per lunar day 

of 24.84 hours) with a small, semidiurnal component (Mariner 1954) . The 

relative magnitudes of the diurnal and semidiurnal components result in 

mixed tides off the Calcasieu Pass, Louisiana area and primarily diurnal tides 

from the Mississippi Delta eastward and to the west of Galveston, Texas (Kelly 

1988). The tidal range averages about 60 cm throughout the Gulf. 

2 .2 General Oceanography of the Region 

2 .2.1 Circulation Patterns 

The Gulf of Mexico is a semi-enclosed basin with only two openings, the 

Yucatan Channel and the Straits of Florida. Water flow through the two 

openings is further restricted to the upper portion of the water column by 

sills that are 1900 m and 800 m deep, respectively. The circulation of the 

eastern Gulf is governed by the Yucatan Current and the Florida Current. The 

Yucatan Current flows into the Gulf through the Yucatan Channel, and the 

Florida Current flows out of the Gulf through the Straits of Florida. The 

subsequent clockwise flow of water thus created extends northward into the 

Gulf and unites the two currents. This circulatory feature is referred to as the 

Loop Current. 

12 
The Gulf is dominated by two major circulation features: the Loop Current 

system in the eastern Gulf, which sheds eddies as a result of instability 

processes, and an anticyclonic cell (warm-core eddy) of circulation in the 

western Gulf (Nowlin and McLellan 1967, Behringer et al. 1977, Merrell and 

Vazquez 1983). The Loop Current enters the Gulf in a nearly annual cycle. The 

extent of its intrusion into the Gulf varies with season, but reaches a maximum 

in the summer, at which time an anticyclonic eddy usually separates from the 

Loop and drifts westward (Hofmann and Worley 1986, Merrell and Vazquez 

1983). The eddy can and often does reattach itself to the Loop Current. High 

fluctuations in the frequency of eddy formation ranging from 8 to 17 months 

have been reported by Behringer et al. (1977). Advanced Very High Resolution 

Radiometer (AVHRR) data from NOAH satellites have produced maps that show 

Gulf warm-core eddies originating as pinched-off, northward penetrations of 

Loop current meanders. After their separation from the Loop Current, these 

anticyclonic rings drift westward until their progress is eventually 

constrained by shoaling topography that leaves them in a "graveyard" over 

the northwestern continental slope of the Gulf (Vukovich and Hamilton 1989). 

These warm eddies interact with the steep topography of the Mexican and 

Texan continental slope and generate secondary cyclonic (cold-core) eddies . 

These warm-core and cold-core eddies remain in the region, slowly decaying 

or coalescing with another approaching eddy. 

The ability to locate Gulf warm-core rings by their sea surface temperature 

(SST) anomaly is usually limited seasonally to the period. November through 

May. For the remainder of the year, the sea surface temperature for the entire 

Gulf is uniform, and so eddies cannot be distinguished by temperature. During 

the summer months, clouds and water vapor further limit the detection of 

eddies by making clear satellite images of the Gulf difficult to obtain. 

Vukovich and Hamilton (1989), using infrared satellite data (1976-1980), 

showed that the Loop Current covers more than 50% of the oceanic area of the 

Gulf east of 90°W over 50% of the time. They also found that the highest 

probability for warm rings occurred at about 25°N and 92°W. Warm rings are 

also common in the southwestern Gulf, but are not easily detected by satellites 

because of the rapid warming of the region in the spring and the effects of 

cloud cover. 

Vidal et al. (1992) have shown that the weakening of the western Gulf's 

anticyclonic rings' relative vorticity is due to their collision against the 

western Gulf of Mexico's continental slope . Hence, anticyclonic ring 

interactions with the western Gulf boundary give rise to cyclonicanticyclonic ring pairs . Recent field work has shown that when an 

anticyclonic (warm-core) eddy is present in the northwestern corner of the 

Gulf, there are often one or more regions of local, cold cyclonic circulation 

about its perimeter (Biggs et al . 1988, Vidal et al. 1990 and 1992). In addition, 

recent studies have described different types of rings or eddies, including 

anticyclonic eddies, cyclonic eddies, cyclonic-anticyclonic eddy pairs (Merrell 

and Morrison 1981, Brooks and Legeckis 1982), and cyclonic-anticycloniccyclonic triads (Vidal et al. 1994, Jockens et al. 1994) . 

Less is known about the circulation in the western Gulf relative to the eastern 

Gulf (Merrell and Morrison 1981). In general, the large-scale circulation 

13 
consists of a clockwise (anticyclonic) gyre which is most prominent in the 

upper 500 m of the Gulf (Hofmann and Worley 1986). Eastward to westward 

transport associated with the gyre is approximately equal to S x 106 m3 " s-1. 

Variations to this flow are created by the Loop Current in the eastern Gulf and 

a cyclonic eddy in the northwestern Gulf. Two primary mechanisms for 

maintaining the western Gulf anticyclonic gyre have been suggested. The 

first mechanism is thought to maintain the gyre by separated Loop Current 

eddies which have drifted to the west (Ichiye 1967, Schroeder et al. 1974) . The 

second mechanism postulates that the gyre is driven by a curl of wind stress 

and thus is analogous to the world's major ocean gyres (Sturges and Blaha 

1976). The relative contribution of each mechanism may vary greatly from 

year to year. An equal contribution of both mechanisms has been suggested by 

Merrell and Morrison (1981). Figure 2.3 shows a schematic of the Loop Current 

and warm-core eddies pinching off the Loop Current. 

2.2.2 Water Temperature and Salinity 

In 1916, Helland-Hansen (Sverdrup et al . 1949) introduced the study of 

temperature-salinity (T-S) diagrams for analyzing and identifying a complex 

system of water masses. The first attempt to establish a T-S characteristic for 

the Gulf was made by Parr (1935) using the Mabel Taylor winter cruise data. 

Parr was able to recognize two separate water masses within the Gulf: that 

derived from the Caribbean and Gulf water. Maximum sampling depth for 

these data was 200 m, so the distinction of these water masses was confined to 

the this portion of the water column . The first complete coverage of the Gulf 

was with the R/V Hidalgo survey (winter 1962). Nowlin and McLellan (1967) 

analyzed these data and for the first time, constructed T-S diagrams below 

1500 m. 

A temperature-salinity diagram constructed for the Gulf reveals a distinct 

maximum (36.60 to 36.70 psu with a temperature of 22.5°C) and minimum {34.84 

to 34.88 psu} salinity. These salinity signatures are characteristic of 

Subtropical Underwater (SUW) and Antarctic Intermediate Water (AAIW), 

respectively. Each of these water masses is found in the adjacent Cayman Sea 

and enters the Gulf through the Yucatan Channel (Nowlin and McLellan 1967). 

Usually the SUW salinity maximum is centered at about 200 m. The AAIW 

salinity minimum in the eastern Gulf occurs between depths of 800 to 1,000 m 

(shallower in the western Gulf) . Waters below the AAIW are isosaline at 

34.97 psu. This salinity concentration is consistent with that found in the flow 

of North Atlantic Deep Water (NADW) over the Yucatan sill into the Gulf 

(Nowlin and McLellan 1967, Morrison et al. 1983). Hofmann and Worley (1986) 

used these three water masses as a basis for a three-layer system to investigate 

the circulation of the Gulf. 

The SUW is found in the region of the Loop Current and the rings derived from 

the current. These rings constitute the principal mechanism by which 

Caribbean Sea water enters the central and western Gulf (Elliot 1982). Thus the 

SUW can be used as a tracer to identify the presence of anticyclonic rings 

within the Gulf. The collision of Loop Current anticyclonic rings against the 

western continental slope of the Gulf constitutes the principal mechanism 

responsible for the dilution of SUW core water and its conversion to Gulf 

14 
-96 -94 -92 -90 -88 -86 -84 -82 -80 

30 

28 

26 

24 

22 

20 

S I t 1 

MS L 

I 30 

l 

28 

26 

24 

z 

22 

., 20 

-96 -94 -92 -90 -88 -86 -84 -82 -80 

Figure 2 .3 . Schematic of the life-cycle of a Loop Current anticyclonic or 

warm-core eddy (W) as it is pinched off or separated from the 

Loop Current, and its possible paths through the western Gulf of 

Mexico. A time series representation of the Loop Current shows it 

in three possible positions (1-3) . The third position represents 

the most northerly intrusion into the Gulf (this event usually 

occurs in summer), at which time an anticyclonic eddy may 

pinch off from the Loop Current. After its formation, the warmcore eddy may follow one of two paths : a westerly (A) or 

southwesterly (B) path. Cyclonic or cold-core eddies (C) are 

frequently associated with a warm-core eddy . Regardless of 

whether an eddy follows path A or B, anticyclonic eddies spin 

down or fade away in an area of the NW Gulf known as the eddy 

graveyard. This is due to loss of vorticity from collision with the 

continental margin. 

15 
Common Water (GCW). GCW is also formed during the winter by intense 

vertical mixing in the upper 200 to 300 m of the water column. Water in the 

upper 250 m of the western Gulf is then characterized by salinity 

concentrations of 36.4 to 36.5 psu and are then designated as GCW (Morrison et 

al. 1983). 

2.2.3 Mississippi River Influence 

The Mississippi River is the largest river in North America and the sixth 

largest worldwide in terms of discharge (Milliam and Meade 1983). The 

Mississippi River discharges into the northern Gulf through the Balize and 

Atchafalaya delta regions. Approximately 30% of the Mississippi River flow 

enters the northern Gulf through the Atchafalaya, and the remaining 70g'o 

goes through the Balize bird-foot delta. The fresh water influence of this river 

has been observed as far away as 800 km from its source, near Port Aransas, 

Texas (Smith, 1980). In the summer of 1993, the fresh water flow reached the 

Straits of Florida and the east coast of the U.S . (Walker et al. 1994). 

Nearly two-thirds of the U.S. mainland and half the area of Mexico drains into 

the Gulf (Weber et al. 1990) . The Mississippi and other rivers with their 

associated pollutants, nutrients, and sediment loads have a great impact on all 

aspects of continental shelf oceanography in the northern Gulf. The input of 

nutrients ensures high phytoplankton production and thus higher 

zooplankton productivity (Lohrenz et al. 1990) . Twenty-eight percent of the 

total U.S. commercial fish catch is from the Louisiana/Texas shelf (Walker and 

Rouse 1993) . Spawning of key species, such as Gulf menhaden, is also 

concentrated around the Mississippi delta. 

River discharge into the Gulf is distinctly seasonal, with the highest flow 

occurring from March through May, and the lowest flow occurring from 

August through October. Walker and Rouse (1993) utilized four years of AVHRR 

data (1989-1992) to quantify which areas of the continental shelf and slope of 

the Gulf are most influenced by the river. The eighty-three satellite images 

revealed that the Mississippi River plume area varied from 450 km2 to 7,700 

km2 . Under medium discharge conditions (10,001 to 20,000 M3-S-1) the mean 

extent of the river plume covered 2,200 km2 and exhibited a southwestnortheast orientation following the 200 m isobath. Under maximum discharge 

(20,001 to 35,000 m3~s-1), the river's plume covered an extensive area of the 

continental shelf-slope (13,207 km'-) and extended from 88°20'W to 90'50'W and 

offshore to the 1,000 m isobath. 

Walker and Rouse (1993) identified wind as a major force for sediment 

transport. Under the influence of strong northeasterly winds, shelf water can 

be rapidly forced away from the delta and onto the continental slope. The 

Mississippi River plume is then subject to oceanic forcing by eddies and 

filaments detached from the Loop Current. Walker and Rouse also documented 

large, persistent anti cyclonic-cycIonic Loop Current eddies east of the delta. 

The current associated with these eddies can augment the off-shore movement 

of water from the continental slope. 

16 
v 

45 

.. 

g 3s 

k 

v 30 

v 25 

v 

20 

v15 

oa 

X10 

S 

0 

0 

Figure 2 .4 . 

-- ".. 

'*-~ ` _. ,. 

2 4 6 8 10 12 

Time (months) 

Min 1930-

- - 1992 

Max 1930-

1992 

Mean 1930-

1992 

Mean 1993 

Comparison of 1993 total Mississippi River discharge volume to that of the minimum, maximum, 

and mean river discharge volumes (in cubic meters/second x 1,000) for 1930-1992 (data courtesy 

of the LATEX-A program) . 
The 1992-1993 Mississippi River flow was anomalous in its seasonality and flow 

(USGS 1992 and 1993 a and b). High rainfall during the spring of 1993 caused 

the ground to become saturated, so that when the rainfall continued to be 

unusually high throughout the summer, extreme flooding occurred along the 

Mississippi River valley in the late summer of 1993. The result was that the 

highest rainfall and therefore outflow from the Mississippi and its tributaries 

occurred unseasonably in August of 1993 . GulfCet's Cruise 6 (RN Pelican) took 

place during August-September of 1993, and very high fresh water 

concentrations were found from the surface to a depth of three meters on the 

easternmost track-line . The seasonality of the Mississippi River is shown in 

Figure 2.4. This shows the total discharge volume of the river using daily data 

from 1932 to 1992 . Figures 2 .5 and 2 .6 show the flow of the river from 

November 1979 to June 1986 with a time series of chlorophyll pigments from 

the Coastal Zone Color Scanner (CZCS) satellite. The data clearly indicate a 

positive correlation between the Mississippi River flow and the interannual 

variations in chlorophyll concentration, which in turn influence the 

development of high primary productivity in the Gulf. 

Another consequence of the fresh water influence in the Gulf is the hypoxic 

condition (i .e ., oxygen concentrations below 2 mg/1) of waters found along the 

Louisiana coastline west of the Mississippi delta (Rabalais et al. 1991). Two 

events have been suggested to cause this condition. The first event may be 

initiated by an increase in phytoplankton biomass during the summer (a 

bloom), which is fueled by the high nutrient content of the fresh water. The 

sinking and subsequent degradation of this increased biomass causes the 

hypoxia. The second scenario may occur when the river's widespread lowsalinity plume is rapidly heated by solar radiation, resulting in a very stable, 

stratified water mass on the continental shelf. Mixing of the water column is 

prevented by this stratification and leads to stagnant, hypoxic conditions in 

the lower portion of the water column. The effect of this hypoxic condition 

certainly impacts the benthic community, but its affect on the fish community 

has not yet been determined (Rabalais et al . 1991) . 

Wind forcing and shelf currents are major factors controlling the distribution 

of Mississippi River outflow onto the continental shelf. Loop Current eddies 

and filaments provide the major control of plume circulation over the 

continental shelf-slope and into the northern Gulf. The fresh water of the 

Mississippi affects the spatial and temporal distribution of areas of higher 

primary production which may also influence the distribution of cetaceans in 

the Gulf of Mexico. 

2.2.4 Summary 

The Gulf of Mexico is a dynamic body of water dominated by two major 

circulation features. The Loop Current, formed by the interconnection of the 

Yucatan and Florida Currents, governs the circulation of the eastern Gulf. In 

the western Gulf, a warm water anticyclonic eddy with associated cold water 

cyclones is the primary circulatory feature . Temperature-salinity diagrams 

demonstrate the complexity of the Gulf of Mexico. Waters of the Gulf of Mexico 

are derived from three water masses : Subtropical Underwater, Antarctic 

Intermediate Water, and North Atlantic Deep Water. Each of these water masses 

18 
Chl a ----- River Flow 

18- 

16- 

14- 

,-. 12 -

M 

10- 

4- 

2- 

-1800 

-1600 

-1400 

-1200 

-1000 0 

-OOO 

-600 

-400 

-200 

0 0 

11/78 11/79 Ti/80 ' 11/81 11/82 11/83 11/84 11/85 

Time 

, . 

., 

. . ~~ , 

. . . , . ., 

. , . , ~ ., 

., 

. ~ ~ . . 

. . 

. " ~ 

U.

~, . . ,,~ 

. . 

Figure 2 .5 . Time series 1979-198G of Mississippi River flow and chlorophyll pigment data from C7_CS (data in 

close proximity to Pelican/Longhorn station 12-125) . 
Chi a ----- River Flow 

18- 

16- 

14- 

12- 

10- 

`/ 

4- 

2- 

-1800 

-1600 

-1400 

-1200 

-1000 0 

-VOO 

-600 

:.. 

-400 

- 200 

0 0 

11/78 11/79 11/80 11/81 11/82 11/83 11/84 11/85 

Time 

. . ' . , . 

1 

., 

. . . . ,, . . . . ., 

V IJ V --,J V-\ I 

Figure 2 .6 . Time series 1979-1986 of Mississippi River flow & chlorophyll pigment data from CZCS (data in 

close proximity to Pelican/ Longhorn station 11-106) . 
has its own characteristic temperature and salinity range. These 

characteristics allow the identification and tracking of these water masses in 

the Gulf. As mixing and dilution of these water masses occurs, the conversion 

to Gulf Common Water takes place. Gulf Common Water is identified by a 

salinity of 36.436.5 psu. The dynamics of the Gulf are made more complex by 

the large fresh water inflow. Nearly two-thirds of the continental U.S . and 

half of Mexico's land area drains into the Gulf. The associated nutrient input 

from this fresh water inflow increases the level of primary production with a 

subsequent increase in secondary production as well. The overall resulting 

circulation of the Gulf of Mexico is remarkable because of its interannual 

variability and intensity. 

2 .3 Historical Overview of the Distribution of Cetaceans in the 

Offshore Gulf of Mexico 

2.3 .1 Introduction 

This section provides an overview of previous knowledge about the status of 

marine mammals in the north-central and western Gulf of Mexico. This 

overview serves as a bench mark for present and future studies and provides 

information Gulf-wide instead of only in the northern part of the Gulf. 

Many types of marine mammal distribution records were reviewed and 

analyzed for this section of the report. It is important to remember that what a 

particular record indicates about the distribution of a species depends on the 

record type. Sightings (with very few exceptions) can be assumed to indicate 

the true location and movement of the animals. The animals are at a specific 

location because they swam there of their own volition. Not all sightings are 

indicative of the normal range of the species, however, as animals may get lost 

or occasionally venture to areas outside their normal range during unusual 

circumstances. 

Similarly, most direct captures (reports of incidental captures in fishing gear 

are rare in the Gulf) first involve a sighting of a group of animals and can 

generally be assumed to represent a true location . The major exception to this 

generalization involves some of the Gulf records where an animal was 

captured outside its normal habitat, usually very near to shore, possibly in the 

process of stranding. 

Finally, strandings should not be assumed to indicate anything more than a 

very general region of occurrence (and nothing at all of the habitat 

preference of a species) . Reasons for this assumption are that strandings often 

involve sick or injured animals that are behaving abnormally, swimming or 

being carried (sometimes after death) by currents many hundreds of 

kilometers from their normal range. One need only look at the numerous 

stranding records for offshore species, such as sperm whales and beaked 

whales, to understand the significance of this . 

Strandings involve at least one other major bias: they are highly dependent on 

physical features that bring the animals to shore . Currents and weather 

patterns will affect when and where (and even if) an animal strands. In the 

Gulf, for instance, a dolphin that dies off the coast of Louisiana may wash up 

21 
on a beach in southern Texas due to the current system in that part of the Gulf. 

There are also many reports of cetacean strandings in the Gulf of Mexico 

coinciding with the passage of hurricanes or other large oceanic storms 

(Lowery 1943, 1974, Gunter 1955, Waldo 1957, Caldwell and Caldwell 1969, 

Schmidly et al . 1972a, Schmidly and Melcher 1974, Schmidly and Shane 1978, 

Davis 1978, Gruber 1981, Harris 1986). Thus, it should not be assumed that the 

stranding of a cetacean in a certain area at a certain time means that the 

species naturally occurs in that area at that time. 

There have been few systematic surveys of marine mammals in the Gulf of 

Mexico, especially in the offshore areas. For species other than the bottlenose 

dolphin, what is known of their natural history in the Gulf comes mostly from 

occasional strandings or opportunistic sightings, and for at least one species, 

old whaling records. The first large-scale vessel surveys to assess marine 

mammal distribution and abundance in the Gulf of Mexico began in 1990. 

Two reports have previously summarized information on historical cetacean 

records for the Gulf of Mexico . Schmidly (1981) presented distribution maps 

for all species of cetaceans known to occur in the Gulf of Mexico. This analysis 

has continued to be useful, but it is now out-of-date. In addition, because 

Schmidly was not able to verify species identification for many records, there 

were many mistaken identifications . Recently, Jefferson et al. (1992) updated 

Schmidly's maps for a cetacean field guide of the Gulf of Mexico. This guide 

provided more recent information, but suffered from similar verification 

problems with some of the historical records. It is thus suggested that the maps 

of Schmidly and Jefferson et al. not be cited, but that those included in this 

section be used instead. 

This section reviews and summarizes what is known of the historical 

distribution and seasonal occurrence of offshore cetaceans in the Gulf of 

Mexico. Offshore cetaceans are defined here to include all those members of 

the order Cetacea found in the Gulf, with the exception of the bottlenose 

dolphin. This species was not included because it has primarily been 

recognized as a coastal species, and its distribution in the Gulf has been 

comparatively well studied (Leatherwood et al. 1978, Barham et al. 1980, Odell 

and Reynolds 1980, Leatherwood and Reeves 1983, Scott et al. 1989, Mullin et al. 

1990). However, the GulfCet program has found what may be a larger form of 

bottlenose dolphin which is present quite commonly in the offshore waters of 

the Gulf, and more detailed information may be found in Appendix A sighting 

records as well as in Chapter 3 . The analyses in this section are based on all 

available records, published and unpublished, except those resulting from the 

GulfCet program (except where these have already been published: 

Leatherwood et al. 1993, Mullin et al . 1994a, and Mullin et al. 1994b). The 

GulfCet records form a continuous, homogeneous database, based on systematic 

surveys. 

99 
2 .3.2 Data Acquisition 

2.3.2.1 Area of Coverage 

For the purposes of this section, the Gulf of Mexico study area was demarcated 

in the east by a line from the southern tip of Florida to the Cuban coast, 

running along longitude 80°30'W, and in the southeast by the shortest line 

from the northeastern tip of the Yucatan Peninsula to Cabo San Antonio (or 

western), Cuba (refer to Figure 2.7) . 

2.3.2 .2 Sources of Data 

All available cetacean records of strandings, sightings, and captures from the 

waters of the Gulf of Mexico were compiled. A total of 1,223 records were 

available for this analysis. These records came from the following sources: 

Published and unpublished literature - There is a moderate amount of Gulf of 

Mexico cetacean literature available . Most of the published papers report 

strandings or opportunistic sightings, but there are a few reports of livecaptures, specimens collected for research, and whaling catches (e.g., Cuni 

1918, Moore 1953, Layne 1965, Lowery 1974, Schmidly and Melcher 1974, 

Schmidly and Shane 1978). In recent years, there have also been several 

research projects that conducted systematic visual surveys of cetaceans in 

offshore waters of the Gulf (Fritts and Reynolds 1981, Fritts et al. 1983, Mullin 

et al. 1991 - see Figure 2.7 for locations of survey blocks). Identifications were 

verified whenever possible. 

Southeastern United States Marine Mammal Stranding Network (SEUS MMSN) 

Database - The Southeastern U.S . Stranding Network maintains a database of 

reported strandings from Gulf coast states (Texas, Louisiana, Mississippi, 

Alabama, and Florida) (see Odell 1991). Most records in this database are 

unpublished, and although species identifications were not always confirmed, 

most of the data were collected by experienced marine mammal biologists and 

are considered accurate . Data for the years 1977 to 1991 were available 

courtesy of D.K. Odell and N.B. Barros, both of Sea World of Florida 

Texas Marine Mammal Stranding Network (TMMSN) Database - The marine 

mammal stranding network in Texas (Tarpley 1987) has records from the 

inception of the program in 1981 through 1994. These records were made 

available courtesy of G.A.J . Worthy and E. Haubold, each of whom is affiliated 

with Texas A&M University at Galveston (TAMUG). Most records in this database 

are also in the SEUS MMSN database, and it was possible to verify species 

identification for many records by examining TMMSN data and photographic 

files. 

,J.G. Mead's Stranding and Historical Record Database - J.G. Mead, of the National 

Museum of Natural History, Smithsonian Institution, maintained a database 

through 1977 of marine mammal records from the U.S . East and Gulf coasts . The 

majority of records in the database are of strandings, and most are from the 

published literature (see Mead 1975). Not all identifications in this database 

have been verified . 

23 
-96 -94 -92 -90 -88 -86 -84 -82 -80 

30 - 

28 

26 

24 

N 

f 

22 

20 

-_- 

30 

28 

26 

I 

24 

A. Fntts and Reynolds 1981 

B. Fries et al . 1983 

C. Oregon II (SEFSC, unpbl.) 

D. Mullin et al . 1991 

E. Esher et al . 1992 

1 C 

22 

20 

-96 -94 -92 -90 -88 -86 -84 -82 -80 

Figure 2 .7 . Map of the study area showing locations of survey blocks from previous studies. The bold dashed 

lines demarcate the easterly and southeasterly boundaries of the area used in this overview. 
D.J. Schmidly's Gulf of Mexico marine mammal files - Files compiled by D.J . 

Schmidly of TAMUG, primarily during the preparation of his 1981 report 

(Schmidly 1981), were searched and reviewed . Information principally 

consisted of newspaper clippings and previously unpublished stranding 

records. Often photos or other information were available to verify species 

identifications. 

WE Perrin's compilation files of data on dolphins of the genus Stenella -

Several binders of data on dolphins of the genus Stenella, collected by D.K. and 

M.C. Caldwell, G.H . Lowery, and D.J . Schmidly, are in the possession of W.F . 

Perrin, Southwest Fisheries Science Center, NMFS. These binders were 

searched for records of dolphins in the Gulf of Mexico. Examination of photos, 

drawings, or skeletal material allowed verification of most of these records. 

Museum holdings - Gulf of Mexico specimens from several museums were 

examined. The entire cetacean osteological collections of the following 

museums were examined by T.A . Jefferson: the Texas Cooperative Wildlife 

Collection (TCWC), Houston Museum of Natural Science (HMNS), University of 

Southwestern Louisiana (USWL), and Louisiana State University Museum of 

Zoology (LSUMZ). A portion, primarily Stenella specimens, of the Florida 

Museum of Natural History (UF), and National Museum of Natural History 

(USNM) collections were also examined. 

National Oceanic and Atmospheric Administration (NOAH) Vessel Bridge logs -

NOAA vessels stationed at the SEFSC, Pascagoula laboratory (Oregon, Oregon II, 

Researcher, and Chapman) often kept logs of opportunistic sightings of 

marine mammals in the Gulf of Mexico during cruises dedicated to other types 

of research. These data were provided courtesy of K.D. Mullin, K. Rademacher, 

and W.L. Perryman; some records from the Oregon and Oregon II were 

previously plotted in Lowery (1974) . These records cover the years 1950-1992. 

The data were collected by many different people, with widely different 

abilities to identify marine mammals. Many of the identifications could not be 

verified and were not used. 

Oregon II marine mammal surveys - Prior to the present GulfCet study, two 

marine mammal surveys were conducted by the SEFSC using the NOAH Ship 

Oregon II during the spring seasons of 1990 and 1991 (see Figure 2 .6) . These 

marine mammal surveys had a systematic sighting effort and used highly 

trained observers (thus all identifications are verified). Unpublished data 

from these surveys were provided by K.D. Mullin and L.J . Hansen (SEFSC, 

unpubl. ) . 

Several marine mammal surveys, which provided almost no data that could be 

used in this section, have been conducted in the Gulf of Mexico. Esher et al. 

(1992) sighted several large herds of dolphins that were identified as either 

Atlantic spotted dolphins or pantropical spotted dolphins, and groups of 

unidentified whales and dolphins (see Figure 2 .7 for survey blocks) . It was also 

stated in this report that sperm whales, pilot whales, and common dolphins 

were detected by acoustic methods (sonobuoy drops from the survey aircraft), 

but except for sperm whales, these detections were rejected as unreliable 

because of the lack of visual confirmation. The sperm whale detections were 

accepted as verified records because of the unique species-specific 

25 
characteristics of their vocalizations. However, they were not plotted or used 

in the present analyses, because Esher et al . (1992) did not provide positions 

for these records. 

Scott et al . (1989) surveyed out to 9.3 km beyond the 183 m contour. However, 

very few animals other than bottlenose dolphins were sighted, and data on 

these other species were not presented in the report. Similarly, Mullin et al . 

(1990) surveyed coastal and offshore waters (within 37 km of shore), but 

reported no sightings other than bottlenose dolphins. There have been several 

other surveys for bottlenose dolphins in coastal waters, but they have 

provided no data on offshore species of cetaceans (Leatherwood et al. 1978, 

Barham et al. 1980, Odell and Reynolds 1980, Leatherwood and Reeves 1983, 

Mullin et al . 1990). 

Finally, the Galveston laboratory of the NMFS maintains a database of marine 

mammal sightings by observers from their Oil Platform Removal Observer 

Program. These unpublished data, mostly collected over the continental shelf, 

were provided by E. Klima, SEFSC . The database was checked, but since the 

observers were not trained in marine mammal identification, species 

identification was not available (although the vast majority of sightings were 

apparently of bottlenose dolphins). 

2.3.2.3 Verification of Species Identification 

There are many errors in species identification for the older records, and even 

some for recent records from stranding network data. In addition, the 

taxonomy of some groups (such as Kogia and Stenella) has only recently been 

clarified. Thus, for each record, the species identification was questioned and 

an attempt made to verify it . Verification was done in one of several ways: 

review of photographs, drawings, or detailed descriptions of the animals 

demonstrating diagnostic features ; examination of voucher materials, such as 

skulls collected from specimens stranded or captured; identifications made by 

highly trained observers; or identifications made by relatively inexperienced 

observers, but of highly distinctive species (such as sperm whales or killer 

whales) . The ease of identifying each species was kept in mind when judging 

the extent of an observer's identification experience. 

For many records, it was not possible to verify the species identification. 

Unless there was strong reason to believe that the identification was in error, 

these records were included in plots, using a unique symbol (X) indicating 

questionable accuracy. For many other records, the genus could be verified, 

but not the species. Such records were not plotted. Despite this treatment of the 

data, there may still be some errors in a few of the records that have been 

considered verified. These are pointed-out in the text where applicable. Of the 

1,223 records, 1,044 were considered to be verified, 104 were questionable, and 

75 could only be identified to genus. 

2.3 .2 .4 Plotting of Distribution A7aps 

After species verification, records were plotted for each species represented 

by at least two verified records. It should be mentioned that it was not always 

possible to obtain an exact position for each record. In cases where the 

26 
position could be localized to a small area (such as a county or a region of less 

than about SO a 50 km), a position in the middle of that area was chosen for 

plotting. Thus, a small number of records on the maps may appear some 

distance from their true location. For records in which only a very general 

region (such as "the northwestern Gulf of Mexico") was available, the record 

was not plotted. A bar graph of the seasonal distribution of the verified records 

has been overlaid onto each species map. Questionable records were not 

included in these graphs . Distribution maps which are not provided as figures 

in the following section are provided in Appendix A. 

2.3 .3 Species Distribution 

2.3 .3 .1 Sighting and Stranding Records 

Northern Right Whale 

There were only two confirmed records of northern right whales in the Gulf 

of Mexico, a spring sighting off Florida (Moore and Clark 1963) and a winter 

stranding in Texas (Schmidly et al. 1972b). In addition, there were three 

questionable records. Townsend (1935), in his report on nineteenth century 

whaling grounds and whaling catch records, did not show any catches in the 

Gulf of Mexico. Clark (1884), however, did identify the central Gulf as a 

whaling ground for right whales, but did not present any specific records of 

their occurrence there, and we know of no other information that documents 

whaling of this species in the Gulf. 

From the above information, it is concluded that the northern right whale is 

not a normal inhabitant of the Gulf of Mexico. Existing records probably 

represent extralimital strays from the wintering grounds of this species off 

the southeastern U.S . coast from Georgia to northeastern Florida (see Kraus et 

al . 1987). 

Rorquals 

There are five species in the genus Balaenoptera, and they are all 

cosmopolitan, occurring in all oceans and major seas . All five have been 

reported from the Gulf of Mexico. In addition to the records summarized below, 

which were considered to be at least tentatively identified to species, there 

were 14 others that were not supported by enough evidence, or were collected 

by inexperienced observers, and thus could not be confidently assigned to any 

particular species. 

There were only two reliable records of blue whales in the Gulf, both of 

strandings (Lowery 1974, Mead unpubl.), and two additional questionable 

reports. Possibly, some records of unidentified balaenopterids were of this 

species, but there appears to be little justification for considering the blue 

whale to be a regular inhabitant of the Gulf of Mexico. 

Seven reports of fin whales in the Gulf of Mexico, from Louisiana to Florida, 

were considered reliable, but four others were of questionable accuracy. 

Apparently fin whales are not abundant in the Gulf of Mexico, but it is possible 

that the Gulf represents a portion of the range of a low latitude western 

27 
Atlantic population (or a sub-area of it) . Alternatively, the more likely 

consideration is that the fin whale records may be extralimital . All records of 

fin whale occurrence in the Gulf were from summer, fall, and winter. 

Sei whales occur primarily in temperate waters, with lower densities in the 

tropics and near the poles (see Gambell 1985). They were represented in the 

Gulf by only four reliable records, near Louisiana, and one questionable one. 

This species should be considered most likely to be of accidental occurrence in 

the Gulf, although it is worth noting that three of the four reliable records 

were from strandings in eastern Louisiana. 

The Bryde's whale was represented by more records than any other species of 

baleen whale (15 verified records in the northern Gulf, and three 

questionable) . The frequency with which Bryde's whales have been identified 

in recent years suggests that many of the older records of unidentified 

balaenopterids may be of this species. It is interesting that all the sightings 

were from the shelf edge near the De Soto Canyon . Bryde's whales are known 

to be year-round inhabitants of tropical and subtropical waters (see Cummings 

1985). Stranding records for the Gulf of Mexico were scattered throughout the 

year, with the three spring sightings all resulting from the Oregon II surveys. 

It is likely that the Gulf of Mexico represents at least a portion of the range of 

a dispersed, resident population of Bryde's whales. This appears to be the most 

common species of baleen whale in Gulf waters. 

There were 10 reliable and two questionable records of minke whales in the 

Gulf of Mexico, from Texas to Florida, all were of strandings. Seven out of eight 

records for which the season was known were from winter or spring . It is 

suggested that either minke whales migrate into the Gulf regularly in winter, 

but in small numbers, or that these records represent strays from low-latitude 

breeding grounds elsewhere in the western North Atlantic. The latter 

explanation was proposed by Mitchell (1991), and it is considered the more 

likely of the two. 

Humpback Whale 

Seven records of humpback whales, from Texas to Florida, were considered 

reliable either because they were supported by photographic or written 

evidence, or because sightings were made by observers with enough 

experience to recognize this highly distinctive species . Two records were 

questionable. The records were all sightings, several of which have been 

noted to be of small animals (see Weller et al . in press) . All except one (Aguayo 

1954) of these sightings were in shallow, nearshore waters. 

The West Indies breeding grounds of the western North Atlantic stock of 

humpbacks is well-known (Katona and Beard 1991). Although most of the 

winter population is located at the Silver and Navidad banks north of the 

Dominican Republic, some whales venture as far south of the normal breeding 

grounds as the coast of Venezuela (Katona and Beard 1991) . It seems likely that 

some humpbacks stray into the Gulf of Mexico during the breeding season or 

on their return migration northward. This hypothesis is supported by the time 

of year in which the sightings occurred (all six in winter and spring), and the 

small size of the animals involved in many sightings (most likely 

28 
inexperienced yearlings on their first return migration) . This is similar to the 

occurrence of gray whales in the northern Gulf of California, reported by 

Tershy and Breese (1991), except that gray whales' use of the Gulf of California 

may be a more regular occurrence. 

Sperm Whale 

Sperm whales have an extensive deep water distribution, ranging from the 

tropics to the Arctic ice edges in both hemispheres (Rice 1989). There were 

numerous records of sperm whales in the Gulf of Mexico (189 reliable records 

and two questionable ones), more than for any other species of offshore 

cetacean, except the Atlantic spotted dolphin (Figure 2.8). Townsend (1935) 

identified the Gulf of Mexico as a significant sperm whaling ground for 

nineteenth century Yankee whalers. Although the exact dates were not 

reported by Townsend, most of the catches occurred from spring to summer in 

the area between the Straits of Florida and the Mississippi River delta. The 

historical records show no strong seasonal pattern (see Figure 2.8) . The low 

number of sightings in winter probably resulted from decreased offshore 

human activity and poorer sighting conditions during that time of year. 

Sperm whales were found primarily in deep waters beyond the edge of the 

continental shelf (although there are a few records from over the shelf) . In 

the Gulf of Mexico, this generally puts their distribution offshore several tensto-hundreds of kilometers. However, in areas where the continental shelf is 

very narrow (such as off the Mississippi River delta), sperm whales may be 

seen close to shore. 

It appears likely that there is a resident population of sperm whales in the 

Gulf of Mexico, but only identification and or long-term tracking of individual 

whales will tell us how much interchange there may be with populations in 

the Atlantic Ocean. There is no doubt, however, that sperm whales are the most 

common large whales in the Gulf of Mexico, and that they can be found there 

at any time of year. 

J~vemy and Ihvarf Sperm Whale 

Two species of Kogia are currently known, although it was not until Handley's 

(1966) study that the dwarf sperm whale classification was widely recognized 

as valid. Due to the taxonomic problems and the continuing difficulty in 

distinguishing these animals by many observers (especially in sightings at 

sea), there were many (48) records that could not be confidently assigned to 

either species. In addition, it is likely that a few of the records assigned to one 

or the other species were, in fact, misidentifications . 

Historical records of pygmy sperm whales in the Gulf of Mexico were 

exclusively of strandings or presumed strandings from Texas to Florida (61 

reliable records and two questionable ones) . This species was previously 

considered to be rare (see Schmidly 1981), but the number of records would 

indicate otherwise. Alternatively, it may be due to an increased tendency to 

strand or increased mortality in colder months. Whatever the case, these data 

do not indicate that this species is rare in the Gulf. The absence of sightings 

29 
Legend: 

Spring 

Strandings 

Spring 

Sightings 

Summer 

Strendings 

Summer 

Sightings 

Fall 

Strandings 

Q Fall 

Sightings 

Winter 

Strandings 

Winter 

Sightings 

Unknown 

Strandings 

+ Unknown 

Sightings 

X Questionable 

Records 

-y4 -Yl -`JU -8K -86 -254 -8l -8U 

n . . _ 

p 

p p Q %++ +~- + to ++ D 

O p ~ ++ 

op o ~ + d~+++ t + 

O O -iD 

+++~+~~ 

O 

D 

so 

40-

2o 

0 - TTT 

S pr Bum Fell Wint Unk 

-1 1 I I 

-94 -92 -90 - -86 -84 -82 -80 

Figure 2 .8 . Records of sperm whales in the Gulf of Mexico. Seasonality is represented by the bar graph in 

the lower right hand corner, where sightings are indicated by shaded bars and strandings are 

unshaded bars. 
probably has to do with the cryptic behavior and difficulty in positively 

identifying this species at sea. Many of the sighting records identified as Kogia 

sp. on recent shipboard and aerial surveys were probably of this species. 

The dwarf sperm whale, like its congener, was previously thought to be rare 

in the Gulf of Mexico. However, there are numerous records (39 reliable ones) 

from this body of water, many of them strandings. In recent years, the first 

reliable sightings at sea of this species in the Gulf have been reported. There 

are stranding records from all four seasons. 

Cuvier's Beaked Whale 

Cuvier's beaked whale is the most cosmopolitan of all the beaked whales 

(Heyning 1989) . It was reported at least 24 times from the Gulf of Mexico (with 

one additional questionable record), and may be the most common beaked 

whale in these waters. Most of the records are of strandings and are scattered 

more-or-less equally throughout the four seasons. 

Mesoplodonts 

Three of the 13 species in the genus Mesoplodon have been reported from the 

Gulf of Mexico. This is a problematic group because the taxonomy is still in a 

state of flux, and because identification presents major challenges due to poor 

documentation of diagnostic characteristics. One species was first described in 

1991 (Reyes et al. 1991). Only rarely have at-sea sightings been identified to 

species (although the frequency of such identifications is increasing as our 

knowledge grows), and even specimens "in hand" often can not be identified 

without museum preparation. Because of these difficulties, any assessments of 

mesoplodont distribution in the Gulf must be considered highly tentative. 

Other species, such as True's beaked whale, which has not yet been recorded in 

the Gulf of Mexico but is known from the nearby Bahamas (Mead 1989), may be 

shown to occur in the Gulf in the future . 

There were only three confirmed records of Blainville's beaked whale from 

the Gulf, plus one questionable record . This species has the widest distribution 

of all the mesoplodonts (Mead 1989) . All of the Gulf records were of strandings, 

and all the reliable records were from December and January. 

Sowerby's beaked whale was represented in the Gulf of Mexico by only one 

record, a stranding in Florida (Bonde and O'Shea 1989). This species normally 

occurs much further north in cool temperate waters (Mead 1989), and this 

record was thus considered extralimital . 

Gervais' beaked whale is the most common mesoplodont stranded along the 

Atlantic coast of the United States, and most records of its occurrence are from 

the western North Atlantic (Mead 1989) . There were more records for Gervais' 

beaked whale in the Gulf of Mexico than for any other species of Mesoplodon. 

There were 16 reliable records (all strandings), plus another questionable one. 

This species is probably the most common mesoplodont in these waters . 

However, this conclusion must be considered tentative, as the sample sizes are 

still small and many Mesoplodon records remain unidentified to species. 

31 
ded Whale 

Melon-headed whales occur throughout the tropical and subtropical waters of 

the world (Perryman et al. 1994) . The first records of this species from the Gulf 

of Mexico were obtained only recently. These consisted of two strandings, one 

in Texas in 1990, the other in Louisiana in 1991 (Baryon and Jefferson 1993). 

There have been a number of recent sightings, all associated with the GulfCet 

program, (Mullin et al . 1994b), bringing the total number of reliable Gulf 

records to 12 . Most of the sightings have been in deep waters, well beyond the 

edge of the continental shelf. Records exist for all seasons except fall . 

Evgm,v Killer Whale 

Pygmy killer whales occur around the world in tropical and subtropical waters 

(Ross and Leatherwood 1994). These animals do not appear to be very common 

in the Gulf of Mexico; there were only 19 reliable records, most of them 

strandings. Records were found for all four seasons. 

False Killer Whale 

False killer whales are found in tropical to warm temperate waters of the world 

(Stacey et al. 1994) . There were 27 reliable records from the Gulf of Mexico. 

Several sightings have occurred over the continental shelf, although the 

majority appeared to be in oceanic waters. Stacey et al. (1994) mentioned that 

inshore movements associated with movements of prey and warm-water 

currents have been documented. Strandings have occurred in all four seasons. 

Killer Whale 

Killer whales are found in all oceans and seas and probably have the most 

extensive distribution of any cetacean (see Heyning and Dahlheim 1988). 

There were 15 reliable records of killer whales in the Gulf of Mexico (plus two 

others that were questionable), mostly of sightings at sea. There were records 

for all four seasons . In recent years (since 1989) there have been at least nine 

sightings (mostly resulting from GulfCet cruises and aerial surveys and thus 

not summarized in this section), and some have involved resightings of 

previously seen pods or individuals (Roden et al. 1993) . Most of these sightings 

have been in oceanic waters greater than 200 m deep (Roden et al . 1993), 

although there were other sightings from over the continental shelf. 

Katona et al . (1988) located records of only four sightings, four strandings, and 

two fishery catches from the Gulf of Mexico. This report stated that killer 

whale use of the Gulf was unclear, but considered killer whales uncommon in 

the Gulf. On the basis of the gathered data in this report, it seems likely that 

there are a small number of pods that use the offshore waters of the Gulf of 

Mexico as all or part of their normal range. 

Short-finned Pilot Whale 

Two species of pilot whales are currently recognized, the long-finned pilot 

whale and the short-finned pilot whale. The taxonomy of this genus is still 

somewhat controversial, and in this section, all of the pilot whale records from 

39 
the Gulf are assumed to be the short-finned pilot whale, the more tropical of 

the two species. This assumption is based on the currently known distributions 

of the two species and their habitat preferences (see Bernard and Reilly in 

press) . However, it should be kept in mind that the identifications of many 

specimen records and most or all sightings have not been unequivocally 

shown to be of the short-finned species. 

Based on historical records (mostly strandings), the short-finned pilot whale 

would be considered to be one of the most common offshore cetaceans in the 

Gulf of Mexico, with more records than any species except the sperm whale, 

and Risso's, pantropical spotted, and Atlantic spotted dolphins. A total of 64 

records were accepted as reliable, and an additional 17 were considered 

questionable. However, recent aerial and shipboard surveys in the northern 

Gulf of Mexico have not borne out the conclusion that pilot whales are 

common in the Gulf, as they have only been occasionally sighted. This is 

evidence that strandings are not necessarily a good indicator of a species' 

relative abundance. Since pilot whales tend to strand in mass, they are more 

likely to be discovered and reported, and so may be disproportionately 

represented in stranding records. 

One potential explanation for the preponderance of pilot whales in the older 

records and their surprising rarity in recent surveys is that many of the old 

records were misidentifications of other "blackfish" (i .e ., pilot, killer, false 

killer, pygmy killer, or melon-headed whales, or sometimes Risso's dolphins), 

most likely false killer whales. This is a possibility, but many of the older 

records are supported by photographs or voucher specimens (skulls) that 

were collected from stranding sites . 

Rough-toothed Dolphin 

Rough-toothed dolphins are found in tropical to warm temperate waters of the 

world (Miyazaki and Perrin 1990 . Although not very common, the historical 

records nonetheless indicate that the rough-toothed dolphin occurs in the Gulf 

of Mexico throughout the year. The 21 verified records (plus one additional 

questionable record) were from all four seasons. The apparent peak in spring 

sightings most likely results from the effort associated with the Oregon II 

surveys. 

's Dolphin 

This is a tropical species, found worldwide on the high seas and nearshore in 

some areas where deep water approaches the coast (Penin et al . 1994b). There 

are very few records from the Atlantic Ocean (see Leatherwood et al. 1993) . 

Until 1992, there was only a single record from the Gulf of Mexico, a mass 

stranding in the Florida keys (Hersh and Odell 1986) . The first sightings in the 

Gulf were made in 1992, and since then there have been a number of others, 

mostly associated with the GulfCet program (Leatherwood et al. 1993). The 

seven verified records come from all four seasons. The recent spring and 

summer sightings reflect the bias in sighting effort during this time of year, 

and the three strandings occurred one each in spring, fall, and winter. 

33 
's Dolphin 

This species is found in all major oceans in tropical to warm temperate waters 

(Kruse et al . in press) . Although the first record for this species in the Gulf of 

Mexico was only documented in 1966 (Paul 1968), and Risso's dolphins were 

previously considered to be rare in the Gulf, there are now numerous records 

(Figure 2 .9) . A total of 97 reliable records were located, the vast majority of 

them sightings, and most of these from Mullin et al. (1991). There is a large 

peak in sightings during the spring months, and this may be indicative of 

increased abundance on the upper continental slope in this season. 

Atlantic Spotted Dolphin 

Atlantic spotted dolphins are endemic to the tropical to warm temperate 

Atlantic Ocean (Perrin et al. 1987, 1994a) . Although Atlantic spotted dolphins 

have been recorded near oceanic islands and far offshore, they occur in the 

Gulf of Mexico almost exclusively over the continental shelf and the shelf edge 

(Figure 2.10) . The animals from offshore and around oceanic islands are 

smaller and more lightly spotted, and may represent a different form than the 

coastal animals (Perrin et al. 1994a). There are more Gulf records of Atlantic 

spotted dolphins than there are for any other species of offshore cetacean. 

There were 194 records that were considered reliable and an additional seven 

that were questionable. This is the only species, other than the bottlenose 

dolphin, that commonly occurs over the continental shelf. It is also the only 

species for which there is adequate information to assess its occurrence in 

Mexican waters, which is also primarily over the continental shelf. The 

apparent peak in sightings for the spring months may be real, as sightings 

per unit effort also increase in the spring (Mills et al. 1993) . However, 

sighting rate is not necessarily a good indicator, because it does not account 

for bias due to seasonal effects on sightability . One interesting aspect of this 

species' distribution was the low number of strandings; almost all records were 

of sightings or captures. It is unclear why a species as common as this one 

appears to be is so poorly represented in the stranding record, but apparently 

they seldom strand. 

Pantropical Spotted Dolphin 

This is a tropical species, known from the Pacific, Indian, and Atlantic oceans 

(Perrin and Hohn 1994). Most historical and recent evidence indicates that this 

species is the most common and abundant delphinid in the oceanic (deeper 

than 200 m) waters of the Gulf of Mexico (Figure 2.11) (Johnson et al. 1991, 

Jefferson and Lynn 1990. A total of 112 records have been located, mostly of 

sightings . Many of these sightings were the result of recent deep-water 

surveys in the northern Gulf (Mullin et al . 1991, SEFSC unpublished) . The 

previous conclusion that this species was uncommon (see Schmidly 1981) was 

probably the result of this species' confused taxonomic status . Many older 

records of Stenella that could not be identified to species were probably 

pantropical spotted dolphins. Since its re-description (Perrin et al. 1987), there 

have been numerous reports of this species in the Gulf. 

3.4 
w Vll 

Legend: 

Spring 

Strendings 

0 Spring 

Sightings 

Summer 

Strandings 

0 Summer 

Sightings 

Fall 

Strendings 

Q Fall 

Sightings 

Winter 

Strandings 

Winter 

Sightings 

Unknown 

Strandings 

+ Unknown 

Sightings 

X Questionat 

Records 

Figure 2 .9 . 

-94 -92 -90 -88 

D Q 

O 

o ° 

-84 -82 -80 

W50 -

40-

30 -

0 

20 -

10 -

3pr Bum Fell Wmt Unk 

-94 -88 -86 -82 -80 

Records of Risso's dolphins in the Gulf of Mexico. Seasonality is represented by the bar graph in 

the lower right hand corner, where sightings are indicated by shaded bars and strandings are 

unshaded bars. 
-96 -94 -92 -90 -88 -86 -84 -82 -80 

w 

Legend: 

Spring 

Strandings 

0 Spring 

Sightings 

Summer 

Strandings 

Summer 

Sightings 

0 Fall 

Strandings 

O Fall 

Sightings 

Winter 

Strandings 

Winter 

Sightings 

Unknown 

Strandings 

Unknown 

Sightings 

X Questionable 

Records 

O 

O 

N ~_p ! ~Y 

O 0 

60 

+ o AOo 000 

1

0 

+ 

o . . 

2° 

o 

Spr Sum Fell Wml Unk 

I `'~I - I I ~~ I I I I 

-96 -94 -92 -90 -88 -86 -84 -82 -80 

Figure 2 .10. Records of Atlantic spotted dolphins in the Gulf of Mexico. Seasonality is represented by the bar 

graph in the lower right hand corner, where sightings are indicated by shaded bars and 

strandings are unshaded bars . 
-96 -94 -92 -90 -88 -86 -84 -82 -80 

w 

v 

Legend: 

Spring 

Strandings 

0 Spring 

Sightings 

Summer 

Strandinge 

[] Summer 

Sightings 

Fall 

Strandings 

Q Fall 

Sightings 

Winter 

Straadinge 

0 Winter 

Sightings 

Unknown 

Strandings 

+ Unknown 

Sightings 

X Questionable 

Records 

A'IqK7°~ O 

O 

CT-1A 

o er ear c$ 0 0 

D ~ 

O 

D 

D A 

O 

eo 

ao 

20 

0 

Spr Sum Fell Wml Unk 

I I ~~ I I I I 

-94 -92 -90 -88 -86 -84 -82 -80 

Figure 2.11 . Records of pantropical spotted dolphins in the Gulf of Mexico. Seasonality is represented by the 

bar graph in the lower right hand corner, where sightings are indicated by shaded bars and 

strandings are unshaded bars. 
The sightings reported from the mid-continental shelf area off the Yucatan 

Peninsula by Fuentes et al. (1986) do not agree with what is known of the 

preferred oceanic habitat of the pantropical spotted dolphin. These sightings 

may be misidentifications of Atlantic spotted dolphins. 

Striped Dolphin 

Striped dolphins occur from tropical to warm temperate waters in all three 

major ocean basins (Perrin et al . 1994c) . There were relatively few verified 

records (27), but many questionable ones (41) (Figure 2 .12) . Many of the 

questionable records were from aerial surveys by Fritts et al. (1983). There 

were records from all seasons. 

In most areas of the world, the primary habitat of striped dolphins appears to 

be deep water. Thus, there is reason to question the accuracy of the many 

sightings reported from the west Florida shelf by Fritts et al. (1983). These 

were made from aircraft, in which species identification can be difficult (as 

described by Mullin et al. 1991) . It is possible that many, if not all, of these 

sightings were misidentifications of Atlantic spotted dolphins. Groups of 

young spotted dolphins, which are not heavily spotted, may be easily mistaken 

for striped dolphins (especially from the air), since both have prominent 

spinal blazes . 

Spinner Dolphin 

Spinner dolphins are pantropical animals (Perrin and Gilpatrick 1994) . In the 

Gulf of Mexico, only 19 verified and 10 questionable records were found. This is 

fewer than for any other species of Srenella. There were records from all four 

seasons. 

The distribution of this species in the tropical Atlantic appears to be primarily 

oceanic, with the exception of movements into shallow, nearshore waters, 

such as around the island of Fernando de Norohna off Brazil (Lodi and Fiori 

1987). Thus, some of the continental shelf sightings reported for the west coast 

of Florida (Fritts et al . 1983) and the Campeche Bank, northwest of the Yucatan 

Peninsula (Urban-Ramirez and Aguayo-Lobo 1983) may be misidentifications . 

Clvmene Dolphin 

The clymene dolphin is an Atlantic endemic, found in tropical and subtropical 

waters (Perrin et al . 1981, Perrin and Mead 1994). There were 50 verified 

records for the Gulf of Mexico (plus one questionable one from the southern 

Gulf), indicating that this species is not rare in this body of water (Figure 

2.13). The rarity of clymene dolphin records in the past (see Schmidly 1981) 

was probably only a result of its recently clarified taxonomic status and the 

tendency of observers to confuse it with other species (see Perrin et al . 1981). 

Records were from all seasons of the year. The large spring sighting peak is 

probably due to the seasonal bias in survey effort of the Oregon II and in the 

present study (Mullin et al . 1994a) . 

38 
Legend: 

A Spring 

Strendings 

Spring 

Sightings 

Summer 

Strandings 

0 Summer 

Sightings 

Fall 

Strandings 

Fall 

`9 Sightings 

Winter 

Strandings 

Winter 

Sightings 

Unknown 

Strandings 

-i- Unknown 

Sightings 

X Questionable 

Records 

-96 -94 -92 -90 -88 -86 -84 -82 -80 

3 0 

2 X o 

Ch 

~ 8 

X x _Y 

D 

X 

2 X ~ X X 6 

X X 

24- 24 

22 22 

h'o''~'r 

e 

s 

0 

z 

0 

Spr Sum fell Wint Unk 

-96 -94 -92 -90 -88 -86 -84 -82 -80 

Figure 2 .12 . Records of striped dolphins in the Gulf of Mexico. Seasonality is represented by the bar graph in 

the lower right hand corner, where sightings are indicated by shaded bars and strandings are 

unshaded bars. 
-96 -94 -92 -90 -88 -86 -84 -82 -80 

Legend: 

Spring 

Strandings 

Spring 

Sightings 

Summer 

Sh'Blldltlgs 

Summer 

Sightings 

Fall 

Strandings 

Q Fall 

Sightings 

Winter 

Strendings 

40 Winter 

Sightings 

Unknown 

Strandings 

-}- Unknown 

Sightings 

X Questionable 

Records 

AAA 

o 0 

n 

I I ~~ I 

-92 -90 -88 -86 

g 

s 

4 -

" 

0 

- ii ~] 

Sp Sum Fell Wmt Unk 

-84 -g2 

Figure 2 .13. Records of clymene dolphins in the Gulf of Mexico. Seasonality is represented by the bar graph 

in the lower right hand corner, where sightings are indicated by shaded bars and strandings are 

unshaded bars. 
bin 

Common dolphins occur in tropical to warm temperate waters of all three 

major oceans (Evans 1994) . Although long-beaked and short-beaked varieties 

of common dolphins have long been recognized, until recently they were 

classified as one species by most biologists . However, Heyning and Perrin 

(1994) have discovered that the two types represent separate species in the 

eastern North Pacific. These are the short-beaked common dolphin and the 

long-beaked common dolphin. These differences appear to apply to other 

ocean basins as well. Short-beaked common dolphins are known from the east 

coast of Florida, while the nearest occurrence of the long-beaked species is 

from Venezuelan waters (Heyning and Perrin 1994). 

Only three alleged specimen records of common dolphins from the Gulf of 

Mexico were located. These consist of a live-capture of two specimens from 

near St. Petersburg, Florida in the spring of 1965 (R-G-I-SIS), which was 

mentioned by Caldwell and Caldwell (1973) . These animals were later reidentified as "short snouted spinner dolphins" (Caldwell and Caldwell 1975), 

and photographs published therein leave no doubt that these animals were 

indeed clymene dolphins. A stranding at Sabine Pass, Texas, on 16 May 1974 

(TCWC 28286) was identified at the time as a common dolphin, but was later reidentified as a spinner dolphin by Schmidly and Shane (1978). Finally, a livestranding of a common dolphin in Galveston, Texas on 30 March 1979 (TCWC 

50849), was briefly mentioned in Schmidly (1981) . The skulls of the latter two 

specimens have been examined by T.A. Jefferson and the identifications were 

confirmed as spinner dolphin (TCWC 28286) and clymene dolphin (TCWC 

50849), respectively (see Jefferson et al. 1995). Thus, there are no valid 

specimen records of the genus Delphinus from the Gulf of Mexico. 

There have also been several reported sightings of common dolphins in Gulf 

of Mexico waters (Cuni 1918, Caldwell 1955, Caldwell and Caldwell 1973, Lowery 

1974, Fritts and Reynolds 1981, and Dorf 1982). However, none of these was 

accompanied by photographs, sketches, or detailed descriptions of diagnostic 

characteristics used in identification. Most of the reports were made by 

untrained observers and were made at a time when the taxonomy and 

diagnostic features of the long-snouted tropical dolphins were poorly known. 

All of these sightings occurred prior to 1981, when the re-description of the 

similar-appearing clymene dolphin was published (Perrin et al. 1981) . 

Further, the sightings by Fritts and Reynolds (1981) and Dorf (1982) were made 

from aircraft, a platform from which identification can be difficult (see 

Mullin et al. 1991) . 

In conclusion, all reported records of common dolphins from the Gulf of 

Mexico are rejected as either incorrect or questionable. Common dolphins 

should not, at this time, be considered a species known to occur in the Gulf of 

Mexico. This conclusion is supported by the results of the GulfCet surveys. 

2.3 .4 Summary and Conclusions 

There are often strong geographic and seasonal biases in sighting and 

stranding recovery efforts. For essentially all types of records, almost none 

41 
were available for the southern Gulf of Mexico. Thus, distribution maps 

showing records primarily from the northern Gulf of Mexico do not, in any 

way, imply that these species do not occur in the southern Gulf. On the 

contrary, there is no reason to believe that any species known from the 

northern Gulf does not also occur in the southern part of the Gulf of Mexico. 

Areas of the northern Gulf coast with low human population density also show 

a conspicuous lack of strandings. 

Likewise, seasonal graphs must be viewed with several important biases kept 

in mind. For the most part, effort to document strandings since the inception 

of the Southeastern U.S. Marine Mammal Stranding Network in 1977, has been 

relatively even throughout the year. However, since stranding recoveries 

depend heavily on reports from the general public, the summer season (when 

more people are at the beach) would be expected to produce more stranding 

records than other seasons. This is probably even more true of older stranding 

records from a time prior to the establishment of systematic data collection, 

and for species, such as pilot whales, which mass strand and were therefore 

more likely to be discovered and reported. Finally, opportunistic sightings 

tend to be biased towards the time of year when the weather is good and more 

people are venturing offshore (primarily summer) . In addition, one of the 

major sources of high-quality data for this analysis, the Oregon II surveys 

(SEFSC unpubl.), have occurred almost exclusively in the spring. Thus, the 

apparent spring and or summer peaks in sightings for many species may only 

be artifacts of seasonal biases in survey effort. In addition, seasonality of 

strandings is not necessarily directly related to population abundance. It may, 

instead, reflect seasonality in mortality patterns. 

The apparent absence of common dolphins in the Gulf of Mexico is interesting 

and worthy of examination. Schmidly and Scarbrough (1990) noted that 

common dolphins had not been documented from the Gulf in the past decade, 

but apparently did not doubt the accuracy of the older records. Common 

dolphins occur in most tropical to warm temperate waters of the world (Evans 

1994, Heyning and Perrin 1990 . Their status in northeastern Florida (the 

nearest area to the Gulf with confirmed records) was discussed by Caldwell and 

Caldwell (1978) . Common dolphins were once abundant in northeastern 

Florida, but in the past few decades have disappeared from those waters. The 

last known sightings and strandings were in 1958 and 1960, respectively 

(Caldwell and Caldwell 1978). The disappearance of common dolphins from 

northeastern Florida may be the result of natural fluctuations in numbers or 

distribution (possibly associated with oceanographic changes), since there 

have been no known fishery interactions or other human-caused mortality 

events involving common dolphins from the southeastern United States . 

There are other potential historical data sources that have not been examined 

in this section. Some of the most valuable sources are old, nineteenth century 

whaling logbook records. Townsend (1935) has extracted entries of the large 

whale target species (sperm, humpback, and right whales) from many 

logbooks of Yankee whalers . There are undoubtedly many more logbooks that 

have not yet been examined, and it is likely that other information may also be 

available from those Townsend did examine. For instance, other large whales 

that were too quick to be primary targets of whalers (balaenopterids), and 

other species of small cetaceans were often mentioned in old whaler's 

42 
logbooks. Although species identification would be very difficult to verify for 

many of these records, there may be a number of records of highly distinctive 

species (such as killer whales) that could be extracted. Thus, these whaling 

logbooks represent a potentially valuable untapped source of data for analyses 

such as this . 

Understanding of the marine mammal fauna of the Gulf of Mexico is still 

incomplete. It is probable that there are still undocumented species that may 

occur in the Gulf; beaked whales are the most likely candidates. Although 

knowledge of Gulf marine mammals is still in a rudimentary stage, a great deal 

has been learned in the past decade, and the pace at which this knowledge is 

acquired continues to increase. This has come about only through the hard 

work and dedication of a small group of interested individuals. Recent studies, 

such as the GulfCet program described in the remainder of this report, have 

added greatly to the efforts of those early workers who painstakingly 

documented strandings and occasional sightings of these relatively poorly 

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Walker, N.D., LJ. Rouse, G.S . Fargion, and D. Biggs. 1994. Circulation of 

Mississippi River water discharged into the northern Gulf of Mexico by 

the great flood of summer 1993. EOS 75:4109-4115. 

Weber, M., R.T. Townsend, and R. Bierce . 1990. Environmental quality in the 

Gulf of Mexico: A citizen's guide. Center for Marine Conservation, 

Washington, D.C. 130 pp. 

59 
Weller, D.W., A.J. Schiro, V.G. Cockcroft, and Wang Ding. In press. First record 

of a humpback whale (Megaptera novaeangliae) in the western Gulf of 

Mexico. Mar. Mamm. Sci. 12(l). 

53 
In. VISUAL SURVEYS ABOARD SHIPS AND AIRCRAFT 

LJ. Hansen, K.D. Mullin, T.A. Jefferson, and G.P. Scott 

3 .1 Introduction 

Studies of continental slope waters of the northern Gulf of Mexico (Fritts et al. 

1983, Mullin et al. 1994c) indicated that cetaceans were diverse (at least 18 

species) and that some species (e.g., sperm whale, Risso's dolphin, pantropical 

spotted dolphin), at least seasonally, were relatively abundant. However, these 

studies were restricted to relatively small geographic areas and the results 

could not be meaningfully extrapolated to a broader region of the Gulf of 

Mexico. Therefore, information on the seasonal abundance and distribution of 

cetaceans in the slope waters of the entire north-central and western Gulf of 

Mexico (i.e., the GulfCet study area) was not available to the MMS. In order to 

meet these information needs, seasonal line transect surveys of the GulfCet 

study area from ship and aircraft platforms were conducted. Line transect 

surveys are the established and standard method for assessing cetacean 

density and abundance over large geographic areas (Buckland et al. 1993). 

Ships were used to survey the entire GulfCet study area. Aircraft were used to 

provide faster but more fine-scale seasonal surveys of a subregion of the study 

area. The primary objectives of the GulfCet visual aerial and ship surveys 

were: 1) to obtain data on the cetacean species composition in the GulfCet study 

area, 2) to obtain a minimum population estimate of each cetacean species 

encountered in order to establish a baseline for monitoring trends in 

abundance over time, 3 ) to study the seasonal abundance and distribution 

patterns of each species, and 4) to collect location data for use in cetacean 

habitat studies. The surveys were conducted for two years as a first step in 

studying interannual and intraseasonal variation in the diversity, abundance, 

and distribution of cetaceans in the north-central and western Gulf. 

Additionally, line transect data from each sea turtle species sighted during the 

aerial surveys were used to estimate sea turtle abundance. 

3.2 Methods 

3.2.1 Data Acquisition 

3.2.1 .1 Ship Surveys 

Shipboard surveys were conducted seasonally, lasting from 10 to 55 days per 

survey. Seasons were defined as follows for both the ship and aerial surveys: 

summer, July-September; fall, October-December; winter, January-March; and 

spring, April-June. The survey tracks followed one of three designs that 

sampled the entire GulfCet study area. The TIO surveys followed fixed northsouth track-lines that were designed to accommodate oceanographic sampling 

as well as visual and acoustic sampling of marine mammals (Figure 3.1). The 

ship transited the track once each survey for 24 hours a day, and visual 

sampling occured during daylight hours on the north-south track-lines or on 

transit between the track-lines. The SEFSC surveys followed two transect line 

55 
-96 -95 -94 -93 -92 -91 

30 

29 

20 

V` 

lr I 

26 

25 

-90 -89 -88 -87 

-~.. 

12 

iiij, 

9 M.ii %iii 5 6 3 4 

45411 - -------------- .9% K ... .... ..... IN M 

M H&MIN, :R X", -4 .;ff 'a-K -.%~M 

FAIR R g % iiR i~iti- .iiiiM 

. . . . . 

IN, 

El Gulfcet study Area 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 

30 

29 

GO 

27 

26 

25 

7 

Figure 3.1 . R/V Longhorn and R/V Pelican marine mammal visual and acoustic track-lines. This transect 

was completely or partially surveyed during spring, summer, and fall of 1992, and the winter, 

spring, summer, and fall of 1993. The 100 and 2,000 m isobaths define the northern and southern 

borders of the study area. 
designs, sampling the study area three times each survey period. The spring 

SEFSC surveys consisted of one or two transits of equidistant north-south 

tracks (surveyed during daylight hours) with a random start (Figure 3 .2), and 

one or two transits of a predetermined track for sampling ichthyoplankton 

stations, which were transited 24 hours a day (Figure 3 .3). The winter SEFSC 

survey transited the north-south tracks three times. Visual sampling of the 

ichthyoplankton track could be latitudinal or longitudinal, or a combination of 

both. The SEFSC north-south tracks were designed specifically for visual 

sampling of marine mammals along transects perpendicular to the depth 

gradient. 

Visual sampling methods of the TIO and SEFSC surveys were similar except for 

the slower vessel speed on the TIO surveys (varying from 9.3-17 km/hr) than 

the SEFSC surveys (18 lm/hr) . Marine mammal sighting data were collected by 

two teams of three observers during daylight hours, weather permitting (i.e ., 

no rain, Beaufort sea state <6), using standard vessel survey data collection 

methods for cetaceans developed by the Southwest Fisheries Science Center 

(NMFS) (Holt and Sexton 1987). Each team had at least two members trained and 

experienced in shipboard marine mammal observation and identification 

techniques. Two observers searched for marine mammals using high-power 

(25X), large format "Big Eye" binoculars mounted on the ship's flying bridge. 

The third observer maintained a search of the area near the track-line either 

without visual aids or with handheld binoculars, and recorded data. The 

observers rotated through each of these three stations every 30-40 minutes, 

and each team alternated two-hour watches throughout daylight. 

Sighting data were recorded on a computer interfaced with either a global 

positioning system (GPS) or LORAN-C navigation receiver via a data 

acquisition program during the SEFSC surveys . On TIO surveys data were 

recorded on standard marine mammal visual sampling forms developed by 

NMFS (see Hill et al. 1991). Data collected included species, group size, bearing 

and reticle (a measure of radial distance) of a sighting, and data on 

environmental conditions (e .g ., Beaufort sea state, sun position, etc.) which 

could affect the observers' ability to sight animals. The reticle relative to a 

sighting was measured using an eyepiece with a graduated scale in the 

binoculars. The bearing of a sighting relative to the track-line was measured 

using a 360° graduated scale attached to the base of the binoculars (Figure 3 .4). 

Ancillary data were also collected and included, but were not limited to, time of 

day, latitude and longitude, behavior, and associated animals. Typically, on the 

SEFSC surveys, the vessel was diverted from the track-line to identify species 

and obtain group size estimates. For each sighting, the final group size 

estimate was the average of the independent estimates made by individual 

observers and entered in a personal notebook. 

During both the ship and aerial surveys, cetaceans were identified to the 

lowest taxonomic level possible based on descriptions in field guides and 

scientific literature (e.g., Leatherwood et al. 1976, Leatherwood and Reeves 

1983, Perrin et al . 1987) . The ability to make an identification was dependent 

on water clarity, sea state, and animal behavior. Identifications to species were 

not possible for some genera or groups of species. In some cases, cetaceans 

could only be identified as large whales (>7 m long), small whales (nondolphin, <7 m), dolphins, or odontocetes. 

57 
-2i8 -86 -84 

.. 

Figure 3 .2 . Representative NOAH Ship Oregon II marine mammal cruise 

track. A systematic set of lines from a random start similar to this 

track was completely or partially surveyed three times during 

winter 1993, and one time each during spring 1992-1994. The 100 

and 2,000 m isobaths define the northern and southern borders of 

the GulfCet study area. 

30 

28 

26 

24 

30 

28 

26 

24 

30 

28 

26 

24 

Figure 3 .3 . NOAH Ship Oregon II standard ichthyoplankton/marine mammal 

cruise track. This track was completely or partially surveyed two 

times each spring from 1992-1994 . The 100 and 2000 m isobaths 

define the northern and southern borders, respectively, of the 

GulfCet study area. 

98 -96 -94 -92 -90 -88 -86 -84 -82 

T 

:r.:::::.:._::: .:: . . . . . . . .::. . :. :: . . . . . . . . . . .y,:.. ; :::.::x:.:: IY l 

V . . . 

... 

. .yry 

n .~C~ " ::f}.v 

. . /.f 

nh ~~ri 

O 

~~J U \ / 

D Gu1f1Cet Study Area ° 

i i 

)8 -96 -94 -92 -90 -88 -86 -84 -82 

58 
A. B . 

Perpendicular ~ i Aircraft 

sighting distance 

Cetacean Radial 

i group e~ sighting distance 

I 

Cetacean 

group 

`Water surface 

'b Radial Perpendicular 

+' sighting distance sighting distance 

a 

Ship 

w 0 

0 

/ Trackline 

v 

L 

Figure 3.4. Diagram of perpendicular sighting distance (PSD) and other 

sighting parameters for shipboard (A) and aerial (B) surveys (b 

and 8 = angle between track-line and cetacean group, 

h = altitude) . 

The SEFSC data were error-checked daily with programs designed to detect 

inappropriate entries in each data field. The original data file was copied, and 

corrections were made on the copy only. The TIO data were checked by hand, 

and corrections were noted on the original data sheets . 

The sighting and effort data were summarized by survey for the line transect 

distance sampling analysis. The sample unit for analyses was one day's survey 

effort . The length of track-line sampled was determined using LORAN or GPS 

positions (latitude and longitude) collected at regular intervals (usually every 

two minutes during the SEFSC surveys, and every 30 minutes during the TIO 

surveys) along the transect. In some cases, the positions were known to be in 

error and effort was determined using the elapsed time and average vessel 

speed. Effort and sighting data were pooled across environmental conditions 

that may have had different sighting rates due to effects on observers' 

abilities to sight animals (i .e ., sighting rates tended to decrease as wind and 

wave height increased). 

59 
3 .2 .1 .2 Aerial Surveys 

Based on several considerations, including projected availability of acceptable 

survey conditions, available funding, and flight times to the study area, the 

aerial surveys were designed to survey track-lines totaling about 6,400 km 

each season. Given this constraint and the width of both the continental shelf 

and slope in the northwestern Gulf, it would have been logistically difficult to 

uniformly cover the entire GulfCet study area and keep the distance between 

transects small for both efficient use of flight-time and finer scale coverage. 

Therefore, the aerial surveys did not sample the entire GulfCet study area. The 

aerial survey study area (85,815 1:m2) only included waters from 100-1,000 m 

deep west of 90°00.0'W. However, the entire continental slope was surveyed 

east of 90°00.0'W because: (1) it was logistically more feasible (i .e., both the 

slope and shelf are narrow); (2) the area was of special interest because 

previous aerial surveys of the area (Mullin et al . 1994c) indicated that cetacean 

diversity, distribution, and abundance in this area were seasonally variable; 

and (3) this area is oceanographically and physiographically complex (e.g., 

Mississippi Canyon, DeSoto Canyon, Mississippi River Delta, and the Loop 

Current) . 

Aerial surveys were conducted once each season for two years, from summer 

1992 through spring 1994 (eight seasonal surveys). During each season, the 

aerial survey study area was covered uniformly by flying 74 track-lines 

placed equidistantly apart from a random start. Track-lines were oriented 

perpendicular to the bathymetry and consisted of 60 north-south track-lines 

off the Alabama coast, west, through northern Texas, and 14 east-west tracklines off of southern Texas (Figure 3 .5) . Track-lines were 13 .5 km apart. A 

window of 45-days and about 100 flight hours was allocated for each seasonal 

survey. Survey flights were conducted only on days with good visibility (i .e ., 

no rain or fog) and when there were no or few whitecaps (Beaufort Sea State 

0-3) . 

The survey platform was a NOAA-operated DeHavilland Twin Otter (twinengine turbo-prop) aircraft modified with a large bubble window on each side. 

These windows provided observers with track-line visibility . This aircraft was 

used in previous aerial surveys of the Gulf of Mexico in 1989-90 (Mullin et al . 

1994c) . Because a NOAA Twin Otter was not available for the first survey 

(summer 1992), a similarly modified Partenavia aircraft was used. This aircraft 

had a flight time of only 4.5 hours. As the transit time to the study area was 

about one hour, this limited the amount of survey time per flight . The Twin 

Otter, with a flight time of 6 .5 hours, was used for the fall 1992 and all 

subsequent GulfCet aerial surveys. 

Survey flights typically began at 0800 hours and were 4.5-6.5 hours in 

duration. Surveys were conducted from an altitude of 229 m (750 feet) and at a 

speed of 204 km/hour (110 knots). A pilot, co-pilot, and three observers 

participated in each flight . At least two observers on each flight were trained 

and experienced in aerial survey techniques for marine mammals. The 

observers were stationed at each of the two bubble windows and at a computer 

(data entry) station . Observers searched waters primarily on and near the 

track-line and scanned periodically out to the horizon. Only sightings made 

60 
3 f 9g 

30 -

29 -

a, 28 

27 - 

26 -

25 

249 

31 

30 

29 

28 

27 

26 

25 

24 

Figure 3 .5 . GulfCet aerial survey track-lines. A systematic set of lines from a random start similar to this 

track was completely or partially surveyed seasonally from summer 1992 through spring 1994. 

-96 -94 -92 -90 -88 -86 -84 -82 
from the bubble window stations were used in the abundance estimates. To 

avoid fatigue, observers rotated through stations about every 30 minutes . Pilots 

and observers communicated through headsets with voice-activated 

microphones. 

Data were entered on a computer interfaced with a GPS/LORAN-C navigation 

receiver via a data acquisition program. Sighting data included species, group 

size, and sighting angle or interval (for estimating perpendicular sighting 

distance) . A suite of data characterizing survey conditions (e.g., sea state, 

weather, visibility, water color), effort status, and observer positions were 

updated throughout the day whenever a change in any parameter occurred. 

The date, time, and location were automatically recorded with each sighting. 

The sighting angle between the group and the track-line was measured with 

an inclinometer if the angle was less than 60°. Each bubble window was also 

divided into seven 10° intervals and one interval >70° corresponding to 

interval endpoints with perpendicular sighting distances (PSD) equal to 40, 83, 

132, 192, 273, 397, and 629 or >629 m (see Figure 3 .4) . If the inclinometer 

malfunctioned or the sighting angle was greater than 60°, the interval was 

recorded. 

When a cetacean group was sighted, sighting angle or the interval was noted, 

a dye-marker was usually dropped to mark the position, and the aircraft was 

diverted to circle the group . Before continuing the transect, the species was 

identified and group size was estimated by a consensus of the three observers. 

The identifying characteristics of each species and any anecdotal information 

were noted on a standardized form. 

3.2.2 Data Analysis Techniques 

line transect methods (Buckland et al. 1993), implemented with the program 

DISTANCE' (Laake et al. 1993), were used to make two independent sets of 

abundance estimates; one set based on the aerial surveys and the other based 

on the ship surveys. For aerial surveys, estimates were made for the following 

species and temporal strata : (1) each species for the entire study (i .e ., all eight 

seasonal surveys combined); (2) all species combined for each year of the 

study (Year 1= 8/92-4/93, Year 2= 8/93-5/94), each seasonal survey (e.g ., winter 

1993), and each season (e.g ., summer 1992 and summer 1993 combined); and (3) 

each species with 20 or more on-effort sightings for each year of the study 

and for each season. For the ship surveys, estimates were made for each 

species by season and overall (all seasons combined) . 

The formula used to estimate density (D) vas: 

D= n-S.f(0) 

2- L 

where n= number of on-effort group sightings 

S= mean group size or expected group size 

f(0)= sighting probability density function at 0 perpendicular distance 

L =length of track-lines sampled within a stratum . 

62 
Abundance was estimated as the density multiplied by the size of the area 

surveyed (ship survey area = 154,621 km2, aerial survey area = 85,815 km2) . 

The log-normal 95% confidence intervals were computed for each abundance 

estimate. 

The parameter f(0) was estimated using a hazard-rate model and a half-normal 

model (Buckland 1985, Buckland et al . 1993) . The program fit the f(0) 

parameter using a maximum likelihood estimator with exact sighting distances 

for the ship surveys and with grouped sighting distances for the aerial 

surveys. Model selection of f(0) was determined using Akaike's Information 

Criterion (AIC, see Buckland et al. 1993) . No attempt was made to estimate the 

probability of sighting animals on the track-line (go), which was assumed 

equal to one for all species. The resulting estimates do not account for animals 

that were not sighted due to observer error (i .e ., on the transect and missed) or 

that may have been unavailable for sighting (i .e ., on the transect but 

submerged) . This effect could result in conservative estimates of abundance. 

Variance of D was estimated as: 

var(D) _ (D)'` C var(n) + var(S) + var[f(0)] 

n' S

, 

f (0)Z 

and coefficient of variation (CV) was estimated as: 

CV(D) -_ 

var(D) 

D 

The sampling unit for the ship surveys was a day's visual sighting effort, and 

for the aerial surveys, a transect. The variance of n was estimated based on the 

variation in the number of on-effort group sightings between sampling units 

within each stratum. Since the sampling units were of variable length, the 

estimated variance of n was length weighted. The estimated variance of S was 

based on the variation in group size within each stratum. The estimated 

variance of f(0) was based on the variation between expected versus actual 

perpendicular sighting distance (PSD) distributions pooled across strata . 

The group sizes for some species tended to be inversely related to PSD, a feature 

that results from size bias (i .e ., larger groups are easier to see at distance than 

are small groups) . Therefore, the arithmetic mean of group size was probably 

an overestimate of the true mean group size and could have lead to positively 

biased abundance estimates . The program DISTANCE''' used a regression of the 

group size by sighting distance to generate a mean "expected group size." The 

expected group size was used in the density estimation if it was significantly 

different from the arithmetic mean group size (p<0 .15, Student's t-test, see 

Buckland et al. 1993) . 

63 
3 .2.2 .1 Estimation of Perpendicular Sighting Distance and f(0) 

Ship Surveys 

The perpendicular sighting distance (PSD) was estimated using bearing and 

reticle measurements . Examination of the bearing and reticle measurements 

indicated that most were rounded to the nearest 5 units (5 degrees for bearing, 

0.5 for reticle readings). The bearing and reticle data for each sighting were 

smeared by adding a randomly selected value between -5 and S for the bearing, 

and between -0.5 and 0.5 for the reticle readings. This was done to reduce the 

potential for artificial grouping of sighting distances due to rounding of 

measurements by observers . 

The smeared reticle readings were converted to radial sighting distances (R) 

using a model which was derived from empirical data (Hansen et al. 1995) . 

Perpendicular sighting distances (PSD) were calculated as (Figure 3 .4): 

PSD= R-sin(b) 

where b = smeared angle between sighting and trackline. 

An exploratory analysis indicated that sightings made at small radial distances 

(generally < 0.46 km) resulted in a poor fit of the sighting probability density 

function. Exclusion of these sightings resulted in better fits and more precise 

estimates of f(0). Most of these sightings were probably of animals that were 

attracted to the vessel to bowride. One requirement for unbiased estimates of 

abundance is that the sighting target(s) should not move in response to the 

observer or the observation platform (Burnham et al. 1980, Buckland et al. 

1993). To reduce the potential for bias due to attraction to the vessel, only 

sightings made at radial distances of >_ 0.46 km were included in the data used 

for estimating abundance. 

The sample sizes (number of groups sighted) of most species were considered 

insufficient to obtain accurate and precise estimates of f(0) . Sightings of 

species with similar sighting characteristics (i .e ., body size, group size, 

behavior) were pooled to estimate f(0) . (Table 3 .1) . For example, f(0). for 

Cuvier's beaked whale vas estimated by pooling with sightings of Blainville's 

beaked whale, unidentified beaked whales (family Ziphiidae), and dwarf and 

pygmy sperm whales of group sizes less than five. Seven species did have 

sufficient sightings (30 or more, including non-GulfCet study area sightings) 

to estimate species f(0) without pooling; these were the sperm whale, dwarf 

sperm whale, bottlenose dolphin, Risso's dolphin, Atlantic spotted dolphin, 

pantropical spotted dolphin, and striped dolphin. The estimated values of f(0) 

and associated statistics are listed in Table 3.2. 

Aerial Surveys 

The perpendicular sighting distance (PSD, Figure 3 .4) from the track-line to 

the group was calculated as: 

PSD = h ~ tan(o) 

64 
Table 3 .1 . Pooling categories for estimating f(0) . "X" indicates sightings pooled (all sightings used) . 

"<#" and ">#" indicate the group sizes of sightings used. Abbreviations : Bede=Bryde's 

whale, Pmac=Sperm whale, Kbre=pygmy sperm whale, Ksim=dwarf sperm whale, 

KsKb=Kogia spp., Zcav=Cuvier's beaked whale, Ziph=Unidentified Ziphiidae, Pele=melonheaded whale, Fatt=pygmy killer whale, FaPe=pygmy killer/melon-headed whale, 

Pcra=false killer whale, Oorc=killer whale, Gmac=short-finned pilot whale, Sbre=roughtoothed dolphin, Lhos=Fraser's dolphin, Ttru=bottlenose dolphin, Ggri=Risso's dolphin, 

Sfro=Atlantic spotted dolphin, TtSf-bottlenose/Atlantic spotted dolphin, Satt=pantropical 

spotted dolphin, Scoe=striped dolphin, Slon=spinner dolphin, Scly=clymene dolphin, 

Ular=unidentified large whale, Usml=unidentified small whale. 

a~ 

Bede Pmac 

--- -- 

Kb-re- Ksim KsKb Zcav Zi h Pete Fatt Pcra Oorc Gmac Sbre Lhos Ttru G ri Sfro Sact Scoe Slon Sc ty 

Bede 

- - - - - -- - - -- -- 

_ _ 

- -- 

Pmac X 

Kbre X < S < 5 ' 

Ksim X X <S <5 

KsKb X <S <S 

Zcav X 

TIP-h_ 

Yele 

--- - -- - - 

X 

- 

> 29 

Fatt - ---- - - 

FaPe 

Pcra 

>29 <30 

< 16 X 

Oorc X X 

Gmac 

Sbre 

-- -<-3-0 -- 

X < 16 

Lhos > 29 

Ttru >9<30 <21 

Ggri 

- -- - - 

>9<30 X < 16 

- 

>6<30 

--- - - 

Sfro <21 

- - - -- -- - - - - 

TtSf < 21 

Satt > 29 > 29 X 

Scce > 29 > 29 X X 

Slon > 29 > 29 

SC ~1- ----- --- - ---- - 

> 29 

- --- ----- - ---- 

> 29 

- - - -- - --- 

~C 

-- --- - --- - 

War X 

-- -------- ----- - - 

Usml 

< S < 5 - --- -- - --- --- --- --- ---- ----- ---- -- _.__ . _ .---- - 

. . _ - - 
Table 3 .2 . The estimated value of f(0) and associated statistics . CV = coefficient of variation, 

n(f(0)) = number of sightings used to estimate f(0) (* indicates species which 

required pooling with other species to estimate f(0)) n(D) = number of sightings used 

for estimating density and abundance, n(ALL) = total number of on effort sightings, 

and ANIMALS = total estimated number of individuals sighted . 

Species f(0) CV n(f(0)) n(D) n(ALL) ANIMALS 

(km-1) 

Bryde's whale 0.470 0.20 86* 1 1 2 

Sperm whale 0.435 0.15 108 73 73 201 

Pygmy sperm whale 0.480 0.12 65* 9 9 11 

Dwarf sperm whale 0.525 0.13 36* 22 22 47 

Dwarf/pygmy sperm whale 0.493 0.13 G5* 15 16 34 

Cuvier's beaked whale 0.567 0.07 78* G G 7 

Unidentified Ziphiidae 0.493 0.08 8G* 15 16 35 

Melon-headed whale 0.411 0.10 184* lU lU 1407 

Pygmy killer whale 0.433 0.14 104* 2 2 23 

False killer whale 0.419 0.13 108* 2 3 9 

Killer whale 0.439 0.14 83* 4 4 45 

Short-finned pilot whale 0.519 0.17 74* 9 9 138 

Rough-toothed dolphin 0.602 0.07 284* 8 8 106 

Fraser's dolphin 0.411 0.10 183* 1 2 64 

I3ottlenose dolphin 0.760 0.11 206 83 97 999 

Risso's dolphin 0.448 0.14 89 44 40 411 

Atlantic spotted dolphin 0.786 0.22 80 18 25 472 

Pantropical spotted dolphin 0.537 0.11 191 80 98 4998 

Striped dolphin 0.751 0.42 37 21 22 782 

Spinner dolphin 0.500 0.10 260* 10 10 470 

Clymene dolphin 0.492 0.10 263* 23 25 1310 
where e = angle between the track-line and the group 

h = altitude, always 229 m. 

Due to inclinometer malfunction, no angle was recorded for 26 groups sighted 

from 0-60°. For these and sightings from 60-70°, the midpoint of the interval 

was used as the PSD and was treated as an exact distance. Sightings at angles > 

70° were bounded by the horizon. However, we assumed they occurred at less 

than 1,300 m (about 80°), and the midpoint of the interval 70°-80° was used. In 

trial runs, the value of the midpoint made little difference in estimates of D or 

f(0), and therefore these sightings were used because they constituted a 

significant portion of the data (9%). 

All analyses were made with the data left truncated at 50 m. The frequency 

distribution of PSD of all sightings pealed near PSD = 50 m and decreased as the 

PSD approached 0 m, whereas the expected distributions showed no such peaks. 

This deviation from the expected distributions may have been a result of better 

sighting conditions at PSD = 50 than at PSD = 0, due to glare or other conditions. 

Including the sightings between 0-50 m could have resulted in negatively 

biased abundance estimates (see Alldredge and Gates 1985) . The 45 sightings 

with a PSD < 50 m were excluded from analyses and the left-truncation option 

was implemented on DISTANCE . 

The number of sightings for most species was too small to obtain an accurate 

and precise estimate of f(0) . Therefore, species with similar sightability from 

aircraft (i .e ., body size and surface behavior) were pooled into four categories 

(Table 3 .3) and an estimate of f(0) was made for each category (Table 3.4) . For 

each species, the value of f(0) for the category to which it belonged and its 

associated variance were used in abundance estimates . Exploratory analyses 

using exact PSDs and various PSD distance interval combinations were 

performed to achieve a good fit of the model to the data (i .e ., low X2 value and 

decreased CV[f(0)]) . For each estimate, except those involving species from 

Category 4, a model was fit to PSD data grouped into intervals: 50-150, 151-250, 

251-400, 401-630, and 631-1,300 m. PSD intervals for Category 4 were: 50-200, 

201-400,401-630, and 631-1,300 m. 

3.2.2.2 Sea Turtle Density 

Five species of sea turtles are known to occur in the Gulf of Mexico: 

leatherback, loggerhead, Kemp's ridley, green, and hawksbill (Weber et al. 

1992). Sea turtle densities were estimated from aerial survey data only, using 

the program DISTANCE . All sea turtle sightings were of large, probably adult 

turtles. Species were identified on the basis of shell shape and color and the 

head size relative to the overall size. Data collection procedures for sea turtles 

were essentially the same as those for cetaceans except that sea turtles were 

not circled. All sea turtle sightings were pooled in order to estimate a common 

f(0) which was then applied to each species. Analyses methods were the same 

as those for cetaceans with the following exceptions: (1) data were not lefttruncated (a left-truncated estimate of f(0) at 50 m was made, but since both 

estimates of f(0) were similar the non-left-truncated estimates were used in 

order to maintain as large a sample size as possible) (2) data were 

67 
Table 3.3 . Categories of cetaceans with similar sighting 

characteristics for which sightings were pooled to 

estimate the parameter f(0) for aerial surveys. 

Category 1 : Inactive at the surface and <7 m in length 

Pygmy/dwarf sperm whale Unidentified dolphin 

Cuvier's beaked whale Unidentified ziphiid 

Unidentified small whale Unidentified odontocete 

Category 2 : Active at the surface and <7 m in length 

False killer whale 

Short-finned pilot whale 

Rough-toothed dolphin 

Bottlenose dolphin 

Risso's dolphin 

Atlantic spotted dolphin 

Bottlenose/Atlantic spotted 

dolphin 

Category 3: Very active at the surface and <7 m in length 

Melon-headed whale Spinner dolphin 

Pantropical spotted dolphin Clymene dolphin 

Striped dolphin Stenella spp. 

Category 4: Large whales, >7 m in length 

Bryde's/sei whale 

Sperm whale 

Unidentified large whale 

right-truncated at 629 m, and (3) exact distances were used instead of intervals. 

Sea turtles do not occur in social groups as cetaceans do, and each sighting was 

usually of a single turtle . If more than one turtle was sighted at a time, a PSD 

was measured to each turtle . Therefore, there is no variance in group size. 

Table 3 .4. Estimate of f(0) for each category of cetaceans with 

similar sighting characteristics (n = number of 

sightings, P = number of parameters in the model, 

CV = coefficient of variation) . 

f(0) 

n Model P (km-1) CV(f(0)1 

Category 1 69 Hazard Rate 2 4.365 0.24 

Category 2 141 Hazard Rate 3 3.538 0.09 

Category 3 80 Half-normal 2 2.753 0.09 

Category 4 30 Half-normal 1 1.986 0.15 

All cetaceans 310 Half-normal 2 3.390 0.05 

68 
3 .3 Results 

3 .3.1 Cetacean Abundance from Ship Surveys 

A total of 21,350 km of transect was visually sampled during the GulfCet ship 

surveys (Table 3 .5). The transect kilometers sampled during each survey 

varied from 418-4,217 km. The cumulative survey effort each season was: 

spring - 13,507 lm, summer - 2,085 lm, fall - 1,275 km, and winter - 4,483 km. 

Overall, 19 cetacean species were identified in 683 sightings made on-effort 

(Table 3 .6) . All sightings are listed in Appendix A. The number of on-effort 

sightings each season ranged from 509 during spring to 14 during fall (Tables 

3 .7-3 .10) . Most of the survey effort occurred during the spring, with the least 

effort in the fall . The survey effort by cruise and season are also listed in Table 

3.5. 

Sighting rates of cetacean groups were consistent during the spring surveys, 

averaging about 4.0 groups/100 km (range = 3 .3-4.7) (Table 3 .5). The summer 

sighting rates were 2 .7 and 7 .6 groups/100 km. The fall sighting rates were 0.8 

Table 3.5 . Shipboard visual survey effort and cetacean groups 

sighted by vessel and season . Sighting 

rate = groups/100 km effort . 

Vessel Dates Effort #Groups Sighting 

(km) sighted rate 

on effort 

Spring 

R/V Longhorn 15 Apr-1 May 1992 418 17 4.1 

NOAH Ship Oregon II 17 Apr-8 Jun 1992 4,217 170 4.0 

NOAH Ship Oregon II 3 May-15 Jun 1993 4,102 137 3.3 

R/V Pelican 23 May-5 Jun 1993 957 45 4.7 

NOAH Ship Oregon II 15 Apr-10 Jun 1994 3,813 140 3.7 

Total 13,507 509 3.8 

Summer 

R/V Pelican 10 Aug-28 Aug 1992 1,037 28 2.7 

R/V Pelican 25 Aug-5 Sep 1993 1,08 80 7.6 

Total 2,08 108 5.2 

Fall 

R/V Pelican 8 Nov- 22 Nov 1992 536 4 0.8 

R/V Pelican 3 Dec-14 Dec 1993 739 10 1 .4 

Total 1,27 14 1 .1 

Winter 

NOAH Ship Oregon II S Jan-14 Feb 1993 3,964 43 1 .1 

R/V Pelican 12 Feb-27 Feb 1993 529 9 1.7 

Total 4,493 52 1.2 

Total 21,360 683 3.2 

69 
Table 3 .6. Overall sighting and group-size statistics used for estimating cetacean abundances from 

shipboard surveys (n = number of sightings, n/L - groups encountered per 1,000 km 

[survey effort a 21,588], G = mean group size, S - size-bias adjusted group size [group 

sizes denoted with an * indicate which size estimate was used in density calculations], 

CV = coefficient of variation, N = abundance estimate, D s density estimate per 

1,000 km2, LCI = lower log-normal 9596 confidence interval, UCI = upper 9596 confidence 

interval. The size-bias adjusted group size was used in the density calculations if it was 

significantly different from the mean size at p < 0.15 . 

OVERALL n n/L CV G S CV group N D CV LCI UCI 

Species n/L size 

Bryde's whale 1 0.05 0.78 2 * - - 3 0.02 0.81 1 14 

Sperm whale 73 3.38 0.19 2 .7* 3 .1 0.14 313 2.02 0.25 192 508 

Pygmy sperm whale 9 0.42 0.36 1 .2* 1 .3 0.12 19 0.12 0.40 9 40 

Dwarf sperm whale 22 1 .02 0.27 2 .1* 1 .9 0.17 88 0.57 0.34 46 170 

v Pygmy/dwarf sperm whale 15 0.69 0.33 2 .0* 1.7 0.15 53 0.34 0.39 25 111 

° Cuvier's beaked whale 6 0.28 0.38 1 .17* 1.25 0.14 14 0.09 0.41 7 31 

Unidentified Ziphiidae 26 1 .20 0.25 2 .4* 2 .2 0.13 124 0.81 0.29 71 218 

Melon-headed whale 10 0.46 0.26 140.7* 167.7 0.19 2,067 13.38 0.34 1,071 3,988 

Pygmy killer whale 2 0.09 0.61 11 .5* 11.5 0.13 36 0.23 0.64 11 113 

False killer whale 2 0.09 0.60 3 .5* 3 .5 0.14 10 0.07 0.63 3 33 

Killer whale 4 0.19 0.43 11 .2* 11 .0 0.04 71 0.46 0.46 30 167 

Short-finned pilot whale 9 0.42 0.34 15.3 13 .7* 0.33 215 1 .39 0.50 82 563 

Rough-toothed dolphin 8 0.37 0.29 13.2 10.3* 0.18 177 1 .14 0.35 89 351 

Fraser's dolphin 1 0.05 1.16 44.0* - - 65 0.42 1.17 10 400 

Bottlenose dolphin 83 3 .84 0.20 11 .2* 11.2 0.12 2,538 16.43 0.26 1,543 4,174 

Risso's dolphin 44 2 .04 0.17 9.2 7 .5* 0.14 529 3.42 0.26 317 881 

Atlantic spotted dolphin 18 0.83 0.26 22.6* 19.5 0.15 1,145 7.41 0.37 562 2,332 

Pantropical spotted dolphin 80 3 .71 0.15 59.8 46 .2* 0.11 7,105 45 .99 0.22 4,661 10,831 

Striped dolphin 21 0.97 0.27 37.0* 49.2 0.14 2,091 13.53 0.52 788 5,544 

Spinner dolphin 10 0.46 0.43 47.0* 52.0 0.41 840 5.44 0.60 274 2,580 

Clymene dolphin 23 1.07 0.22 54.3 41 .8* 0.28 1,695 10.97 0.37 827 3,474 

ALL I - - - - - - 19,198 124.26 0.12 10.619 36.523 

1 (The CV of ALL (E tv) was estimated as: cv(E tv) _ ry, (cvtv)2 /J tv ) 
Table 3 .7 . Summer sighting and group-size statistics used for estimating cetacean abundances 

from shipboard surveys (n = number of sightings, n/L m groups encountered per 

1,000 km [survey effort = 2,251], G a mean group size, S = size-bias adjusted group size 

[group sizes denoted with an * indicate which size estimate was used in density 

calculations], CV = coefficient of variation, N m abundance estimate, D = density estimate 

per 1,000 km2, LCI = lower log-normal 9596 confidence interval, UCI = upper 95% 

confidence interval. The size-bias adjusted group size was used in the density 

calculations if it was significantly different from the mean size at p < 0.15 . 

Season: SUMMER n n/L CV G S CV group N D CV LCI UCI 

Species n/L size 

Bryde's whale 0 - - - - - - - - - - 

Sperm whale 20 8.88 0.36 3 .0* 4.3 0.16 880 5.70 0.42 391 1,984 

Pygmy sperm whale 1 0.44 0.86 2 .0* - - 33 0.21 0.87 7 156 

Dwarf sperm whale 2 0.89 0.74 5 .5* - 0.27 198 1 .28 0.80 46 855 

Pygmy/dwarf sperm whale 2 0.89 0.83 1 .0* - 0.00 34 0.22 0.84 7 153 

Cuvier's beaked whale 0 - - - - - - - - - - 

Unidentified Ziphiidae 8 3.55 0.48 2 .8* 2.9 0.16 579 3.68 0.46 231 1,397 

Melon-headed whale 1 0.44 0.85 250.0* - - 3,522 22.79 0.86 756 16,407 

Pygmy killer whale 0 - - - - - - - - - - 

False killer whale 0 - - - - - - - - - - 

Killer whale 0 - - - - - - - - - 

Short-finned pilot whale 0 - ' - - - - - - - - - 

Rough-toothed dolphin 1 0.44 0.83 14.0* - - 289 1 .87 0.84 63 1,318 

Fraser's dolphin 0 - - - - - - - - - - 

Bottlenose dolphin 16 7.11 0.47 8.2 7.5* 0.19 3,131 20.26 0.52 1,161 8,444 

Risso's dolphin 2 0.89 0.76 22 .0* - 0.59 677 4.38 0.98 98 4,681 

Atlantic spotted dolphin 1 0.44 0.76 SS .O* - - 1,484 9.61 0.79 351 6,270 

Pantropical spotted dolphin 3 1.33 0.58 39.3* 61 0.46 2,175 14.08 0.74 504 9,378 

Striped dolphin 2 0.89 0.76 37 .5* - 0.87 1,933 12.51 1.23 135 27,594 

Spinner dolphin 1 0.44 1 .15 9.0* - - 154 1.00 1.16 23 1,048 

Clymene dolphin 1 0.44 1 .28 85 .0* - - 1,436 9.29 1.28 185 11,131 

ALL 1 - - - - - - 16,515 106.89 0.31 3,958 90.816 

1 (The CV of ALL (EN) was estimated as: CV(EN) _ ~(cvtv)2/~tv) 
Table 3.8 . Fall sighting and group-size statistics used for estimating cetacean abundances from 

shipboard surveys (n = number of sightings, n/L = groups encountered per 1,000 km 

[survey effort = 1,247], G = mean group size, S = size-bias adjusted group size [group sizes 

denoted with an * indicate which size estimate was used in density calculations], 

CV = coefficient of variation, N = abundance estimate, D = density estimate per 

1,000 km2, LCI = lower log-normal 9596 confidence interval, UCI = upper 9596 confidence 

interval. The size-bias adjusted group size was used in the density calculations if it was 

significantly different from the mean size at p < 0.15 . 

Season: FALL n n/L CV G S CV group N D CV LCI UCI 

Species n/L size 

Bryde's whale 0 - - - - - _ _ _ _ _ 

Sperm whale 5 0.40 0.64 3.2 1 .7* 0.25 224 1.45 0.70 60 842 

Pygmy sperm whale 0 - - - - - - - - - 

Dwarf sperm whale 0 - - - - - - - - _ _ 

Pygmy/dwarf sperm whale 0 - - - - - - - - - 

Cuvier's beaked whale 0 - - - - - - - - - 

^' Unidentified Ziphiidae 2 1.60 0.51 1 .0* - 0.00 68 0.44 0.52 24 192 

Melon-headed whale 0 - - - - - - - - _ _ 

Pygmy killer whale 0 - - - - - - - - _ _ 

False killer whale 0 - - - - - - - _ _ _ 

Killer whale 0 - - - - - - - _ _ _ 

Short-finned pilot whale 2 1.60 0.56 11 .0* - 0.82 667 4.32 1.00 19 23,960 

Rough-toothed dolphin 0 - - - - - - - - _ _ 

Fraser's dolphin 0 - - - - - - - _ _ _ 

Botclenose dolphin 1 0.80 2.02 15.0* - - 706 4.6 2.02 47 10,726 

Risso's dolphin 0 - - - - - - - - _ _ 

Atlantic spotted dolphin 0 - - - - - - - _ _ _ 

Pantropical spotted dolphin 0 - - - - - - - - _ _ 

Striped dolphin 0 - - - - - - - - - 

Spinner dolphin 0 - - - - - - - - _ _ 

Clymene dolphin 0 - - - - - - - _ _ _ 

ALL 1 - - - - - - 1 ,665 10.78 0.95 150 35.720 

1 (The CV of ALL (E rv) was estimated as: cv(E tv) _ ~ (cvtv)2 /1 tv ) 
Table 3 .9. Winter sighting and group-size statistics used for estimating cetacean abundances from 

shipboard surveys (n = number of sightings, n/L = groups encountered per 

1,000 km [survey effort = 4,548], G a mean group size, S = size-bias adjusted group size 

[group sizes denoted with an * indicate which size estimate was used in density 

calculations], CV = coefficient of variation, N = abundance estimate, D = density estimate 

per 1,000 km2, LCI = lower log-normal 9586 confidence interval, UCI = upper 9596 

confidence interval . The size-bias adjusted group size was used in the density 

calculations if it was significantly different from the mean size at p < 0.15 . 

W 

Season : WINTER n n/L CV G S CV group N D CV LCI UCI 

Species n/L size 

I3ryde's whale 0 - - - - - - - - - - 

Sperm whale 9 1.98 0.56 2 .9* 3.5 0.16 192 1.24 0.60 63 582 

Pygmy sperm whale 3 0.66 0.75 1 .0* 1.0 0.00 24 0.16 0.76 6 95 

Dwarf sperm whale 0 - - - - - - - - - - 

Pygmy/dwarf sperm whale 0 - - - - - - - - - - 

Cuvier's beaked whale 0 - - - - - - - - - - 

Unidentified Ziphiidae 0 - - - - - - - - - - 

Melon-headed whale 1 0.22 0.85 60.0* - - 418 2.71 0.85 94 1,866 

Pygmy killer whale 0 - - - - - - - - - - 

False killer whale 0 - - - - - - - - - - 

Killer whale 0 - - - - - - - - - - 

Short-finned pilot whale 2 0.44 0.88 16 .5* - 0.03 274 1.78 0.90 58 1,298 

Rough-toothed dolphin 0 - - - - - - - - - - 

Fraser's dolphin 0 - - - - - - - - - - 

Bottlenase dolphin 5 1 .10 0.44 5 .8* 480.8 0.27 374 2.42 0.53 136 1,032 

Risso's dolphin 0 - - - - - - - - - 

Atlantic spotted dolphin 3 0.66 0.47 9.3* 22.1 0.31 374 2.42 0.60 118 1,186 

Pantropical spotted dolphin 4 0.88 0.62 28 .0* 30.4 0.09 1,022 6.61 0.64 315 3,316 

Striped dolphin 1 0.22 1.08 28 .0* - - 357 2.31 1.16 56 2,284 

Spinner dolphin 0 - - - - - - - - - - 

Clymene dolphin 2 0.44 0.82 6.5* - 0.23 109 0.70 0.85 24 487 

ALL 1 - - - - - - - 3,144 20.36 0.30 870 12.146 

1 (The CV of ALL (E tv) was estimated as: cv(E tv) _ ~ (cvtv)2 /~ tv ) 
Table 3.10 . Spring sighting and group-size statistics used for estimating cetacean abundances 

from shipboard surveys (n = number of sightings, n/L = groups encountered per 1,000 

km [survey effort = 13,723], G = mean group size, S = size-bias adjusted group size 

[group sizes denoted with an * indicate which size estimate was used in density 

calculations], CV = coefficient of variation, N = abundance estimate, D = density 

estimate per 1,000 km2, LCI = lower log-normal 9586 confidence interval, UCI - upper 

95% confidence interval. The size-bias adjusted group size was used in the density 

calculations if it was significantly different from the mean size at p < 0.15 . 

Season : SPRING n n/L CV G S CV group N D CV LCI UCI 

Species n/L size 

Bryde's whale 1 0.73 0.83 2 .0* - - 5 0.03 0.85 1 23 

Sperm whale 39 2.84 0.24 2 .6* 2.9 0.11 248 1 .60 0.30 139 444 

Pygmy sperm whale 5 0.36 0.45 1 .2* 1 .3 0.17 16 0.11 0.50 6 42 

Dwarf sperm whale 20 1.46 0.30 1 .8 1 .6* 0.14 93 0.60 0.35 47 183 

Pygmy/dwarf sperm whale 13 0.95 0.38 2 .2* 2 .0 0.16 79 0.51 0.43 35 76 

Cuvier's beaked whale 6 0.44 0.39 1 .2* 1 .3 0.14 23 0.15 0.43 10 51 

Unidentified Ziphiidae S 0.36 0.43 1 .4* 1 .6 0.17 98 0.63 0.37 48 200 

Melon-headed whale 8 0.58 0.30 147.6* 137.1 0.20 2,569 16.63 0.38 1,230 5,363 

Pygmy killer whale 2 0.15 0.65 11 .5* - - 57 0.37 0.68 17 192 

False killer whale 2 0.15 0.64 3 .5* - - 17 0.11 0.67 S 56 

Killer whale 4 0.29 0.45 11 .3* 11.0 0.04 113 0.73 0.48 46 275 

Short-finned pilot whale 5 0.36 0.43 16 .6* 17.2 0.22 232 1.50 0.51 88 607 

Rough-toothed dolphin 7 0.51 0.32 13.1 9.2* 0.19 221 1.43 0.39 105 467 

Fraser's dolphin 1 0.07 1 .23 44.0* - - 103 0.67 1 .24 15 699 

Bottlenose dolphin 61 4.45 0.24 12.5 12 .6* 0.14 3,335 21.58 0.30 1,877 5,925 

Risso's dolphin 42 3.06 0.18 8.6 7 .5* 0.14 803 5.20 0.27 478 1,349 

Atlantic spotted dolphin 14 1.02 0.34 23 .1* 23.7 0.13 1453 9.05 0.42 653 3,232 

Pantropical spotted dolphin 73 5.32 0.16 62.4 45 .6* 0.11 10,191 65.96 0.23 6,553 15,850 

Striped dolphin 18 1.31 0.31 37 .5* 60.9 0.15 2,891 18.72 0.54 1,049 7,968 

Spinner dolphin 9 0.66 0.48 51 .2* 64.8 0.41 1,314 8.50 0.64 403 4,286 

Clymene dolphin 20 1 .46 0.24 57.6 52 .1* 0.28 2,924 18.93 0.38 1,398 6,119 

ALL I - - - - - - - 26,785 173.02 0.13 14.203 53.407 

1 (The CV of ALL (EN) was estimated as: CV(EN) _ Ty_(cvtv)2/Y_ tv) 
and 1 .4 groups/100 km. The winter sighting rates were similar to fall, and were 

1 .1 and 1 .7 groups/100 km. However, the shipboard survey effort was not 

designed to provide information on seasonal occurrence of cetaceans. More 

than SOg'o of the total effort and 75% of all sightings occurred during the 

spring. The relatively small amount of effort and the resulting small number 

of sightings collected during the other seasons preclude valid comparisons 

between seasons; any variation in species distribution and abundance between 

seasons relative to the ship surveys may be a result of the inconsistent 

seasonal survey effort . 

The sperm whale, bottlenose dolphin, and pantropical spotted dolphin were the 

most commonly sighted species during ship surveys. Each species was sighted 

more than 70 times (Table 3 .G) . Dwarf sperm whale, unidentified ziphiid, 

Risso's dolphin, striped dolphin, and clymene dolphin were each sighted 21-44 

times, with the other species sighted fewer than 20 times. Average group sizes 

ranged from 1 .2 for pygmy sperm whale and Cuvier's beaked whale to 140.7 for 

melon-headed whale (Table 3.G) . 

The overall estimate of cetacean abundance (with CV in parentheses) in the 

GulfCet study area was 19,198 (0.12) animals (Table 3.6) . The most common 

species was the pantropical spotted dolphin with an estimated overall 

abundance of 7,105 (0.22) animals. The bottlenose dolphin was the next most 

common species with 2,538 (O.ZG) animals and was followed by the striped 

dolphin and the melon-headed whale, with 2,091 (0.52) and 2,067 (0.34) 

animals, respectively. The clymene dolphin and Atlantic spotted dolphin 

estimates were 1,695 (0.37) and 1,145 (0.37) animals, respectively, and were the 

only other species with estimates of over 1,000 animals. Relatively precise 

estimates were achieved for the sperm whale with 313 (0.25) animals and 

Risso's dolphin with 519 (0.26) animals. The only other species with estimates 

of more than 200 animals were the spinner dolphin and the short-finned pilot 

whale with estimates of 840 (0.60) and 215 (0.50) animals, respectively. 

3.3 .2 Cetacean Abundance ftom Aerial Surveys 

A total of 49,960 km of transect was visually sampled during the eight GulfCet 

aerial surveys (Tables 3 .11 and 3 .12) . Except for fall 1992, all of the proposed 

aerial track-lines were completed each survey. During fall 1992, high winds 

and rain persisted throughout most of the survey window and only 80g'o of the 

proposed survey effort was completed (eight track-lines between 89°47'W and 

90°44'W were not surveyed) For the entire GulfCet study, 97% of the proposed 

aerial survey effort was completed. The transect kilometers sampled each 

survey ranged from 5,330-G,592 km, and each season, from 11,756-12,942 km. 

In total, 351 cetacean groups were sighted on-effort during aerial surveys 

(Tables 3 .11 and 3 .12) . The number of sightings per survey ranged from 24 to 

61, and the number of sightings per season ranged from 49 to 109. Except for 

fall, group sighting rates were generally similar each season and ranged from 

0.73-0.86 groups/100 km. During fall, the group sighting rate was lower (0.42 

groups/100 km). The animal sighting rates were much more variable and 

ranged from 5 .1 to 23 .1 animals/100 km for fall and winter, respectively. 

75 
Table 3.11 . Summary of the on-effort results of each GulfCet aerial survey . 

Summer Fall Winter Spring Summer Fall Winter Spring 

1992 1992 1993 1993 1993 1993 1994 1994 

Starting date 

Ending date 

Days in window 

survey 

weather 

travel/transit 

mechanical 

rest 

other 

Flight hours 

Percent completed 

Transect kilometers 

"l,ransects 

Mean 13caufort Sea State 

Number of sightings 

Number of animals 

Group sighting rate 

(groups/100 km) 

Animal sighting rate 

(animals/100 km) 

Mean group size 

Off-effort sightings 

Number of species 

11 Aug 3 Nov 

18 Sep 16 Dec 

40 44 

15 10 

17 31 

6 3 

1 0 

0 0 

1 0 

97 80 

100 80 

6,592 5,330 

77 66 

1 .5 2 .4 

SO 24 

905 226 

0.76 0.45 

1 Feb 25 Apr 

22 Mar 1 Jun 

so 38 

12 16 

28 17 

6 4 

3 0 

0 1 

1 n 

90 100 

100 100 

6,184 6,264 

74 74 

1 .8 1 .5 

37 51 

912 1,159 

0.59 0.81 

1 Aug 31 Oct 

21 Aug 16 Dec 

21 47 

14 12 

1 30 

4 

1 1 

1 0 

0 0 

92 88 

100 1(0 

6,350 6,426 

74 74 

1 .3 2.1 

45 25 

749 372 

31 Jan 

16 Mar 

45 

12 

29 

4 

U 

0 

0 

9G 

100 

6,433 

74 

1.5 

61 

712 

2 May 

2 Jun 

32 

13 

13 

S 

1 

0 

0 

89 

100 

6,381 

74 

1 .2 

58 

1768 

0.71 0.39 0.95 0.91 

13.7 4.2 14.7 18.5 11 .8 5.8 26.6 27.7 

17.4 9.4 24.6 20.7 16.3 14.3 26.8 30.0 

7 1 4 G 7 0 6 9 

10 9 10 12 12 8 14 11 
Table 3 .12 . Summary of on-effort seasonal results of GulfCet 

aerial surveys. 

Summer Fall Winter Spring Total 

Percent completed 100 90 100 100 97 

Transect kilometers 12,92 11,756 12,616 12,645 49,960 

Transects 151 140 148 148 587 

Average Beaufort 

Sea State 1 .4 2.3 1.7 1 .3 1.5 

Number of sightings 95 49 98 109 351 

Number of animals 1,654 59 2,624 2,927 7,803 

Group sighting rate 

(groups/100 km) 0.73 0.42 0.78 0.86 0.70 

Animal sighting rate 

(animals/100 km) 12.8 5.1 20.8 23 .1 15.6 

Average group size 17.4 12.2 26.7 26.8 22.2 

Off-effort sightings 14 1 10 15 40 

Number of species 12 10 14 13 16 

At least 17 cetacean species were identified during GulfCet aerial surveys 

(Table 3.13) . The only sighting of killer whales during aerial surveys was offeffort . Aerial sightings are listed in Appendix A. All species sighted during 

aerial surveys also sighted during ship surveys . Seasonally, the number of 

species sighted ranged from 11 in fall to 15 in winter. Eight species were 

identified in all four seasons, two in three seasons, four in two seasons and 

four in only one season (Table 3.13) . Five species that were each sighted 20 or 

more times accounted for 71% of the identified sightings: sperm whales, 

pygmy/dwarf sperm whales, bottlenose dolphins, Risso's dolphins, and 

pantropical spotted dolphins. 

Overall, there were an estimated 16,986 (CV = 0.14) cetaceans in the GulfCet 

aerial survey study area (Tables 3 .14 and 3.15). There were an estimated 12,690 

(0.23) cetaceans the first year and 20,669 (0.18) the second. Most of the 

difference between years vas a consequence of the two winter and the two 

spring estimates. In both cases, the point estimates were about two times as 

large the second year compared to the first. Seasonally, the overall cetacean 

abundance was about the same in winter (21,894 [0.27]) and spring (19,215 

[0.25]), a little less in summer (14,959 [0.24]), but two-three times lower in the 

fall (6,051 [0.32]) . 

Overall, the pantropical spotted dolphin was the most abundant species in the 

aerial survey study area (5,251 [0 .22]) followed by the melon-headed whale 

(2980 [0.60]), bottlenose dolphin (2,890 [0 .?0]), and Risso's dolphin (1,214 [0.24]) 

(Tables 3 .16 and 3.17) . The overall sperm whale population was estimated to be 

87 whales (0.27) and pygmy/dwarf sperm whales, 176 (0.31) . All the other 

delphinid species were represented by less than 1,000 individuals each, and 

77 
Table 3 .13 . Number of on-effort sightings of cetacean species during aerial surveys by temporal 

strata (Year 1 = summer 1992 to spring 1993, Year 2 = summer 1993 to spring 1994, 

T = total of 1 & 2, S = number of surveys/number of seasons a sighting was made). 

00 

Species 

Bryde's/sei whale 

Sperm whale 

Pygmy/dwarf sperm whale 

Cuvier's beaked whale 

Unidentified ziphiid 

Melon-headed whale 

Melon-headed/pygmy killer 

False killer whale 

Short-finned pilot whale 

Rough-toothed dolphin 

Fraser's dolphin 

Bottlenose dolphin 

Risso's dolphin 

Atlantic spotted dolphin 

Bottlenose/ntlantic spotted 

Pantropical spotted dolphin 

Striped dolphin 

Spinner dolphin 

Clymene dolphin 

Stenella spp . 

Unidentified dolphin 

Unidentified small whale 

Unidentified large whale 

Unidentified odontocete 

Summer Fall Winter Spring Years 

1 2 T 1 2 T 1 2 T 1 2 "1' 1 2 T S 

0 0 0 0 0 0 1 0 1 0 0 0 1 0 1 1/1 

2 4 6 1 6 7 0 6 6 7 2 9 15 18 28 7/4 

7 7 14 1 1 2 3 3 6 7 8 15 18 19 37 8/4 

0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 1/1 

1 1 2 2 2 4 0 3 3 3 0 3 6 6 12 4/4 

0 0 0 0 0 0 0 1 1 2 1 3 2 2 4 3/2 

whale 1 0 1 0 2 2 0 2 2 0 0 0 1 4 5 3/3 

1 1 2 0 0 0 0 0 0 0 0 0 1 1 2 2/1 

2 1 3 2 1 3 1 2 3 2 0 2 7 4 11 7/4 

2 0 2 1 0 1 0 1 1 2 3 5 5 4 9 5/4 

0 0 0 0 0 0 0 1 1 1 0 1 1 1 2 2/2 

9 18 27 4 4 8 11 13 24 10 14 24 34 49 83 8/4 

2 2 4 2 1 3 5 11 16 7 9 16 16 23 39 8/4 

2 1 3 1 0 1 3 2 5 3 0 3 9 3 12 6/4 

iolphin 0 1 1 0 0 0 1 2 3 3 0 3 4 3 7 6/3 

11 4 15 1 3 4 4 8 12 4 12 16 21 26 47 8/4 

0 0 0 0 2 2 1 3 4 0 2 2 1 7 8 4/3 

0 1 1 0 0 0 1 2 3 0 0 0 1 3 4 3/2 

0 1 1 0 0 0 1 1 2 2 2 4 3 4 7 5/3 

2 1 3 1 0 1 2 1 3 0 3 3 5 5 10 6/4 

4 0 4 3 1 4 1 0 3 2 0 2 12 1 13 6/4 

5 0 5 1 1 2 0 0 0 2 1 3 8 2 10 5/4 

1 1 2 0 1 1 0 1 1 0 0 0 1 3 4 4/3 

2 2 4 1 S n > > 0 1 1 4 rN n sia 

Total sightings 52 46 98 24 26 50 37 64 101 57 59 116 170 195 365 

Species sighted 10 12 12 9 8 10 10 14 14 12 11 13 15 15 16 
Table 3 .14. Estimates of the parameters used to estimate the abundance of all cetaceans combined 

in the aerial survey study area. Year 1 = Aug 1992-May 1993, Year 2 = Aug 1993-Jun 

1994, n = groups sighted, n/L = group sighting rate per 1,000 km, S = size-bias adjusted 

group size, G = average group size, CV = coefficient of variation, P = probability that 

group size vs. PSD regression was significant. 

Stratum n n/L CV(n/L) S CV(S) G CV(G) P 

Years 1 & 2 

Year 1 

Year 2 

Summer 1 & 2 

Summer 1 

Summer 2 

Fall 1&2 

Pall 1 

Fall 2 

Winter 1 & 2 

Winter 1 

Winter 2 

Spring 1 & 2 

Spring 1 

Spring 2 

310 G.20 0.07 18.8 0.12 20.9 0.11 0.08 

145 5 .95 0.10 14.0 0.17 20.2 0.18 0.02 

1G5 6.45 0.09 24.1 0.1G ' 21 .4 0.14 0.54 

85 G.57 0.12 15 .7 0.20 17 .2 0.13 0.07 

44 6.G8 0.18 13.2 0.29 17 .8 0.18 0.04 

41 6.46 0.15 19.5 0.30 16.6 0.18 0.42 

40 3.40 0.21 22 .6 0.29 12.2 0.24 0.99 

19 3.56 0.33 20.2 0.45 10.7 0.29 0.92 

21 3.27 0.27 33.7 0.45 13.G 0.3G 0.97 

88 6.98 0.12 21.6 0.24 2G.8 0.19 0.07 

33 5.34 0.17 18.5 0.38 2G.1 0.30 0.08 

55 8.55 0.1G 26.4 0.32 27.2 0.2G 0.23 

97 7.G7 0.12 17.2 0.21 22.3 0.23 0.10 

49 7.82 0.19 11 .9 0.30 22.2 0.40 0.05 

48 7.52 0.16 24.G 0.30 22.E 0.23 0.38 
Table 3 .15 . Density (D = animals/1,000 km2) and abundance (N) 

estimates of all cetacean species combined in the 

aerial survey study area. CV = coefficient of variation 

of D and N, CI = confidence interval, Year 1 = Aug 

1992-May 1993, Year 2 = Aug 1993-Jun 1994. 

Stratum D N CV Log-normal 95% CI 

Years 1 & 2 197.95 16,986 0.14 12,869-22,419 

Year 1 147.88 12,690 0.23 8,081-19,929 

Year 2 240.87 20,669 0.18 14,650-29,161 

Summer 1 & 2 174.33 14,99 0.24 9,390-23,830 

Summer 1 149.83 12,857 0.35 6,541-25,271 

Summer 2 181.52 15,576 0.24 9,747-24,890 

Fall 1 & 2 70.51 6,051 0.32 3,265-11,215 

Fall l 64.88 5,567 0.43 2,44412,682 

Fall 2 75.18 6,451 0.46 2,683-15,510 

Winter 1 & 2 255.14 21,894 0.27 13,023-36,805 

Winter 1 167.11 14,340 0.42 6,362-32,320 

Winter 2 393.96 33,806 0.31 18,621-61,374 

Spring 1 & 2 223.93 19,215 0.25 11,935-30,936 

Spring 1 157.83 13,544 0.37 6,818-26,906 

Spring 2 285.32 24,483 0.30 14,218-42,161 

balaenopterids and ziphiids, less than 100 individuals each. Mean group sizes 

ranged from 315 for melon-headed whales to less than four for sperm whales, 

pygmy/dwarf sperm whales, and ziphiids (Table 3 .16) . 

3 .3.3 Cetacean Distribution 

Cetaceans were found throughout the GulfCet study area (Figures 3 .6 and 3 .7). 

While the perception from the distribution of all cetaceans from ship surveys 

is that of a marked reduction in sightings in the extreme western part of the 

study area (Figure 3 .6), ship survey effort was not uniformly distributed. 

However, aerial survey effort was uniformly distributed and, while not as 

marked, fewer sightings were made in the extreme west (Figure 3.7) . 

Seasonally (based on aerial surveys only), except for one sighting, no cetacean 

groups were sighted in the extreme eastern portion (DeSoto Canyon) of the 

study area during summer and fall (Figures 3 .8-3 .11). Very few cetacean 

groups were sighted in the western portion during spring. Cetacean groups 

were sighted throughout the study area during winter. 

80 
Table 3 .16. Estimates of the parameters used to estimate the abundance of cetaceans in the aerial 

survey study area for the entire study (n = groups sighted, n/L = group sighting rate, 

S = size-bias adjusted group size, G = average group size, CV = coefficient of variation, 

P = probability that group size vs PSD regression was significant) . 

Species n n/L CV(n/L) S CV(S) G CV(G) P 

00 

Bryde's/sei whale 1 0.00002 1.07 - - 1 .0 1.00 - 

Sperm whale 25 0.00050 019 2.1 0.13 2 .0 0.12 0.52 

Pygmy/dwarf sperm whale 33 0.00066 0.18 1 .4 0.08 1 .4 0.09 ().47 

Cuvier's beaked whale 1 0.00002 0.67 - - 3 .0 0.58 - 

Unidentified ziphiid 11 0.00022 0.46 3.2 ' 0.19 3 .1 0.11 0.=43 

Melon-headed whale 3 0.(X)006 0.70 526.0 0.98 311.7 0.22 0.43 

Melon-headed/pygmy killer whale 2 0.00004 0.72 - - 14.5 0.72 - 

False killer whale 2 0.00004 0.66 - - 27.5 0.27 - 

Short-finned pilot whale 10 0.00020 0.43 23.8 0.21 22.5 0.17 0.50 

Rough-toothed dolphin 7 0.00014 0.38 11.2 0.44 15.0 0.39 0.11 

Fraser's dolphin 2 0.00004 0.88 - - 31.0 0.45 

Bottlenose dolphin 70 0.00140 0.14 13.7 0.16 13.6 0.12 0.25 

Risso's dolphin 34 0.00068 0.18 11 .7 0.14 12.0 0.19 0.55 

Atlantic spotted dolphin 11 0.00022 0.32 17.4 0.30 17.8 0.21 0.31 

Bottle iiose/Atlantic spotted dolphin 5 0.00010 0.41 9.7 0.77 8.2 0.52 () .a4 

Pantropical spotted dolphin 43 0.00086 0.16 43.3 0.15 50.2 0.11 0.07 

Striped dolphin 6 0.00012 0.45 62.7 0.67 52.5 0.39 0.37 

Spinner dolphin 4 0.00008 0.50 250.1 0.74 91 .3 0.40 0J1 

Clymene dolphin 5 0.00010 0.36 29.7 0.46 35.0 0.21 020 

Unidentified stenellid 8 0.00016 0.40 53.3 0.45 28.5 0.31 0.88 

Unidentified dolphin 10 0.00020 0.33 6.0 0.44 5.0 0.36 05C> 

Unidentified small whale G 0.00012 0.41 3.9 0.18 2.2 ()18 () .90 

Unidentified large whale 4 0.00008 0.47 1 .5 0.27 1 .3 019 0.41 

Unidentified odontocete 8 0.00016 0.35 2.2 0.59 3.0 0.57 0.25 
Table 3.17. Overall density (D = animals/1,000 km2 ) and 

abundance (N) estimates of cetacean species in the 

aerial survey study area for all seasons combined 

(CV = coefficient of variation of D and N, 

CI = confidence interval). 

Species D N CV 

Log-normal 

959'o CI 

Bryde's/sei whale 0.02 2 1.08 a10 

Sperm whale 1.01 87 0.27 52-146 

Pygmy/dwarf sperm whale 2.04 176 0.31 97-317 

Cuvier's beaked whale 0.13 11 0.71 3-40 

Unidentified ziphiid 1.44 123 0.37 43-162 

Melon-headed whale 29.84 2,561 0.74 698-9,396 

Melon-headed/pygmy killer whale 1.02 88 1.03 &925 

False killer whale 1.94 167 0.72 45-614 

Short-finned pilot whale 7.96 684 0.48 284-1656 

Rough-toothed dolphin 2.76 237 0.59 74758 

Fraser's dolphin 1 .69 146 1 .00 26-810 

Bottlenose dolphin 33.67 2,890 0.20 1,955-4,270 

Risso's dolphin 14.1 1,237 0.28 727-2,102 

Atlantic spotted dolphin 6.94 5% 0.38 288-1,233 

Bottlenose/Atlantic spotted dolphin 1 .45 125 0.67 32-478 

Pantropical spotted dolphin 59.40 5,097 0.24 3,207-8,100 

Striped dolphin 10.05 863 0.60 276-2,699 

Spinner dolphin 11 .65 1,000 0.65 291-3,433 

Clymene dolphin 5.59 479 0.43 209-1,101 

Unidentified stenellid 7.28 624 0.52 235-1,660 

Unidentified dolphin 2.18 187 0.55 67-526 

Unidentified small whale 0.56 49 0.51 19-124 

Unidentified large whale 0.10 9 0.53 3-23 

Unidentified odontocete 1.05 90 0.71 23-350 

The distribution of species are summarized below. The distribution of most 

species in the GulfCet study area had a depth component. That is, some were 

generally found only in the more shallow portions of the study area near the 

continental shelf break (100 m isobath) and others were usually only found 

well past the 100 m isobath. In addition to this depth component, some species 

were concentrated in different broad regions of the study area (e.g., central, 

western, or eastern) . There was no evidence that any species shifted 

distribution seasonally within the aerial survey study area (see below) . 

82 
30 

29 

28 

00 w 

27 

26 

25 

-90 -89 -88 - 87 

30 

29 

28 

27 

26 

25 

7 

Figure 3.6. The locations of all cetacean groups sighted during GulfCet ship surveys. 

-96 -95 -94 -93 -92 -91 
30 

29 

14 

28 

27 

26 

25 

-87 

1 30 

29 

28 

27 

26 

25 

.7 

Figure 3 .7 . The locations of all cetacean groups sighted during GulfCet aerial surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

F- I 

30 v ` 30 

29 29 

28 R'p! +~ ' IT 28 

+ ++F~ +t+ +++++ ++ 

+ + 

v 

- 27 

26 26 

25 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .8. The locations of all cetacean groups sighted during GulfCet 

summer aerial surveys. 

30 

29 

28 

.7 

30 

29 

28 

27 27 

26 26 

25 25 

.7 

Figure 3 .9 . The locations of all cetacean groups sighted during GulfCet fall 

aerial surveys. 

85 
30 

29 

28 

27 

26 

25 

.0 

!9 

.g 

;7 

!6 

!5 

Figure 3 .10. The locations of all cetacean groups sighted during GulfCet 

winter aerial surveys. 

30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

IO 

!9 

.g 

!7 

!6 

!5 

Figure 3 .11 . The locations of all cetacean groups sighted during GulfCet 

spring aerial surveys. 

86 
3 .3.4 Species Accounts 

In order to quickly access abundance and distribution information for each 

species from both ship and aerial surveys, summaries are provided below. Each 

summary includes: (1) the number of on-effort sightings (Tables 3 .6 and 3.13), 

(2) estimates of overall mean group size (Tables 3.6 and 3.16), and (3) estimates 

of overall animal density from both ship and aerial surveys (Tables 3 .6 and 

3 .17) ; (4) the range of group sizes ; (5) where applicable, seasonal density 

estimates (Tables 3 .18 and 3 .19); and for the entire GulfCet study area, (6) an 

estimate of overall abundance (Table 3.6), and (7) a summary of distribution 

(Figures 3 .12-3 .50) . As stated earlier, aerial surveys were designed to provide 

information on seasonal densities whereas ship surveys were not. However, 

seasonal density estimates were only made for those species sighted 20 or more 

times during aerial surveys . Although some of the seasonal density estimates 

are variable, they generally are not significantly different (p < 0.05) . Since 

aerial surveys only sampled about 56% of the GulfCet study area, and because 

abundance estimates are the product of density and size of the area sampled, 

overall abundance estimates are from the overall ship surveys which sampled 

the entire study area. Comments on the distribution of each species are based 

on distribution maps from all aerial and ship surveys. 

Balaenopterid whales 

Balaenopterid whales were sighted on two occasions. Two Bryde's whales were 

sighted together from a ship during spring . A whale that was either a Bryde's 

whale or a sei whale vas sighted from the aircraft during winter. The density 

estimates (animals/1,000 km'- (CV)) were 0.02 (0.81) and 0.02 (1 .08) for overall 

ship and overall aerial surveys, respectively. Bryde's whale abundance was 

estimated to be three animals (95% CI = 1-14, where CI = confidence interval). 

Both sightings were made near the 100 m isobath in the west-central part of 

the study area (Figures 3 .12 and 3 .13). 

Sperm whale 

Sperm whales were sighted during all seasons, 73 times during ship surveys 

and 28 times during aerial surveys. Group sizes averaged 2 .7 (0.14) and 2.0 

(0.12) animals as estimated from ship and aerial platforms, respectively, and 

ranged from 1-12 animals. The density estimates (animals/1,000 km2 (CV))were 

2.02 (0 .25) and 1 .01 (0.27) for overall ship and overall aerial surveys, 

respectively. The annual densities of sperm whales were similar, 0.85 (0 .33) 

and 1 .16 (0.33), for Year 1 and Year 2, respectively. Seasonally, the fall and 

spring densities were similar, 1 .26 (0.43) and 1 .41 (0.43), respectively. Summer 

density was intermediate 0.84 (0.52) and winter density was the lowest 0.55 

(0.54). Sperm whale abundance was estimated to be 313 animals (95% CI = 192-

508) . Sperm whales were sighted throughout the study area (Figures 3 .14 and 

3 .15) . However, concentrations occurred near the 1,000 m isobath in the 

vicinity of the Mississippi River delta and on the central slope in the westcentral part of the study area. 

87 
Table 3 .18 . Estimates of the parameters used to estimate the abundance of all cetaceans sighted 20 

or more times in the aerial survey study area by year and season. Year 1 = Aug 1992- 

May 1993, Year 2 = Aug 1993-Jun 1994, n = groups sighted, n/L - group sighting rate 

per 1,000 km, S = size-bias adjusted group size, G = average group size, CV = coefficient 

of variation, P = probability that group size vs . PSD regression was significant. 

Species Stratum n n/L CV(n/L) S CV(S) G CV(G) P 

Sperm whale Years 1 & 2 25 0.50 0.19 2 .1 0.13 2.0 0.12 0.52 

Year 1 10 0.41 0.30 2.0 0.24 2.1 0.19 0.25 

Year 2 15 0.59 0.24 2.4 0.18 2.0 0.16 0.75 

Summer 5 0.39 0.42 2.2 0.39 2.2 0.26 0.26 

Fall 7 0.60 0.31 3.0 0.33 2.1 0.26 0.76 

Winter 5 0.40 0.49 1.5 0.32 1 .4 0.17 0.46 

Spring 8 0.63 0.34 2.2 0.31 2.3 0.21 0.28 

800 Pygmy/dwarf Years 1 & 2 33 0.66 0.18 1.4 0.08 1 .4 0.09 0.47 

sperm whale Year 1 17 0.70 0.24 1.3 0.11 1 .4 0.12 0.34 

Year 2 16 0.63 0.25 1.6 0.11 1 .5 0.11 0.57 

Summer 13 1.00 0.27 1.7 0.16 1 .8 0.13 0.34 

Fall 2 0.17 0.80 - - 1 .5 0.33 - 

Winter 5 0.40 0.50 1.7 0.18 1.2 0.17 0.89 

Spring 13 1.03 0.27 1.2 0.11 1.3 0.10 0.22 

Bottlenose Years 1 & 2 70 1.40 0.14 13.7 0.16 13.6 0.12 0.25 

dolphin Year 1 30 1.23 0.23 11.6 0.22 14.0 0.17 0.08 

Year 2 40 1.56 0.16 16.8 0.23 13 .3 0.16 0.68 

Summer 24 1.85 0.21 21.5 0.30 18.2 0.19 0.45 

Fall 6 0.51 0.63 31.1 0.70 18.2 0.32 0.56 

Winter 20 1 .59 0.22 9.1 0.29 9.0 0.16 0.25 

Spring 20 1 .58 0.28 12.3 0.30 11.3 0.22 0.36 
Table 3 .18 . Estimates of the parameters used to estimate the abundance of all cetaceans sighted 20 

or more times in the aerial survey study area by year and seas on. Year 1 = Aug 1992- 

May 1993, Year 2 = Aug 1993-Jun 1994, n = groups sighted, n/L = group sighting rate 

per 1,000 km, S = size-bias adjusted group size, G = average group size, 

CV = coefficient of variation, P = probability that group size vs . PSD regression was 

significant. (continued) 

Species Stratum n n/L CV(n/L) S CV(S) G CV(G) P 

IZisso's dolphin Years 1 & 2 34 0.68 0.18 11 .7 0.14 12.0 0.19 0.55 

Year 1 12 0.49 0.35 10.0 0.20 10.4 0.20 0.33 

Year 2 22 0.86 0.20 13.4 0.19 12.8 0.30 0.66 

Summer 3 0.23 0.58 325 .4 1.60 30.7 0.77 0.80 

Fall 2 0.17 0.70 - - 9.0 0.22 - 

Winter 14 1.11 0.26 9.8 0.19 11 .1 0.17 0.18 

Spring 15 1.19 0.29 9.9 0.19 9.4 0.17 0.53 

Pantropical Years 1 & 2 43 0.86 0.16 43.3 0.15 50.2 0.11 0.07 

spotted dolphin Year 1 19 0.78 0.23 30.3 0.26 40.2 0.17 0.08 

Year 2 24 0.94 0.21 52.4 0.16 58.1 0.14 0.16 

Summer 14 1 .08 0.26 34.8 0.26 35.1 0.17 0.34 

Fall 4 0.34 0.42 41.1 0Z5 54.3 0.24 0.13 

Winter 10 0.79 0.35 41.5 0.32 63.7 0.13 0.07 

Spring 15 1.19 0.26 66.5 0.32 55.1 0.24 0.54 
Table 3.19. Density (D = animals/1,000 km2) and abundance (N) 

estimates of cetacean species sighted 20 or more times 

by year and season in the aerial survey study area. CV 

= coefficient of variation of D and N, CI = confidence 

interval, Year 1 = Aug 1992-May 1993, Year 2 = Aug 

1993-Jun 1994. 

Species 

Log-normal 

Stratum D N CV 95% CI 

Sperm whale Year 1 & 2 1.01 87 0.27 52-146 

Year 1 0.85 73 0.33 35-153 

Year 2 1.16 100 0.33 54186 

Summer 0.84 72 0.51 27-192 

Fall 1.26 109 0.43 47-250 

Winter 0.55 47 0.52 18-127 

Spring 1 .41 121 0.43 54274 

Pygmy/dwarf Year 1 & 2 2.04 176 0.31 97-317 

sperm whale Year 1 1 .96 168 0.36 85-334 

Year 2 2.04 176 0.37 88-352 

Summer 3.87 333 0.38 161-688 

Fall 0.55 48 0.90 10-227 

Winter 1 .03 89 0.58 31-255 

Spring 2.71 233 0.39 114478 

Bottlenose Year 1 8c 2 33.67 2,890 0.20 1955-4270 

dolphin Year 1 25.1 2,158 0.33 1141-4081 

Year 2 36.70 3,150 0.25 1959-5064 

Summer 59.73 5,126 0.30 2867-9163 

Fall 16.0 1,407 0.71 395-5011 

Winter 25.09 2,154 0.29 1241-3737 

Spring 31.61 2,713 0.36 1356-5428 

Risso's dolphin Year 1 & 2 14.41 1,237 0.28 727-2102 

Year 1 9.07 779 0.41 359-1689 

Year 2 19.49 1,673 0.34 861-3249 

Summer 12.7 1,079 0.97 133-8777 

Fall 2.70 232 0.74 62-872 

Winter 21.87 1,877 0.32 1019-3459 

Spring 19.72 1,693 0.35 8743279 

Pantropical Year 1 & 2 59.40 5,097 0.24 3207-8100 

spotted dolphin Year 1 37.67 3,233 0.37 1586-6590 

Year 2 86.86 7,453 0.27 4413-12587 

Summer 58.89 5,03 0.33 26749549 

Fall 22.32 1,915 0.50 726-5050 

Winter 52.49 4,504 0.49 1766-11486 

Spring 104.16 8,938 0.37 4383-18224 

90 
30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -9u -zsy -aa 

200 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 

30 

29 

28 

27 

26 

25 

Figure 3 .12 . The location of the Bryde's/sei whale sighted during GulfCet ship 

surveys . 

30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

-v 

s 

100 N 

Summer 

+ Fall 

Winter 

~£- Spring 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

;0 

" 9 

rg 

?6 

LS 

Figure 3 .13 . The locations of the Bryde's/sei whale sighted during GulfCet 

aerial surveys . 

91 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

30 30 

29 29 

28 28 

27 27 

26 26 

25 25 

7 

Figure 3 .14. The locations of all sperm whales sighted during Gul fCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 ~ 30 

29 ~ ~ 29 

28 ipp ~ 28 

27 + ~ yam"- 27 

26 ~ Summer 26 

Fall 

Winter 

25 `~ Spring 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .15 . The locations of all sperm whales sighted during GulfCet aerial 

surveys. 

92 
whale 

Pygmy sperm whales were sighted during all seasons except fall, a total of 

nine times from ship platforms . (Pygmy sperm whales could not be 

distinguished from dwarf sperm whales from aircraft. See below.) Group sizes 

averaged 1 .2 (0.12) animals and ranged from 1-2 animals. The density estimate 

(animals/1,000 km'- (CV)) was 0.12 (0.40) for overall ship surveys. Pygmy 

sperm whale abundance was estimated to be 19 animals (959'o CI = 9-40) . Pygmy 

sperm whales were sighted in the central part of the study area. No sightings 

occurred in the extreme eastern or western portions of the study area (Figure 

3 .16). 

Dwarf sperm whale 

Dwarf sperm whales were sighted during spring and summer, a total of 22 

times from ship platforms . (Dwarf sperm whales could not be distinguished 

from pygmy sperm whales from aircraft. See below.) Groups sizes averaged 2.1 

(0.17) animals and ranged from 1-7 animals. The density estimate 

(animals/1,000 km'- (CV))was 0.57 (0.34) for overall ship surveys. Dwarf sperm 

whale abundance was estimated to be 88 animals (95% CI = 46-170) . Dwarf 

sperm whales were sighted throughout the study area with most sightings 

occurring in the eastern portion of the study area (Figure 3.17). 

Pvgmv/dwarf sperm whales 

Pygmy/dwarf sperm whales were sighted in all seasons, 15 times during ship 

surveys and 37 times during aerial surveys. Group sizes averaged 2.0 (0.15) and 

1 .4 (0.09) animals as estimated from ship and aircraft platforms, respectively, 

and ranged from 1-4 animals. The density estimates (animals/1,000 km2 (CV)) 

were 0.34 (0.39) and 2.04 (0.31) for overall ship and overall aerial surveys, 

respectively. The sum of the overall ship densities of dwarf, pygmy, and 

pygmy/dwarf sperm whales was 1 .13 . The densities from Year 1 and Year 2 

were similar, 1 .96 (0.36) and 2 .04 (0.37), respectively . Seasonally, densities 

peaked during summer (3 .87 [0.38]) and spring (2 .71 [0.38]), and were much 

lower during fall (0.55 [0.90]) and winter (1 .03 [0.58]). Pygmy/dwarf sperm 

whale abundance was estimated to be 53 animals (95% CI = 25-111). 

Pygmy/dwarf sperm whales were sighted throughout the study area (Figures 

3 .18 and 3.19) . 

Cuvier's beaked whale 

Cuvier's beaked whales were sighted six times during ship surveys and once 

during aerial surveys. All of the sightings were made during spring. The 

group sizes averaged 1 .2 (0.14) animals from ship sightings with a range of 1-2 

animals, and the aircraft sighting consisted of one whale . The density 

estimates (animals/1,000 km-' (CV)) were 0.09 (0.41) and 0.13 (0.71) for overall 

ship and overall aerial surveys, respectively. Cuvier's beaked whale 

abundance was estimated to be 14 animals (95% CI = 7-31). Cuvier's beaked 

whale sightings were distributed throughout the deepest part of the study area 

near the 2,000 m isobath (Figures 3.20 and 3.21) . 

93 
30 

29 

28 

27 

26 

25 

.o 

!9 

rg 

!7 

!6 

!5 

Figure 3 .16. The locations of all pygmy sperm whales sighted during GulfCet 

ship surveys. 

30 

29 

28 

27 

26 

25 

.0 

:9 

.g 

!6 

!5 

Figure 3 .17 . The locations of all dwarf sperm whales sighted during GulfCet 

ship surveys. 

94 
30 

29 

28 

27 

26 

25 

i0 

!9 

rg 

t6 

?5 

Figure 3 .18 . The locations of all pygmy/dwarf sperm whales sighted during 

GulfCet ship surveys. 

30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -90 

Figure 3 .19 . The locations of all pygmy/dwarf spec 

GulfCet aerial surveys. 

-87 

30 

29 

28 

27 

Summer 26 

Fall 

Winter 

Spring 25 

-89 -88 -87 

-m whales sighted during 

95 
30 

29 

28 

27 

26 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

30 

29 

28 

27 

26 

25 r 

1 

25 

7 

Figure 3.20. The locations of all Cuvier's beaked whales sighted during GulfCet 

ship surveys. 

7 

30 30 

29 29 

28 28 

27 27 

26 26 

+ Fall 

Z Winter 

25 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .21 . The locations of all Cuvier's beaked whales sighted during GulfCet 

aerial surveys. 

96 
Unidentified ziphiid whales 

Unidentified ziphiid whales were sighted in all seasons, 15 times during ship 

surveys and 12 times during aerial surveys. Group sizes averaged 2.4 (0.13) and 

3 .2 (0.19) animals as estimated from ship and aircraft platforms, respectively, 

and ranged from 1-7 animals. The density estimates (animals/1,000 km2 (CV)) 

were 0.80 (0.38) and 1 .44 (0.37) for overall ship and overall aerial surveys, 

respectively. During aerial surveys, five sightings occurred in Year 1, three 

in Year 2, and two or more sightings were made in each season except summer. 

Unidentified ziphiid whale abundance was estimated to be 124 animals (95% 

CI = 71-218). Unidentified ziphiid whales were sighted throughout the study 

area, generally well away from the 100 m isobath (Figures 3 .22 and 3.23). 

Melon-headed whale 

Melon-headed whales were sighted during all seasons except fall, 10 times 

during ship surveys and four times during aerial surveys . Melon-headed 

whales were not always distinguished from pygmy killer whales from aircraft . 

Identification of melon-headed whales from aircraft was based on large group 

size (>100 animals) and the presence of Fraser's dolphins (see Mullin et al. 

(1994a) and the Pygmy killer/melon-headed whales account, below) . Group 

sizes averaged 140.7 (0.19) and 311.7 (0.22) animals as estimated from ship and 

aircraft platforms, respectively, and ranged from 30-400 animals. The density 

estimates (animals/1,000 km'- (CV)) were 13 .38 (0.34) and 29.84 (0.74) for 

overall ship and overall aerial surveys, respectively. During aerial surveys, 

two groups were sighted in both Year 1 and Year 2 and three sightings were 

made in the spring. Melon-headed whale abundance was estimated to be 2,067 

animals (95% CI = 1,071-3,988) . Melon-headed whales were sighted in the westcentral portion of the study area, well past the 100 m isobath (Figures 3.24 and 

3.25). 

Pygmy killer whale 

Pygmy killer whales were sighted from ship two times during spring. Pygmy 

killer whales and melon-headed whales could not be distinguished from the 

aircraft (see Pygmy killer/melon-headed whale account, below). Group sizes 

were 10 and 13 animals. The density estimate (animals/1,000 km2 (CV)) was 0.23 

(0.64) . Pygmy killer whale abundance was estimated to be 36 animals (95% 

CI = 11-113) . The pygmy killer whale sightings were in the west-central 

portion of the study area well past the 100 m isobath (Figure 3.26). 

J~vgmv killer/melon-headed whale 

Pygmy killer/melon-headed whales were sighted from aircraft during all 

seasons except spring (Figure 3 .27) . These two species were always 

distinguished from each other during ship surveys (see species accounts 

above) . The five sightings averaged 14.5 (0.72) animals and ranged from 3-25 

animals. The density estimate (animals/1,000 km'- (CV)) was 1 .02 (1 .03) . Four of 

the sightings were made in Year 2 . Two sightings were made in fall, two in 

97 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

'.9 

;8 

:7 

'.6 

!5 

Figure 3 .22. The locations of all unidentified Ziphiids sighted during GulfCet 

ship surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 30 

29 !9 

28 

27 

26 

25 

.g 

!7 

!6 

!5 

Figure 3.23 . The locations of all unidentified Ziphiids sighted during GulfCet 

aerial surveys. 

98 
30 

29 

28 

27 

26 

25 

!6 

!5 

Figure 3 .24. The locations of all melon-headed whales sighted during GulfCet 

ship surveys. 

30 

29 

28 

27 

26 

25 

.0 

:9 

;7 

'.5 

Figure 3 .25 . The locations of all melon-headed whales sighted during GulfCet 

aerial surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

.0 

!9 

.g 

r7 

99 
30 

29 

28 

27 

26 

25 

rg 

!6 

'.5 

Figure 3 .26. The locations of all pygmy killer whales sighted during GulfCet 

ship surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

:9 

:8 

:7 

:6 

5 

Figure 3 .27. The locations of all pygmy killer/melon-headed whales sighted 

during GulfCet aerial surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

30 

!9 

100 
winter, and one was made in summer. Most pygmy killer/melon-headed whales 

were sighted throughout the central portion of the study area well away from 

the 100 m isobath. 

False killer whale 

False killer whales were sighted in spring and summer, twice during ship 

surveys and twice during aerial surveys. Group sizes averaged 3 .5 (0.14) and 

27.5 (0.27) animals as estimated from the ship and aerial platforms, 

respectively, and ranged from 2-35 animals. The density estimates 

[animals/1,000 km2 (CV)] were 0.07 (0.63) and 1 .94 (0.72) for overall ship and 

overall aerial surveys, respectively . False killer whale abundance was 

estimated to be 10 animals (3-33) . False killer whale sightings were not 

concentrated in any particular portion of the study area (Figures 3 .28 and 

3.29) . 

Killer whale 

Killer whales were sighted in spring and summer, four times during ship 

surveys and one time (off-effort) during aerial surveys. Group sizes averaged 

11 .2 (0.04) as estimated from the ship platforms and ranged from 10-12 

animals. The group sighted from the aircraft consisted of 10 animals. The 

density estimate (animals/1,000 km'- (CV)) was 0.46 (0.46) for overall ship 

surveys. Killer whale abundance was estimated to be 71 animals (95% CI = 30-

167) . Killer whales sightings were confined to an relatively small area well 

past the 100 m isobath south and southwest of the Mississippi River delta 

(Figure 3 .30) . The off-effort aircraft sighting was near the 1,000 m isobath 

south of the delta. 

Short-finned pilot whale 

Short-finned pilot whales were sighted in all seasons, nine times during ship 

surveys and 11 times during aerial surveys. Group sizes averaged 13 .7 (0 .33) 

and 22 .5 (0.18) animals as estimated from ship and aircraft platforms, 

respectively, and ranged from 2-50 animals . The density estimates 

(animals/1,000 km'- (CV)) were 1 .39 (0.50) and 7 .96 (0.48) for overall ship and 

overall aerial surveys, respectively. During aerial surveys, seven and four 

groups were sighted during Year 1 and Year 2, respectively, and sightings 

were almost evenly distributed throughout the seasons. Short-finned pilot 

whale abundance was estimated to be 215 animals (95% CI = 82-563). Shortfinned pilot whales were sighted primarily in the west and central portion of 

the study area (Figures 3 .31 and 3.32). 

Rough-toothed dolphin 

Rough-toothed dolphins were sighted in every season, eight times during ship 

surveys and nine times during aerial surveys. Group sizes averaged 10.3 (0.18) 

and 11 .2 (0.44) animals as estimated from ship and aircraft platforms, 

respectively, and ranged from 3-48 animals. The density estimates 

(animals/1,000 km' (CV)) were 1 .14 (0.35) and 2.76 (0.59) for overall ship and 

101 
30 

29 

28 

27 

26 

25 

;o 

!9 

rg 

r7 

!6 

!5 

Figure 3.28 . The locations of all false killer whales sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 .0 

29 :9 

28 - inn !8 

27 

26 

25 

!7 

!6 

S 

Figure 3 .29 . The locations of all false killer whales sighted during GulfCet 

aerial surveys. 

102 
30 

29 

28 

0 w 

27 

26 

25 

Figure 3 .30 . The locations of all killer whales sighted during GulfCet ship surveys . 

i0 

!9 

rg 

i7 

!6 

!5 
30 

29 

28 

27 

26 

25 

s0 

9 

>g 

r7 

!6 

!5 

Figure 3 .31 . The locations of all short-finned pilot whales sighted during 

GulfCet ship surveys. 

30 

29 

28 

27 

26 

25 

.0 

!9 

tg 

!7 

!6 

:5 

Figure 3 .32. The locations of all short-finned pilot whales sighted during 

GulfCet aerial surveys. 

104 
overall aerial surveys, respectively. During aerial surveys, five and four 

sightings were made during Year 1 and Year 2, respectively, and five 

sightings occurred during spring. Rough-toothed dolphin abundance was 

estimated to be 177 animals (95% CI = 89-351) . Rough-toothed dolphins were not 

sighted in the extreme eastern portion of the study area (Figures 3 .33 and 

3.34). 

Fraser's dolphin 

Fraser's dolphins were sighted in winter and spring, twice during ship 

surveys and twice during aerial surveys. Group sizes were 22 and 44 (ship), 

and 17 and 45 (aerial) animals . Fraser's dolphins were associated with melonheaded whales in all four sightings. The density estimates (animals/1,000 km2 

(CV)) were 0.42 (1 .17) and 1 .69 (1 .00) for overall ship and overall aerial 

surveys, respectively. Fraser's dolphin abundance was estimated to be 65 

animals (95% CI = 10-400) . Fraser's dolphins were sighted in the central 

portion of the study area. Two of the four sightings occurred in different 

seasons and were within a radius of SO km (Figures 3.35 and 3 .36). 

Bottlenose dolphin 

Bottlenose dolphins were sighted in all seasons, 83 times during ship surveys 

and 83 times during aerial surveys. Group sizes averaged 11 .2 (0.12) and 13 .6 

(0.12) animals as estimated from ship and aircraft platforms, respectively, and 

ranged from 1-90 animals. The density estimates (animals/1,000 km2 (CV)) 

were 16.43 (0.26) and 33 .67 (0.20) for overall ship and overall aerial surveys, 

respectively . The densities from Year 1 and Year 2 were 2 .15 (0.33) and 36.70 

(0.25), respectively. Seasonally, densities pealed during summer (59.73 [0.30]), 

were similar during winter and spring (25 .09 [0.29], 31 .61 [0.37]) and were 

much lower during winter (1G .40 [0.71]). Bottlenose dolphin abundance was 

estimated to be 2,538 animals (95% CI = 1,53-4,174). Bottlenose dolphins were 

sighted throughout the study area almost exclusively at depths less than 1,000 

m (Figures 3.37 and 3.38) . 

Risso's dolphin 

Risso's dolphins were sighted in all seasons, 44 times during ship surveys and 

39 times during aerial surveys. Group sizes averaged 7 .5 (0.14) and 12.0 (0.19) 

animals as estimated from ship and aircraft platforms, respectively, and 

ranged from 1-78 animals. The density estimates (animals/1,000 km2 (CV)) 

were 3 .42 (0.2G) and 1 .41 (0.28) for overall ship and overall aerial surveys, 

respectively. The density from Year 1 was less than one-half that of Year 2, 

9.07 (0.41) and 19.49 (0.34), respectively. Seasonally, densities were similar 

during winter and spring (21 .87 [0.32], 19 .72 [0.35]), somewhat lower during 

summer (12 .57 [0.97]), and were much lower during fall (2 .70 [0.76]) . Risso's 

dolphin abundance was estimated to be S?9 animals (95% CI = 317-881) . Risso's 

dolphins were sighted throughout the study area. Many sightings occurred 

from the Mississippi Canyon, east (Figure 3.39 and 3.40). 

105 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

30 

29 

28 

27 

26 

25 

Figure 3 .33 . The locations of all rough-toothed dolphins sighted during 

GulfCet ship surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 30 

29 29 

28 28 

27 27 

26 26 

+ Fall 

Winter 

25 - Spring 25 

.7 

Figure 3 .34 . The locations of all rough-toothed dolphins sighted during 

GulfCet aerial sun~eys. 

106 
-9b -95 -y4 

30 

29 

28 

27 

100- 

26 

25 

-yl -91 -yU -tiy -2S2S -2S / 

i i i i 

-92 -91 -90 -89 - 

0 

9 

8 

27 

26 

-~ 11 5 

Figure 3 .35 . The locations of all Fraser's dolphins sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 100 

27 

30 

29 

28 

27 

26 e Summer 26 

- Fall 

Z winter 

25 Spring 25 

1 1 rl I I j 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .36. The locations of all Fraser's dolphins sighted during GulfCet 

aerial surveys . 

107 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

!6 

!5 

Figure 3.37. The locations of all bottlenose dolphins sighted during GulfCet 

ship surveys. 

30 

29 

28 

27 

26 

7 

30 

29 

28 

27 

26 

25 Spring 25 

87 

Figure 3 .38 . The locations of all bottlenose dolphins sighted during GulfCet 

aerial surveys. 

108 
-QA _q5 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

7 

.6 

5 

Figure 3 .39 . The locations of all Risso's dolphins sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 30 

29 ~ ~ 29 

28 1(,* 28 

Z~ 27 

26 ~ 

25 -

-96 -95 -94 -93 -92 -91 -90 

Figure 3 .40. The locations of all Risso's dolphins sigl 

surveys . 

Summer 26 

Fall 

Winter 

Spring 25 

-89 -88 -87 

zted during GulfCet aerial 

109 
Atlantic spotted dolphin 

Atlantic spotted dolphins were sighted in all seasons, 18 times during ship 

surveys and 12 times during aerial surveys. Group sizes averaged 22 .6 (0.15) 

and 17 .8 (0 .21) animals as estimated from ship and aircraft platforms, 

respectively, and ranged from 3-55 animals. The density estimates 

(animals/1,000 km'- (CV)) were 7 .41 (0.37) and 6 .94 (0.39) for overall ship and 

overall aerial surveys, respectively. Atlantic spotted dolphin abundance was 

estimated to be 1,145 animals (95% CI = 562-2,332) . Atlantic spotted dolphins 

were generally sighted throughout the length of the study area but almost 

exclusively near the 100 m isobath. None were sighted in the extreme eastern 

or southwestern portions of the study area (Figures 3 .41 and 3 .42). 

Pantropical spotted dolphin 

Pantropical spotted dolphins were sighted in all seasons, 80 times during ship 

surveys, and 47 times during aerial surveys . Group sizes averaged 46.2 (0.11) 

and 55 .1 (0 .24) animals as estimated from ship and aircraft platforms, 

respectively, and ranged from 3-225 animals. The density estimates 

(animals/1,000 km'- (CV)) were 45.99 (0.22) and 59 .40 (0.24) for overall ship and 

overall aerial surveys, respectively . The Year 1 density (37.G7 [0.37]) was less 

than one-half of the Year 2 density (86 .86 [0.27]) . Seasonally, pantropical 

spotted dolphin densities were similar during summer and winter (58.89 [0.33], 

52.49 [0.491), peaked during spring (104.16 [0.37] ), were at a low during fall 

(22.32 [0.50]) . Pantropical spotted dolphin abundance was estimated to be 7,105 

animals (95% CI = 4,661-10,831) . Pantropical spotted dolphins were sighted 

throughout the study area generally well past the 100 m isobath. However, 

very few of the sightings occurred in the extreme western part of the study 

area (Figures 3 .43 and 3 .44) . 

Striped dolphin 

Striped dolphins were sighted in every season, 21 times during ship surveys 

and eight times during aerial surveys. Group sizes averaged 37.0 (0.14) and 52.5 

(0.39) animals as estimated from ship and aircraft platforms, respectively, and 

ranged from 4-150 animals. The density estimates (animals/1,000 km2 (CV)) 

were 13 .53 (0.52) and 10.05 (0.60) for overall ship and overall aerial surveys, 

respectively. During aerial surveys, seven of the eight sightings occurred in 

Year 2 of the study. Striped dolphin abundance was estimated to be 2,091 

animals (95% CI = 788-5,540. Striped dolphins were sighted throughout the 

study area and were generally well past the 100 m isobath. Ten of the 21 ship 

sightings occurred in the extreme east (DeSoto Canyon) portion of the study 

area (Figures 3 .45 and 3 .46) . 

Spinner dolphin 

Spinner dolphins were sighted in every season except fall, 10 times during 

ship surveys and four times during aerial surveys. Group sizes averaged 47.0 

(0.41) and 91 .3 (0.40) animals as estimated from ship and aircraft platforms, 

110 
30 

29 

28 

27 

26 

29 

28 

27 

26 

25 r 

1 

25 

-96 -95 -94 --93 -92 -91 -90 -89 -88 -87 

Figure 3 .41 . The locations of all Atlantic spotted dolphins sighted during 

GulfCet ship surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 ~ A 30 

29 

28 

27 

26 

25 

!9 

tg 

!7 

!6 

!5 

Figure 3 .42 . The locations of all Atlantic spotted dolphins sighted during 

GulfCet aerial surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

6;~~ 30 

111 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

30 30 

29 29 

28 28 

27 27 

26 26 

25 

rl 1 

25 

.7 

Figure 3.43. The locations of all pantropical spotted dolphins sighted during 

GulfCet ship surveys. 

30 

29 -

28 -

27 

26 

25 

-95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

r Ipp 

q) 

28 

Aviv 

Z -27 

Summer 26 

- Fall 

Winter 

Spring 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .44. The locations of all pantropical spotted dolphins sighted during 

GulfCet aerial surveys. 

112 
96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

3 

2 

100 

++ 

-I- Z0~ 

+ 2 

F ri I I I 

-96 -95 -94 --93 -92 -91 -90 -89 -88 -87 

0 

9 

8 

.6 

S 

Figure 3 .45 . The locations of all striped dolphins sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 ~ 30 

29 29 

28 lpp + 28 

27 Z~ 27 

26 E) Summer 26 

Fall 

Winter 

25 ~`f- Spring 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .46. The locations of all striped dolphins sighted during GulfCet aerial 

surveys. 

113 
respectively, and ranged from 9-215 animals. The density estimates 

[animals/1,000 km- (CV)] were 5 .44 (0.60) and 11 .65 (0 .65) for overall 

ship and overall aerial surveys, respectively. During aerial surveys, three of 

four sightings occurred in Year 2 of the study and three sightings occurred 

during winter. Spinner dolphin abundance was estimated to be 840 animals 

(95% CI = 274-2,580) . Spinner dolphins were generally sighted in the eastern 

one-half of study area usually well away from the 100 m isobath (Figures 3.47 

and 3.48). 

Clvmene dolphin 

Clymene dolphins were sighted in every season except fall, 23 times 

during ship surveys and seven times during aerial surveys. 

Group sizes averaged 41 .8 (0 .28) and 35 .0 (0.21) animals as estimated 

from ship and aircraft platforms, respectively, and ranged from 2-200 

animals . The density estimates (animals/1,000 km2 (CV)) were 10.97 (0.37) 

and 5 .59 (0.44) for overall ship and overall aerial surveys, respectively. 

During aerial surveys, six of seven sightings occurred during winter and 

spring. Clymene dolphin abundance was estimated to be 1,695 animals (95% CI 

= 827-3,474) . Clymene dolphin sightings occurred almost exclusively in a 

central portion of the study area well past the 100 m isobath (Figures 3 .49 and 

3 .50) . 

3 .3.5 Distribution, Density, and Abundance of Sea Turtles 

Three species of sea turtles were sighted: 50 leatherbacks, 13 loggerhead, and 

two Kemp's ridley sea turtles . Twelve unidentified chelonid sea turtles were 

also recorded . Overall, annual, seasonal, and by-survey estimates of 

leatherback abundance were made . However, because of small sample sizes, 

estimates of abundance for the other species were limited to an overall 

estimate (Table 3 .20). 

Leatherback sea turtles were the most abundant sea turtle in the aerial 

survey area with an overall density estimate (CV in parentheses) 

of 1 .79 turtles/1,000 km- (0.19) and an abundance estimate of 153 (0.19) 

turtles. Leatherback sea turtle abundances were similar during Year 1 

and Year 2 with 160 (0.29) and 139 (0.22) turtles, respectively, and during 

each season where estimates ranged from 135 (0.28) to 171 

(0 .43) turtles. However, by survey, estimates were much more 

variable. Estimates for the two summer surveys ranged from 0 to 336 

(0.43) turtles and for the two spring surveys, from 54 (0.63) to 213 (0.31) 

turtles. 

Leatherback sea turtles were sighted throughout the aerial survey study 

area. However, the majority of the sightings (34/50) occurred in the 

eastern part of the study area from Mississippi Canyon east to DeSoto 

Canyon . Eight of these sightings were made on one day during the 

first summer survey; seven of the eight, on one track-line. 

114 
30 

29 

28 

27 

30 

29 

28 

27 

26 26 

25 - 

ri 

- 25 

.7 

Figure 3 .47. The locations of all spinner dolphins sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 v 30 

29 ~ ~ 29 

28 28 lpp 

27 ti~ 27 

26 ~ Summer 26 

-f - Fall 

Winter 

25 -* Spring 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .48. The locations of all spinner dolphins sighted during GulfCet 

aerial surveys. 

115 
30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -8 7 

SO 

!9 

rg 

!6 

'.5 

Figure 3 .49. The locations of all clymene dolphins sighted during GulfCet ship 

surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 30 

29 29 

28 28 

27 27 

26 26 

+ Fall 

winter 

25 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3.50. The locations of all clymene dolphins sighted during GulfCet 

aerial surveys. 

116 
Table 3 .20. Abundance estimates and related statistics of sea turtle species in the aerial survey 

study area. Estimates for years and seasons are for leatherbacks. Year 1 = Aug 1992-

May 1993, Year 2 = Aug 1993-Jun 1994, n = number turtles sighted, n/L = number 

turtles per 1,000 km, D = number turtles/1000 km2, N = number of turtles, 

CV = coefficient of variation, CI = confidence interval. 

Log-normal 

Species Stratum n n/L CV(n/L) D N CV(N) 95% CI 

Loggerhead 12 0.24 0.28 0.48 41 0.29 23-71 

Kemp's ridley 2 0.04 0.71 0.08 7 0.71 2-24 

Unid. chelonid 12 0.24 0.32 0.48 41 0.33 22-77 

Leatherback 

Years 1 & 2 45 0.90 0.18 1.79 153 0.19 105-223 

Year 1 23 0.94 0.29 1.87 160 0.29 91-282 

Year 2 22 0.82 0.20 1.62 139 0.22 91-214 

Summer 1 & 2 13 1 .00 0.42 1.99 171 0.43 76-385 

Summer 1 13 1 .97 0.42 3.91 336 0.42 149-754 

Summer 2 0 0 - 0 0 - - 

Fall 1 & 2 11 0.94 0.32 1 .86 159 0.33 85-300 

Fall l 4 0.75 0.67 1 .49 128 0.68 37-435 

Fall 2 7 1 .09 0.32 2.16 185 0.33 97-355 

Winter 1 & 2 11 0.87 0.31 1 .73 148 0.32 80-276 

Winter 1 4 0.65 0.51 1 .28 110 0.51 42-289 

Winter 2 7 1.09 0.40 2.16 185 0.41 84-407 

Spring 1 & 2 10 0.79 0.27 1 .57 135 0.28 78-231 

Spring 1 2 0.32 0.63 0.63 54 0.63 17-172 

Spring 2 8 1.25 0.30 2 .48 213 0.31 118-387 
Another aggregation of 10-12 sightings occurred in the central part of the 

study area; five of these made on one day (Figure 3.51). 

Estimates of abundance for loggerhead and Kemp's ridley sea turtles were 41 

(0.29) and 7 (0.71) turtles, respectively. There were an estimated 41 (0.33) 

unidentified chelonids (most probably loggerheads). Although loggerhead 

turtles were sighted in all seasons, 10 of 13 sightings occurred in the winter 

and spring. Sightings of Kemp's ridley, loggerhead, and unidentified chelonid 

sea turtles occurred throughout the study area. However, very few sighting 

were made in the DeSoto Canyon area (Figure 3 .52). 

3 .4 Discussion 

3 .4.1 Comparison of Ship and Aircraft Surveys 

The use of line transect theory to assess the density and abundance of 

cetaceans has been developing over the past two decades (Buckland et al. 1993). 

Aerial, ship and land-based platforms have been effectively used. Some of the 

first extensive surveys, both aerial and ship, were conducted in the Eastern 

Tropical Pacific (ETP) in conjunction with the involvement of small cetaceans 

in the yellowfin tuna purse seine fishery (Smith 1983, Holt and Sexton 1990) . 

The refinement of survey design, equipment, and analysis techniques for both 

platforms has improved the reliability of population estimates for cetaceans 

(e.g ., Wade and Gerrodette 1993). 

Ship-based and aerial surveys are the standard techniques used for cetacean 

density and abundance estimates. A ship allows for the identification of more 

cetacean species and for the collection of a more complete suite of 

hydrographic data than do aircraft surveys . Ship-based surveys are, however, 

labor intensive and take considerable time to cover large areas. Aircraft can 

cover large areas in a matter of days rather than weeks and provide a time 

efficient method for monitoring and periodically verifying population 

numbers and distribution . However, depending on the type of aircraft, range 

offshore can limit aerial surveys. Both of these visual methods are limited to 

daylight hours and sea states that are generally less than Beaufort 6 (for ship 

surveys) and less than Beaufort 4 (for aerial surveys) . Rain and reduced 

visibility conditions are also limiting . 

The density estimates of cetacean species were generally similar between the 

aerial and ship survey platforms (Table 3.21) . Except for false killer whales, all 

of the differences in density estimates were less than an order of magnitude. 

Due to the small sample size (total of four sightings), any difference in the 

false killer whale estimates is probably not significant. For most species, the 

estimate from one platform was less than twice the size of the estimate from 

the other platform. Neither platform provided consistently higher or lower 

estimates of density. However, these comparisons are confounded by the 

differences in habitat surveyed from the aircraft and ship. The density 

estimates for short-finned pilot whale, bottlenose dolphin, and Risso's dolphin 

were significantly different (p < 0.05), using the criteria of non-overlap of 

85% confidence interval. These were probably due to differences in the 

proportion of habitats sampled from each platform. 

118 
30 

29 

28 

27 

26 

25 

.o 

9 

.g 

!6 

!5 

Figure 3 .51 . The locations of leatherback sea turtles sighted during all GulfCet 

aerial surveys. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 ~J 30 

29 29 

28 100 0 . 1% A ' + A 28 

" , ; , e 

z zoo z7 

26 ~ ~ 26 

unidentified chelonid 

" loggerhead 

25 + Kemp's ridley 25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 3 .52 . The locations of Kemp's ridley, loggerhead, and unidentified 

chelonid sea turtles sighted during all GulfCet aerial surveys. 

119 
Table 3 .21 . Comparisons of overall density estimates from aerial 

and ship surveys, by species. 

Species 

Visual 

Aerial 

Density per 1,000 km2 

Visual Acoustic 

Shipboard 

Bryde's whale 0.02 0.02 

Sperm whale 1 .01 2.02 2.04 

Pygmy/dwarf sperm whale 2.04 1.03 

Cuvier's beaked whale 0.13 0.09 

Unidentified ziphiids 0.61 0.80 

Pygmy killer whale/Melon-headed whale 30.86 13.60 

False killer whale 1 .94 0.07 

Killer whale - 0.46 

Short-finned pilot whale 7.96 1.39 

Rough-toothed dolphin 2.7G 1.14 

Fraser's dolphin 1 .69 0.41 

Bottlenose dolphin 33.67 16.43 

Risso's dolphin 14.41 3.42 

Atlantic spotted dolphin 6.94 7.41 

Pantropical spotted dolphin 59.40 45.99 

Striped dolphin 10.05 13.53 

Spinner dolphin 11.65 5.44 

Clymene dolphin 5.59 10.97 

All dolphins 186.92 124.26 229.00 

The proportion of habitats sampled by aircraft and ships differed because the 

aerial and shipboard visual survey areas were not equal in size. The aerial 

survey covered about 56% of the total study area, with most of the aerial effort 

in waters from 100 to 1,000 m deep. In contrast, the shipboard visual survey 

effort occurred throughout the entire study area (i .e ., 100 to 2,000 m) . Shipbased density estimates for species whose primary habitat includes waters 

beyond the 1,000 m isobath could be larger than the aerial-based estimates, 

because most of the aerial survey sampling occurred inside the 1,000 m 

isobath. In contrast, aerial-based density estimates for those species whose 

primary habitat includes waters inside the 1,000 m isobath could be larger 

than the ship-based estimates, for the same reason. For example, bottlenose 

dolphins were sighted almost exclusively in waters less than 1,000 m deep (see 

Figures 3.49 and 3 .50) while clymene dolphins were sighted almost exclusively 

in waters greater than 1,000 m deep (refer to Figures 3 .32 and 3 .33) . The 

density estimate of bottlenose dolphins from the aerial survey was larger than 

the ship-based estimate (33 .7 versus 16.4 animals/1,000 km2), and the density 

estimate of clymene dolphins from the ship surveys was larger than the 

aerial-based estimate (11 .0 versus 5.6 animals/1,000 km'-). For some species this 

pattern in density estimates did not appear to hold, and of these, most were 

120 
based on small sample sizes . Sample sizes for several species, however, were 

judged adequate . Atlantic spotted dolphins, for example, were sighted 

relatively frequently and almost exclusively within the 1,000 m isobath (see 

Figure 3.24 and 3 .25), and the ship estimate was similar to that of the aerial 

estimate (7.4 versus 6.9 animals/1,000 km'-). 

3.4.2 Cetacean Distribution 

3.4.2 .1 Temporal Distribution 

Assessments of seasonal variation in density estimates were based on the aerial 

surveys since they were designed to provide equal seasonal sampling. Seasonal 

density estimates were made for all cetaceans combined, and for the four 

species and one genus sighted a total of 20 or more times. Seasonal densities 

were lowest during the fall for all categories (Table 3 .19) except sperm whales, 

which had the lowest densities in the summer and winter. Most of the seasonal 

estimates were not significantly different (p < 0.05) using the criteria of nonoverlap of 85% CI. There were, however, several significantly different 

seasonal estimates and all of them involved a fall estimate. The fall density 

estimate for all cetaceans combined was significantly less than those for the 

other seasons. Significantly different seasonal estimates occurred as follows: 

pygmy/dwarf sperm whales, fall less than summer; bottlenose dolphin, fall 

less than summer; Risso's dolphin, fall less than winter and spring ; and 

pantropical spotted dolphin, fall less than spring. These differences suggest 

that the GulfCet study area was less heavily utilized by cetaceans during fall. 

Although the ship surveys were not designed to provide equal seasonal 

coverage (the SEFSC cruises occurred primarily in the spring), seasonal 

density estimates were made for several species (Tables 3 .7-3 .10) . 

Interestingly, the spring and winter ship-based visual density estimates of 

pantropical spotted dolphins were significantly different, with the spring 

estimate being about 10 times larger than the winter estimate. This contrasts 

with the aerial pantropical spotted dolphin estimates which were lowest in the 

fall . The ship-based winter estimate vas based mainly on one year of sampling, 

whereas the aerial estimate was based on two years of sampling. In addition, 

there were twice as many sightings during the first year of the shipboard 

survey. This may indicate significant interannual variability which could 

obscure long-term trends in seasonal patterns. 

3.4.2 .2 Spatial Distribution 

With the exception of the clymene dolphin, groups of oceanic dolphins (i .e ., 

pantropical spotted, spinner, and striped dolphins) were generally sighted 

more frequently in the eastern part of the GulfCet study area. Common 

dolphins were never sighted and are not known to occur in the Gulf of Mexico. 

Atlantic spotted dolphins, which occur in all but the very nearshore habitats 

on the continental shelf in the Gulf, were only sighted near the 100 m isobath 

during these surveys . Bottlenose dolphins were most commonly sighted in 

association with the continental shelf edge. 

Clymene dolphins, short-finned pilot whales, melon-headed whales, and 

pygmy killer whales were found primarily in the central to western region of 

121 
the GulfCet study area. The distributions of melon-headed whales and clymene 

dolphins throughout the entire northern Gulf of Mexico (Mullin et al. 1994a 

and 1994b) suggests that the GulfCet study area may make up a significant 

portion of the range of both of these species. However, short-finned pilot 

whales were sighted by Mullin et al. (1994c) during one survey day near the 

Mississippi River Delta and were observed opportunistically in this same area 

(see Chapter 8) . It is important to remember that while melon-headed whale 

sightings were relatively uncommon, because of their very large group sizes 

(up to 400) they were the fourth most abundant species in the study area. 

The distribution of Risso's dolphin was concentrated along the upper 

continental slope near the Mississippi River but sightings were made 

throughout the remainder of the study area. Groups of Risso's dolphins were 

the most common species sighted by Mullin et al. (1994c) during surveys near 

the Mississippi River Delta. 

Killer whales were found in a broad, but distinct region just southwest of the 

Mississippi River Delta. Opportunistic sightings of killer whales in the Gulf 

support this distribution. However, two sightings have also occurred off the 

Texas coast (O'Sullivan and Mullin in prep.) . Killer whales were observed 

killing a pantropical spotted dolphin from the NOAA Ship Oregon II in this 

area (O'Sullivan and Mase in prep.) . However, pantropical spotted dolphins are 

much more widely distributed in the study area than killer whales. 

3.4.3 Cetacean Abundance and Group Size 

This project was the first attempt to estimate the abundance of all cetaceans 

along the continental slope in the north-central and northwestern Gulf of 

Mexico. As a result, good historical comparisons are not available. However, 

abundance estimates from small sections of the GulfCet study area (Fritts et al. 

1983, Mullin et al . 1994c) and qualitative indices of abundance based primarily 

on stranding data and sightings (Schmidly and Scarbrough 1990, see Chapter 2 

of this report) do provide a basis for comparison with the results from this 

study. 

The precision of abundance estimates depends in part on the number of 

sightings for each species. As a result, abundance estimates of rarely sighted 

species had very large coefficients of variation (CV) . Only four species 

(pantropical spotted dolphins, bottlenose dolphins, Risso's dolphins, and sperm 

whales) had relatively precise estimates (CV of 30% or less) and eight species 

had a CV of 40% or less (Table 3 .22) . Some estimates, such as for Fraser's 

dolphin and Bryde's whale were very imprecise (CVs of 123.8% and 85 .39'0, 

respectively) which should be considered when comparing the abundance 

estimates. Due to the relatively low number of shipboard seasonal sightings 

and the high CVs, distinguishing seasonal patterns in abundance was not 

appropriate. 

Pantropical spotted dolphins occur around the world in tropical waters (Perrin 

and Hohn 1994). This species vas the most abundant cetacean in the study area, 

and was three times more abundant than the next most abundant species, the 

bottlenose dolphin (Table 3.22) . Although this species was thought to be rare 

122 
Table 3 .22 . Ranked abundances (N) and mean group sizes (G) of 

cetaceans from all ship surveys. CV = coefficient of 

variation. 

Species N CV(N) G CV(G) 

1 . Pantropical spotted dolphin 7,105 0.22 46.2 0.41 

2 . Bottlenose dolphin 2,538 0.26 11.2 0.12 

3 . Striped dolphin 2,091 0.52 37.0 0.14 

4. Melon-headed whale 2,067 0.34 140.7 0.19 

5 . Clymene dolphin 1,695 0.37 41.8 0.28 

6 . Atlantic spotted dolphin 1,145 0.37 22.6 0.15 

7 . Spinner dolphin W 0.60 47.0 0.41 

8 . Risso's dolphin 529 0.26 7.5 0.14 

9 . Sperm whale 313 0.25 2.7 0.14 

10. Short-finned pilot whale 215 0.50 13.7 0.33 

11. Rough-toothed dolphin 177 0.35 10.3 0.18 

12. Unidentified ziphiids 124 0.29 2.4 0.13 

13 . Dwarf sperm whale 88 0.34 2.1 0.17 

14. Killer whale 71 0.46 11.2 0.4 

15 . Fraser's dolphin 65 1.17 44.0 - 

16. Pygmy killer whale 36 0.64 11.5 0.13 

17 . Pygmy sperm whale 19 0.40 1.2 0.12 

18 . Cuvier's beaked whale 14 0.41 1.2 0.14 

19 . False killer whale 10 0.63 3.5 0.14 

20. Bryde's whale 3 0.81 2.0 - 

in the northern Gulf of Mexico (Schmidly and Scarbrough 1990), a 

reevaluation of the historical data (see Chapter 2) and recent surveys by 

Mullin et al. (1994c) indicated that pantropical spotted dolphins were common 

in deeper waters beyond the continental shelf. The results of this study 

confirm that this species is very abundant along the continental slope in the 

north-central and western Gulf of Mexico. The mean group size of 45.6 (Table 

2 .23) was within the range reported by Mullin et al. (1994c), but smaller than 

the range of means (75-149) reported for this species in the ETP (Wade and 

Gerrodette 1993). 

Bottlenose dolphins are the most abundant inshore dolphin species on the U.S . 

continental shelf in the Gulf of Mexico. The bottlenose dolphin is especially 

abundant in shallow bays and estuaries along the mainland and barrier 

islands (Scott et al. 1989, Blaylock and Hoggard 1994) . This was the second most 

abundant species in the study area, although they were mostly confined to 

waters along the continental shelf break and upper continental slope. The 

results from Mullin et al. (1994c) were similar to those of this study. Mean 

group size was 12 .6 animals, which is similar to that reported by Mullin et al. 

(1994c). The mean group size (22 .7) for bottlenose dolphins in the ETP (Wade 

and Gerrodette 1993) was larger than that of the present study. 

123 
Table 3 .23 . Comparison of mean group size estimates from ship 

and aerial surveys, by species. 

Mean group size 

Species Shipboard Aerial 

Bryde's whale - 1 

Sperm whale 2.6 2.0 

Pygmy/dwarf sperm whale 2.2 1.4 

Cuvier's beaked whale 1 .2 3.0 

Unidentified mesoplodont 1 .4 3.5 

Melon-headed whale 147.6 311.7 

False killer whale 3.5 ?7.5 

Killer whale 11.3 10.0 

Short-finned pilot whale 16.6 22.5 

Rough-toothed dolphin 9.2 11.2 

Fraser's dolphin 44.0 31.0 

Bottlenose dolphin 12.6 13.6 

Risso's dolphin 7.5 12.0 

Atlantic spotted dolphin 23.1 17.8 

Pantropical spotted dolphin 45.6 43.8 

Striped dolphin 37.5 52.5 

Spinner dolphin 51.2 91 .3 

Clymene dolphin 52.1 35.0 

Striped dolphins, melon-headed whales, and clymene dolphins were the next 

most abundant species, respectively, in the GulfCet study area. Clymene 

dolphins are known only from the tropical and subtropical Atlantic (Perrin 

and Mead 1994), whereas striped dolphins inhabit tropical and warmtemperate waters around the world (Perrin et al. 1994b). Melon-headed whales 

are found throughout the world in tropical and subtropical waters (Perryman 

et al. 1994). As pointed out by Mullin et al . (1994b), the earlier belief that the 

clymene dolphin was uncommon in the Gulf of Mexico may have resulted from 

confusion with other species (e.g ., spinner and common dolphins) until its 

taxonomic status was clarified by Perrin et al. (1981) . No previous estimates of 

abundance are available for clymene dolphins. The abundance of striped 

dolphins in this study is consistent with earlier reports indicating that they 

are common in the northern Gulf of Mexico (Schmidly and Scarbrough 1990) . 

More surprising was the relatively high abundance of melon-headed whales, 

which prior to this study were thought to be rare in the Gulf of Mexico 

(Schmidly and Scarbrough 1990, Mullin et al . 1994a) . Prior to the GulfCet study, 

there were only two stranding records and no confirmed sightings from the 

Gulf of Mexico (Barron and Jefferson 1993) . Mean group sizes for clymene 

(52.1) and striped dolphins (37.5) were similar to that of pantropical spotted 

dolphins (45.6), but less than one-third the average group size (147.6) for the 

gregarious melon-headed whales . Group size for melon-headed whales varied 

greatly (30-400 animals), and this species was frequently observed in 

association with Fraser's dolphin and rough-toothed dolphins (Mullin et al. 

124 
1994a) . The mean group sizes for striped dolphins (60.9) and melon-headed 

whales (199 .1) reported for the ETP (Wade and Gerrodette 1993) were 

comparable to those for this study. 

Atlantic spotted dolphins and spinner dolphins were the sixth and seventh 

most abundant species in the GulfCet study area. Atlantic spotted dolphins are 

endemic to the tropical and warm, temperate Atlantic Ocean (Perrin et al. 

1994a), while spinner dolphins occur in tropical waters around the globe 

(Perrin and Gilpatrick 1994) . The former was thought to be common in the 

northern Gulf of Mexico, while the latter was considered rare or could not be 

distinguished from other species (Schmidly and Scarbrough 1990, Mullin et al. 

1994c). The average group size of Atlantic spotted dolphins (23 .1) was about 

half that of spinner dolphins (51 .2) . Average group size for spinner dolphins 

in the ETP was much larger and ranged from 111.7-134.1 (Wade and Gerrodette 

1993). 

Schmidly and Scarbrough (1990) considered Risso's dolphin to be uncommon 

in the Gulf of Mexico, although Mullin et al . (1994c) found this to be the most 

common species in the offshore waters of Louisiana out to the 1,000 m isobath. 

This species is found world-vide in tropical and warm-temperate waters 

(Jefferson et al. 1993) . In the present study, Risso's dolphins were not the most 

abundant species, but they were common along the continental slope. Average 

group size was 7 .5 individuals, similar to that reported by Mullin et al. (1994c) 

for the offshore waters along the Louisiana coast and by Wade and Gerrodette 

(1993) for the EI'P . 

Sperm whales are found throughout all deep oceans of the world from the 

tropics to the polar pack ice (Rice 1989) . This species was the most abundant 

large cetacean in the study area with an estimated total of about 313 animals. 

Schmidly and Scarbrough (1990) considered sperm whales to be common in 

the Gulf of Mexico. Fritts et al . (1983) and Mullin et al . (1994c) also found sperm 

whales to be the most abundant large cetacean in the northern Gulf of Mexico. 

Average group size based on the shipboard visual surveys was 2.6 individuals 

which is similar to that reported by Fritts et al. (1983) and Mullin et al. (1994c) 

for the northern Gulf of Mexico, but smaller than the 7 .9 animals per group 

reported for the ETP (Wade and Gerrodette 1993) . 

Short-finned pilot whales are found around the world in tropical and warmtemperate waters (Jefferson et al . 1993) . The abundance of short-finned pilot 

whales was similar to that of sperm whales in the GulfCet study area, but they 

occurred in larger groups averaging 16.6 animals. Schmidly and Scarbrough 

(1990) considered pilot whales to be common in the northern Gulf of Mexico. 

Although Fritts et al. (1983) and Mullin et al. (1994c) did not report large 

numbers of sightings, group size was large (18-52) . Mean group size for this 

genus in the ETP was 18.3, comparable to that reported in this study. At this 

time, pilot whales appear to be less abundant than the melon-headed whales, 

which were once thought to be extremely rare in the northern Gulf of Mexico. 

Rough-toothed dolphins are found in tropical and warm-temperate seas 

around the world (Miyazaki and Perrin 1994) . This species, which was 

previously thought to be rare in the Gulf of Mexico (Schmidly and Scarbrough 

1990, Mullin et al. 1994c), appears to be at least as common as sperm whales and 

125 
pilot whales in the northern Gulf of Mexico . Average group size appears to 

range from 4 (Mullin et al. 1994c) to 9.2 (GulfCet) . This is slightly smaller than 

mean group size of 14.7 animals observed in the ETP (Wade and Gerrodette 

1993). 

Dwarf sperm whales, killer whales, Fraser's dolphins, and pygmy killer whales 

were the next most abundant species, with very different group sizes. Killer 

whales are found world-wide in both coastal and pelagic waters (Jefferson et 

al. 1993). Fraser's dolphin is found in tropical waters around the world (Perrin 

et al. 1994c) . The distribution of dwarf sperm whales is not well established, but 

they may be expected in temperate, warm-temperate, and tropical waters 

around the world (Caldwell and Caldwell 1989). Pygmy killer whales occur in 

tropical waters world-wide (Jefferson et al. 1993). Schmidly and Scarbrough 

(1990) considered these four species to be uncommon or rare (Fraser's dolphin 

had never been seen) in the Gulf of Mexico. However, historical evidence 

exists (Chapter 2) indicating that killer whales and pygmy killer whales did 

occur regularly and that dwarf sperm whales could be common. This view was 

consistent with sightings by Mullin et al. (1994c) in offshore waters along the 

Louisiana coast. Fraser's dolphin was thought to be uncommon in the Atlantic 

Ocean and Gulf of Mexico . Sightings from this study now indicate that this 

species occurs in the Gulf of Mexico. Whereas Fraser's dolphin occurred in 

large groups averaging 44 individuals, average group sizes for killer whales, 

pygmy killer whales, and dwarf sperm whales were smaller at 11 .3, 11 .5, and 

1 .6 animals, respectively . In the ETP, the mean group sizes for Fraser's dolphin 

(394.9) and pygmy killer whales (27.9) were larger than in the northern Gulf 

of Mexico, but comparable in size for killer whales (5 .4) and dwarf sperm 

whales (1 .6). 

Pygmy sperm whales, Cuvier's beaked whales, Mesoplodon spp., false killer 

whales, and Bryde's whales were infrequently sighted and typically occurred 

in groups of less than four individuals. It is interesting to note that, based on 

stranding data, pygmy sperm whales were thought to be more common than 

dwarf sperm whales (Schmidly and Scarbrough 1990). However, the shipboard 

visual surveys in the GulfCet study area now indicate the opposite . Bryde's 

whale was the only baleen whale sighted in the study area, with an estimated 

abundance of only three animals. The results from this study are consistent 

with those of Mullin et al . (1994c). 

A total of 20 species or genera were sighted in the GulfCet study area, which is 

comparable in diversity to that reported for the ETP where 19 species or 

genera were reported (Wade and Gerrodette 1993). The estimated abundance of 

all cetaceans sighted in the GulfCet study area was 19,198 animals. With a total 

area of 15,621 km'-, the cetacean density was 0.12 animals/km2 . This is about 

one-fourth the estimated density (0.52 animals/km2) for cetaceans in the ETP 

(Wade and Gerrodette 1993) . It appears that the GulfCet study area is as species 

rich as the ETP, but supports a lower density of cetaceans. This may be due to 

the more oligotrophic conditions in the Gulf. One striking difference between 

these two areas was that common dolphins were the most numerous species in 

the ETP, but appear to be completely absent from the northern and possibly 

the entire Gulf of Mexico . Common dolphins have a world-wide distribution 

from 40° N latitude to 40° S latitude, including the east coasts of North and 

South America (Evans 1994). Why they have not entered the Gulf of Mexico is 

126 
unknown, but one possible explanation is the absence of large areas of cold, 

upwelling-modified waters typically associated with this species (Reilly 1990). 

3 .4.4 Distribution and Abundance of Sea Turtles 

All five species of sea turtles that occur in the Gulf of Mexico are protected 

under the Endangered Species Act of 1973 . The leatherback, Kemp's ridley, and 

hawksbill sea turtles are listed as endangered and the loggerhead and green 

sea turtles are listed as threatened . The hawksbill is usually associated with 

reefs or similar habitats in tropical and subtropical waters and is thought to be 

rare in the northwestern Gulf of Mexico. The green turtle, a herbivore, is 

restricted to warm shallow waters with sea grass beds (National Research 

Council 1990). Both loggerhead and Kemp's ridley sea turtles are known to 

occur on the continental shelf throughout the northern Gulf of Mexico . 

Studies indicate that adult loggerheads and Kemp's ridley are benthic 

carnivores (Dodd 1988). It is not clear why adults of either species would occur 

in oceanic waters, unless they were transiting to and from foraging sites on 

distant and disjunct areas of the continental shelf. 

The leatherback, which feeds primarily on jellyfish, is the most oceanic of the 

sea turtles. However, its distribution is not entirely oceanic, as was previously 

thought, and it is commonly found in relatively shallow continental shelf 

waters along the Atlantic coast of the United States (Hoffman and Fritts 1982) . 

Leatherbacks also occur on the continental shelf in the northwestern Gulf of 

Mexico (Fritts et al . 1983, Lohoefener et al . 1988, 1990) . This study suggests that 

in the northwestern Gulf the primary habitat of the leatherback sea turtle is 

oceanic (> 200 m) . The sighting rate of leatherback sea turtles on the 

continental shelf in the north-central Gulf was 0.4 turtles/100 lm whereas the 

sighting rates from the upper continental slope in the north-central Gulf 

were 2.0 turtles/100 km, almost five times greater (Lohoefener et al . 1990). 

Also, comparatively few leatherbacks were sighted during fall SEFSC surveys 

of the entire continental shelf in the northern Gulf of Mexico. However, the 

majority of the sightings occurred just north of DeSoto Canyon (NMFS, SEFSCMiami unpublished data). Sighting rates of leatherback sea turtles from the 

GulfCet study were 1 .0 turtles/100 km for the entire aerial survey study area. 

However, the sighting rate of leatherbacks from Mississippi Canyon to DeSoto 

Canyon (i .e ., the north-central Gulfs vas 1 .8 turtles/100 km. 

With the two years of turtle data combined by season, leatherbacks were found 

to occur in similar numbers throughout the GulfCet study area in all seasons 

(Table 3 .20) . However, specific locations could be very important to 

leatherbacks, at least for brief periods of time. Lohoefener et al. (1990) sighted 

eleven turtles during one day in August 1989 just south of the Mississippi 

River delta and 14 on one day during October 1989 in DeSoto Canyon. During 

GulfCet flights, eight were sighted on 30 August 1992 in DeSoto Canyon. It is 

important to remember that these numbers of leatherbacks were sighted while 

surveying a relatively narrow and straight corridor (maximum width for 

sighting leatherbacks, about 1 km) and that true size of an aggregation was 

probably not assessed. An estimated 100 leatherbacks were sighted just 

offshore of Teas in 1956 in association with jellyfish (Leary 1957). 

127 
In summary, the GulfCet aerial surveys provided the first assessment of sea 

turtle abundance and distribution over a large area of the oceanic Gulf of 

Mexico. Three sea turtle species occurred in the study area: loggerhead, Kemp's 

ridley, and leatherback. The significance of the oceanic Gulf to loggerheads 

and Kemp's ridley is not clear. However, the leatherback sea turtle, an 

endangered species, occurred in significant numbers, and the study area was 

inhabited by them during all seasons the year. A portion of the study area, 

particularly the region from Mississippi Canyon to DeSoto Canyon, appears to 

be an important habitat for leatherbacks. 

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distributions. Biometrics 41 :273-280. 

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85. 

Blaylock, R.A., and W. Hoggard . 1994. Preliminary estimates of bottlenose 

dolphin abundance in the southern U.S . Atlantic and Gulf of Mexico 

continental shelf waters. NOAA Tech. Mem. NMFS-SEFSC-356 . National 

Marine Fisheries Service, Miami, FL. 10 pp. 

Buckland, S.T. 1985. Perpendicular distance models for line transect sampling. 

Biometrics 41:177-195. 

Buckland, S.T., D.R. Anderson, K.P. Burnham, and J.L Laake. 1993 . Distance 

sampling: Estimating abundance of biological populations. Chapman 

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Burnham, K.P., D.R. Anderson, and J.L. Laake. 1980. Estimation of density from 

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Caldwell, D.K., and M.C. Caldwell. 1989. Pygmy sperm whale Kogia breviceps 

(de Blainville, 1838): Dwarf sperm whale Kogia simus Owen, 1866, 

pp. 235-260. In S.H. Ridgway and R. Harrison, eds. Handbook of marine 

mammals, volume 4: River dolphins and the larger toothed whales. 

Academic Press, San Diego. 

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Fritts, T.H., A.B . Irvine, R.D. Jennings, L.A. Collum, W. Hoffman, and 

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of Mexico and nearby Atlantic waters. Rept. FWS/OBS-82/65. U.S . Fish 

and Wildlife Service, Office of Biological Services, Washington, D.C. 455 

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abundance in the northern Gulf of Mexico from vessel surveys. 

Contribution No . MIA-94/95-25 . National Marine Fisheries Service, 

Miami, FL 9pp. 

Hill, P.S., R.C. Rasmussen, and T. Gerrodette . 1991 . Report of a marine mammal 

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Starr ,Jordan July 28-December 6, 1990. NOAH Tech. Mem. NMFS-SWFC158. National Marine Fisheries Service, La Jolla, CA. 133 pp. 

Hoffman, W., and T.H. Fritts . 1982. Sea turtle distribution along the boundary 

of the Gulf stream current off eastern Florida. Herpetologica 38:405-409. 

Holt, R.S ., and S.N. Sexton . 1987. Report of a marine mammal survey of the 

eastern tropical Pacific aboard the Research Vessel David Srarr Jordan 

July 29-December 5, 1988. NOAH Tech. Mem. NMFS-SWFC-76. National 

Marine Fisheries Service, La Jolla, CA. 171 pp. 

Holt, R.S ., and S.N. Sexton. 1990. Monitoring trends in dolphin abundance in 

the eastern tropical Pacific using research vessels over a long sampling 

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130 pp. 

132 
N. ACOUSTIC SURVEYS 

J.C. Norris, W.E. Evans, R. Benson, and T.D. Sparks 

4.1 Introduction 

It has been known for several decades that cetaceans produce a variety of 

vocalizations . Acoustic communication is favored because of the excellent 

transmission characteristics of water, and most marine vertebrates, including 

cetaceans, make extensive use of that transmission channel. It was evident 

during WWII that cetaceans were a significant interference to Navy 

Antisubmarine Warfare (ASW) use of passive (listening) sonars for submarine 

detection. This resulted in extensive research on cetacean vocalizations and 

the development of increasingly sophisticated systems for detection and 

identification of biological noise sources (e.g ., Levenson 1974) . 

In the late 1960's and early 1970's, the U.S . Navy began to address the problem 

of tracking mobile underwater targets that make noise in order to identify 

them and determine their abundance and distribution as a function of time. 

The Navy benefited from a transfer of the towed acoustic array technology 

from the oil and gas exploration industry . This technology, coupled with 

rapidly improving computer data analysis capability, produced a sensitive, 

mobile, range detection and tracking passive sonar. With 15 years of Navy 

experience, the stage was set to use this capability for the detection, location, 

and assessment of biological targets . 

In 1980, a towed linear acoustic array specially tuned to optimize the reception 

of biological signals was designed by the Hubbs/Sea World Research Institute 

(Thomas et al . 1986) . This new system was initially tested in the eastern 

tropical Pacific Ocean (ETP) for detecting cetaceans associated with the 

yellowfin tuna purse seine fishery by scientists from Hubbs/Sea World 

Research Institute and the National Marine Fisheries Service Southwest 

Fisheries Center. The tests demonstrated that a properly designed system could 

detect and classify various species of cetaceans, in many cases well beyond 

visual detection range. Tests also showed that acoustic censusing was not 

limited by sea state, weather, or lighting conditions, and that large herds of 

dolphins and in some cases large whales could be detected as well as tracked 

(Thomas et al. 1986). 

The array described above had several distinct advantages over other systems 

that have been used in the past to detect and record vocalizations. Hydrophone 

groups were arranged for improved reception of specific frequency bands to 

optimize detection of certain species. Array depth could be adjusted to enhance 

sound reception as a function of target depth and ambient oceanographic 

conditions. The directional sensitivity of the array was used to minimize ship 

self-noise . The array could be modified to electronically form narrow beams 

that facilitate the tracking operation of specific targets. In addition, the array 

proved to be easily transported, adaptable for use on a variety of boats and 

ships, and easy to repair at sea. These initial trials demonstrated that passiveacoustic analysis techniques coupled with acoustic array technology had 

133 
significant potential for advances in bio-acoustic research and could possibly 

improve the assessment of fisheries resources, including cetaceans. 

There have been several acoustic surveys of marine mammals. Bowhead 

whales were censused as they migrated past a linear hydrophone array that 

was fixed to the shore-fast ice at Pt . Barrow, Alaska (Clark et al. 1986, Clark and 

IIlison 1988) . Later projects used concurrent visual and acoustic censusing 

methods (Zeh et al . 1988). There have also been two attempts to devise acoustic 

surveying techniques to census sperm whales. Investigators have used 

multiple hydrophones in an attempt to determine three-dimensional locations 

of vocalizing sperm whales (Watkins and Schevill 1977, Leaper et al. 1992). The 

latter study used paired arrays to determine sperm whale locations, but found 

that determining the location of one array relative to the other was difficult. 

Whitehead and Weilgart (1990) attempted to determine the density of sperm 

whales around the Galapagos Islands using click rates. 

The purpose of this chapter is to describe the techniques and results of an 

acoustic survey of cetaceans in the GulfCet study area. The towed linear 

hydrophone array was the same one developed by the Hubbs/Sea World 

Research Institute. It was towed behind the survey vessel during each of the 

shipboard visual surveys conducted by Texas A&M University. The technology 

proved to be a valuable adjunct to visual surveys for detecting and estimating 

the abundance of cetaceans. 

4.2 Methods 

4.2 .1 Data Acquisition 

4.2 .1 .1 Equipment 

$vdrophone Arrav and Associated Equipment 

A linear hydrophone array was towed behind the vessel during 

Longhorn /Pelican Cruises 1-7 to record the distinctive underwater 

vocalizations of cetaceans . The 525 m array consisted of two sections: a 290 m 

tow cable and a 235 m wet section that contained the active hydrophone groups 

(Figure 4.1) . A 30 m deck cable connected the shipboard electronics to the 

array via the tow cable. The tow cable both pulled the array and provided 

negative buoyancy which allowed depth control over the positively buoyant 

array. The wet section of the array consisted of four sections: 1) a "dead" 

section, 2) a fore and aft vibration isolating mechanism (VIM) section, 3) a 

fore and aft high frequency hydrophone section, each with depth and 

temperature sensors, and 4) a middle low frequency hydrophone section. The 

dead section contained no hydrophones or VIM elements and functioned as a 

spacer between the tow cable and the remainder of the array. The VIMs were 

elastic sections used to reduce low frequency, self-induced vibration. 

Within the active section of the array, there were 195 hydrophones in 16 

groups . The low frequency sections used Teledyne T-1 low frequency 

hydrophones. The high frequency groups used Benthos AQ 20 hydrophones 

with a frequency sensitivity from 0.5 Hz to 30 kHz. These 16 groups were tuned 

134 
Tow Vessel 

25m Dead 

Section 

(b) Forward High 

Frequency Section 

w Sensors 

v~ *. ., r 

Deck Cable 

----,290m D Tow Cable 

_ ~ Deck Reel 

60m Forward Vibration 60m Aft Vibration 

Isolating Mechanism 60m Low Frequency Section Isolating Mechanism 

15mForward High 

Frequency Section 

Low Frequency Section 

15m Aft High Tail 

Frequency Section Droque 

Aft High 

Frequency Section 

3 .84 kHz Group 

y jv n4 vruups 

W W V~ 

9~SV t1Z,VTOUp ~ ll~pih t emp 

~ ~l~ W W 

0013 14 15 ~1 ~2 

E 

4 5 C~ o 6 a 7 8 C~ ~ a 9 D 10 00D 16 17 18 DD 

5 kHz Groups 

10 kHz Groups 

15 kHz Groups 

Figure 4.1 . Diagram of (a) the 525 m linear acoustic array illustrating its various components (including the 

deck capble, deck reel, tow cable, dead section, vibration isolating mechanisms, hydrophone groups, 

and tail torque) and (b) a detailed diagram of the active section, illustrating the placement of depth 

and temperature sensors, and the various hydrophone groups listed by their respective tuning 

frequencies . 
to six different frequency bands. In the low frequency section, hydrophone 

groups were tuned to three frequency bands: eight hydrophone groups tuned 

to 30 Hz, one group tuned to 480 Hz, and one group tuned to 3 .84 kHz. In each 

fore and aft high frequency section, there were groups tuned to S, 10, and 15 

kHz (Figure 4.2). Each hydrophone group was designed to maximize sensitivity 

and directivity at a particular frequency. To increase directivity, each group 

was optimized to receive signals at four-times the wavelength of the 

frequency of interest. For the S kHz hydrophone group, for example, the 

wavelength of 5,000 Hz is 0.3072 m. Four-times 0.3072 m is 1 .229 m. To increase 

phase and frequency resolution, 20 hydrophones were used in the S kHz 

group, therefore each hydrophone was separated by 0.061 m (1 .229/20) . The 

10 kHz groups contained 10 hydrophones, and the 15 kHz groups contained six 

hydrophones. All hydrophones were acceleration canceling and side-mounted. 

The array's area of maximum sensitivity was located in a torus pattern 

perpendicular to the long axis of the array (Figure 4.3). The array possessed 

little sensitivity to the front and rear of this area (shadow zones) and, as a 

result, received little ship generated noise. 

Signal Amplifier and Recording Equipment 

The amplifier was a SIE, Inc. portable Geophysical Amplifier (model RA-44A) 

with 16 channels, each with its own gain and variable cut-off filters for the 

high frequency channel (Figure 4.4). An eight channel Racal V-Store analog 

tape recorder was used to record the acoustic data. This recorder had seven 

recording speeds, ranging from 15/32 to 30 inches per second (ips) and three 

bandwidth settings for each channel. The 3-3/4 ips recording speed produced a 

2.5 kHz bandwidth for the low frequency hydrophone groups and a 12 .5 kHz 

bandwidth for the high frequency hydrophone groups. At this recording 

speed, approximately 40 minutes of recording time were possible on a T-120 

VHS tape. The 3-3/4 ips recording speed was used to minimize the number of 

tapes used per cruise, while a 7-1/2 ips recording speed was often used when 

good signal-to-noise ratios were available. The 7-1/2 ips recording speed had a 

5 kHz bandwidth for the low frequency and 25 kHz bandwidth for the high 

frequency hydrophone groups . 

Monitoring and Analytical Equipment 

While at sea, acoustic signal processing was conducted with an AST 386 

microcomputer utilizing SIGNAL' software. The software contained a 

subroutine providing real-time spectrograms displayed on the computer 

screen. The signals were auditorially monitored with either speakers or 

headphones concurrently with the spectrogram display. The acoustic operator 

detected and logged all acoustic events via these aural and visual outputs. 

The primary analytical tool was a Kay Elemetrics (model 5500) dual channel, 

real-time spectrograph . This instrument produced the spectrograms 

(frequency versus time displays with relative amplitude signified by shades of 

gray), oscillograms (time versus amplitude), and spectra (frequency versus 

amplitude) used to analyze the tapes. The dolphin whistle classification work 

used SIGNALT"' and Canary' sound analysis software programs. 

136 
Depth Temp 

Sensor Sensor 5 kHz Group 

Transformer ; 

W ~ ~ W 

O ED 

5 kHz Group 

20 Hydropliones 

4*(=1 .224m 

10 kHz Group 15 kHz Group 

Transformer 

W 

Transformer 

W 

10 kHz Group 

10 Hydrophones 

4 * 1= 0 .612 m 

15 kHz Group 

6 Hydrophones 

4*1=0.408m 

Figure 4.2 . Illustration of the high frequency section of the hydrophone array indicating sensors, 

transformers, and frequency groups with their respective number of hydrophones and their 

spacing . 
w 

00 

Tow Vessel 

Shadow Zone_______- -- 

Direction of Travel 

Figure 4.3 . The directivity pattern of the hydrophone array. 

Active Segment 

of the Array 

----Shadow Zone 

Scan Area 
W 

On Board Electronics Depth 

Gauge 

Monitor Signal 

Speakers Signal 

Displaying Processing 

Tape Amplifier 

Spectrogram Computer Filter Recorder 

Ship Board Array 

Handling System 

with Deck 

Connector 

Tow Cable 

Linear Hydrophone Array 

Deck Cable 

Figure 4.4. The configuration of the on-board electronics, array handling system, and the array. 
4.2 .1.2 Array Operation and Recording Procedures 

Array Operation 

The array was deployed while the survey vessel was on a track-line. The 

towing speed ranged from 9 .3-12.4 km/hr for the seven cruises . The speed of 

the vessel and the length of the tow cable determined the depth of the array. 

This placed the array above the thermocline most of the time. The forward 

section of the array traveled significantly deeper than the aft section at any 

given tow speed (t = 43.8, p < 0.001), with a mean difference of 2.28 m. 

The array was retrieved when the vessel was to be stopped, such as at a CTD 

hydrographic stations. Retrieval took approximately 12 minutes, deployment 

10 minutes. Both retrieval and deployment required three people: one to 

operate the winch, one to guide the array on or off the reel, and a third to 

guide the array on or off the stern. 

Recording methods 

Eight channels on the Racal recorder were used to record the output from the 

array: the fore and aft 5, 10, and 15 kHz, as well as two 30 Hz groups. 

Information on the array trace number, tape recorder channel number, 

amplifier gain, and tape recorder attenuation was necessary for later 

determination of equipment sensitivity. These hardware settings, as well as the 

date, time, ship's location, track-line number, recording tape speed, and ship's 

speed were written on the data sheet at the beginning of each VHS tape. 

Whenever a signal was perceived, the tape revolutions, time, location, and any 

appropriate comments were noted. While all recorded tapes were archived, this 

written record was used for subsequent acoustic contact analysis. 

4.2 .1.3 Data Processing 

Cataloging 

The first step in post-cruise data processing was to catalog the recorded tapes. 

Based on the written record made at sea, a tape with a signal of interest was 

located. Information on the recording time, and vessel speed and position were 

entered as a file header into a data management program. Once this header 

information was entered, the tape was forwarded to the signal's location. Each 

acoustic event that had been noted at sea was then analyzed. The tape 

revolution, source identity when known, and any pertinent comments were 

listed for each signal . The data management program calculated the time and 

the ship's location for each acoustic event. This data record contained the 

location and identity of every recorded acoustic event. Each acoustic contact 

often consisted of multiple acoustic events. For example, a single sperm whale 

contact typically consisted of a multiple series of clicks, often with many 

entries per minute. A second data set was generated by importing the Multiple 

Interface Data Acquisition System (MIDAS) location data into the acoustic 

contact database to provide GPS locations for all acoustic contacts at a one 

minute time resolution . All subsequent analyses were performed on this data 

set. Only MIDAS locations were used in the analyses reported here. 

140 
MIDAS Location Data 

Cruises 2-7 were conducted aboard Louisiana Universities Marine Consortium's 

(LUMCON) R/V Pelican. The ship's GPS location and speed were recorded every 

10 seconds using LUMCON's proprietary data collection system, MIDAS (Walser 

et al. 1992). The R/V Longhorn, used on Cruise 1 only, did not have such an 

automated system. Therefore, only location and speed information that had 

been recorded on the data sheets while at sea could be used for subsequent 

analysis of Cruise 1 . 

Acoustic Effort 

Acoustic effort was defined as occurring when the survey vessel was 

traversing a track-line while recording . Off-effort events occurred when 

recording off the track-line . For example, at the request of the visual survey 

team, the ship would occasionally leave the transect-line to verify the 

identification of animals. Effort also stopped for CTD hydrographic stations and 

high noise conditions caused by ship traffic or heavy rain. Acoustic effort 

differed from shipboard visual effort in that it did not include the time spent 

traveling between track-lines but did include track-lines traversed at night, 

and during all Beaufort sea states . 

4.2.2 Data Analysis Techniques 

4.2.2.1 Species Identification 

Schevill and Lawrence (1949) were among the first investigators to note that 

there is considerable difference between the sounds of different cetacean 

species . Their report also proposed that underwater vocalizations might be 

helpful in distinguishing species at sea. Schevill (1964) suggested that sounds 

of cetaceans might be used as taxonomic aids, if and when a sufficient data 

base was available. 

Since the 19G0's, the number of recorded vocalizations of whales, dolphins, and 

porpoises has greatly increased (Watkins et al. 1991, Evans 19G7) . In the case of 

the great whales, the underwater vocalizations are distinctive enough to allow 

a high probability of correct identification (Thompson et al. 1979, Cummings et 

al. 1986, Clark 1990 . Bowhead and gray whale vocalizations are well studied. 

Underwater vocalizations of bowhead whales have been used to estimate the 

Bering Sea population of this endangered species (Clark et al. 1986, Clark and 

IIlison 1988). The rorquals have also been the subject of extensive acoustic 

research. This is true especially for the humpback whale, where populations 

have been differentiated using vocalizations (Winn et al. 1981). The most 

vocally distinctive of the great whales, the sperm whale, has been studied in 

detail (Berzin 1971, Watkins and Schevill 1977, Watkins et al . 1985, Weilgart and 

Whitehead 1988, Whitehead and Weilgart 1990). Sperm whale pulses are 

distinguished from other species by their bandwidth, duration, and rhythmic 

nature. While limited knowledge exists about the vocalizations of the two Kogia 

species, their vocalizations differ from those of sperm whales by being of a 

higher frequency and shorter duration (Norris, personal observation). There 

is little data on ziphiid pulses; there is no indication that they use the 

141 
rhythmic patterns that are commonly used by sperm whales. The bandwidth of 

delphinid pulses are much wider and pulse repetition rates are higher (Au 

1993). 

Based on the historical records of cetacean occurrence and distribution in the 

Gulf of Mexico (see Chapter 2 of this report), the decision was made to 

concentrate acoustic effort on those species that had a high probability of 

accurate identification from underwater vocalizations without visual 

verification . Sperm whales were the main focus of the acoustic effort reported 

here. . 

Dolphin vocalizations have been extensively studied, but because of their 

variability they have been difficult to quickly and accurately identify to 

species (Wang Ding 1993, Fristrup and Watkins 1992) . The most common 

dolphin vocalization, whistles, although different from species to species, are 

difficult to identify to the species level without considerable analysis. Evans 

(1967) noted that most of the smaller pelagic species, including the common 

dolphin and the pantropical spotted dolphin, are distinctive from bottlenose 

dolphins and other larger species because their whistles are of higher 

frequencies. This vas verified by Wang Ding (1993) who found a 0.931 

correlation between maximum frequency of vocalization and body length for 

nine species of dolphin . Hauser (1993) found a similar frequency-to-body size 

relationship among primates. The first comprehensive quantitative study of 

whistles of different dolphin species vas done by Steiner (1980, 1981). Using 

multivariate discriminant analysis, he compared the whistle structures of five 

western north Atlantic dolphin species. He measured six variables for each 

whistle: beginning frequency, end frequency, minimum frequency, maximum 

frequency, duration, and number of inflection points . The results indicated 

that the whistles from the five species studied had consistent species-specific 

characteristics. The relative degrees of species distinctiveness were broadly 

correlated with taxonomic and zoogeographic relations . The greater the 

taxonomic differences between species the greater the difference between 

whistle vocalizations. Differences were generally greater between sympatric 

species than between allopatric species . 

Wang Ding (1993) conducted a detailed comparison of the whistles of six 

dolphin species. He found that whistle structures of dolphins were associated 

with taxonomic relations, body size, habitat, and oceanographic conditions in a 

complex way. The major difference between Wang Ding's and Steiner's (1981) 

studies was that Wang Ding considered habitat. Pelagic species usually emitted 

whistles in a higher frequency range with greater frequency modulation 

than coastal species. Because of the large number of dolphin species recorded 

without visual identification to species and listed as unidentified dolphins, 

Wang Ding's (1993) method was used to differentiate the coastal species and 

pelagic species encountered during the GulfCet cruises . Whistles from 

bottlenose dolphins, clymene dolphins, pantropical spotted dolphins, and 

striped dolphins, recorded during Pelican/Longhorn Cruises 1-3, were 

compared. 

In this study, signal spectral analysis and statistical analysis procedures were 

the same as those used by Wang Ding (1993) . All recordings were analyzed with 

SIGNALT"' software and the analysis frequency range vas 0-20 kHz. All signals 

142 
with suitable signal-to-noise ratios were analyzed. The sonograms had a 

frequency resolution of 98 Hz when the analysis bandwidth was 20 kHz. Ten 

variables were measured for whistles from the four species : beginning 

frequency, end frequency, minimum frequency, maximum frequency, 

duration, number of inflection points (a change in the slope of the frequency 

contour from negative to positive or vice versa), beginning frequency sweep 

and end frequency sweep (up or down), harmonics, and contour break. 

Multivariate discriminant analysis was used to compare overall whistle 

structures to determine if any significant variations existed between the 

species being studied. The SAS-PROC CANDISCT"' program was used to compute 

Mahalanobis distance-squared statistic (D'), F-statistic, and canonical variable 

values. The Mahalanobis D-' statistic is a relatively simple size-independent 

measure of the differences between overall whistle structure as determined by 

the distance between species mean vectors in multivariate space; the greater 

the D2 statistic the greater the differences between species. 

Canonical variables, which are computed from the linear combination of 

quantitative variables entered into the discriminant function, are a 

multivariate measure of the differences in overall whistle structures between 

different species. Each canonical variable is a linear combination of a subset 

of the acoustic variables, with each canonical variable minimizing the 

variance within its particular subset of variables . When the first two 

canonical variables of each species are plotted on an X-Y coordinate plot, the 

relative distance between the positions of each species is proportional to the 

relative differences between their whistles. 

For the purposes of describing distributions and determining number of 

acoustic contacts, species identity was determined by two means. With all 

species except sperm whales, acoustic contact identity was made when an 

animal was concurrently recorded with an identified visual contact. In those 

cases where recordings were made from schools containing more than a 

single species, the contacts were listed as unidentified dolphins . For sperm 

whales, identification was made as described above using acoustic cues such as 

bandwidth, duration, and the rhythmic nature of their pulses . 

4.2.2.2 Abundance and Density Estimation Techniques 

Detection Distance 

Standard line transect methods for estimating abundance require the 

measurement of perpendicular distances (or angles and radial distances) from 

the track-line to the position of the target animal . Using the array 

configuration described, perpendicular distances to vocalizing sperm whales 

could not be determined with sufficient accuracy. Consequently, an 

alternative method, a detection function modified strip transect method, was 

developed to estimate the sperm whale population density in the study area. 

To use this method, it vas necessary to determine the distance from which 

sperm whales could be detected. The hydrophone array was calibrated by 

projecting three signals (sperm whale pulses, rough-toothed dolphin whistles, 

and a S kHz pure tone) from measured distances perpendicular to the array. 

Distances between the array and the signal projector were determined using 

143 
two GPS receivers ; one at the projector and one aboard the Pelican. The 

projector was a calibrated FS6 transducer leased from the Naval Research 

Laboratory. The source level of the projected signal was measured using a 

Bruel and Kjaer (B&K) SPL meter. The source level of sperm whale pulses has 

been estimated as being from 165-185 dB re. 1 gP at 1 m (Watkins 1977, Watkins 

and Schevill 1977, and Watkins et al . 1988) . Corcella and Green (1968) reported 

a source level of 136 dB re 1mP at 1 m. Dunn (1969) reported a mean source 

level of 173.9 dB re 1mP at 1 m (n = 148, SD = 3.51 dB). Levenson (1974) reported 

a mean source level of 171 .2 dB re 1p.P at 1 m (n = 13, SD = 2.9 dB, range = 165.5-

175.3 dB). We chose to broadcast the pulses at the source level reported by 

Levenson (1974). The projected signals were then recorded at the array from a 

known distance. Subsequently, 63 random samples of background noise from 

Cruises 2-7 were measured to adjust the transmission loss model to the average 

noise level during the cruises . It was determined that a sperm whale signal of 

at least 67 dB re 1 uP must be present at the array in order for the signal to be 

detected. A transmission loss (TL) model was used with the form: 

TL = 20 log r + ar 

where r = source range, and a = an attenuation coefficient. Based on the 

transmission loss model, the average maximum detection range for sperm 

whales producing 171 dB signals was 11 .1 km. 

Detection Threshold 

The detection function describes how many sperm whale contacts should be 

detected as a function of distance from the vessel . Earlier it was established 

that signals up to 11 .1 km away from the vessel were detectable. Signals may, 

however, go undetected for reasons associated with the psychoacoustics of 

listening. This is analogous to the problem of determining the visual detection 

function for standard line transect. A proportion of otherwise detectable 

signals are lost due to human factors concerning attentiveness to low 

amplitude signals mixed with background noise . The detection function 

determines the proportion of missed signals so that the density calculations 

can be adjusted. 

An audio recording was made to simulate the experience of listening to the 

hydrophone array at sea. Forty-three minutes of background noise, without 

detectable sperm whale signals, were transferred from a recording made 

during a GulfCet cruise . The sound pressure level of this background noise was 

equilibrated to the average sound pressure level based on the mean dB level 

from 63 segments taken from Cruises 2-7. Thirty-four sequences of sperm 

whale clicks were then digitally mined into this background noise at random 

intervals . The sperm whale click amplitude levels were adjusted to simulate a 

random distribution of animals around a simulated transect line. A data 

acquisition program was written to record the elapsed time, allowing us to 

grade the ability of each listener to detect a signal. Based on their 

performance, a detection threshold representing the probability density 

function for detecting signals vas generated. 

Each listener was trained using two tapes. The first tape contained a series of 

sperm whale pulses that familiarized the listeners with these signals. The 

144 
second tape familiarized them with the test paradigm (Atkinson 1988). Twentyfour participants listened to the 43-minute test tape. From their responses 

(Figure 4.5), a model for the psychoacoustic sperm whale detection function 

was developed. The data was standardized so that there were no responses at 

11 .1 km. Participants had little trouble detecting clicks in marginal or higher 

signal-to-noise ratios . A rapid drop off in the ability to detect clicks occurred 

only after the signal-to-noise ratio became very low. This suggested that very 

few actual sperm whale contacts were missed, even out to the maximum 

detection range of 11 .1 lm. Based on this model, we predicted that only 3.8g'o of 

the sperm whale clicks within 11 .1 km of the array were missed and that the 

unadjusted density estimate should be multiplied by a correction factor of 

1 .038. 

ale C*roun Size 

Central to the accurate estimation of sperm whale numbers is the size of the 

enumerated population unit. The results of the density estimates are given in 

number of groups/km2 . A problem arises when translating this density 

number to the number of animals in the study area: How many animals are in 

a group? This question arises because sperm whales have a hierarchical social 

system containing varying numbers of whales over space and time 

(Whitehead and Kahn 1992). What was the detected population unit - a group or 

a cluster of sperm whales? It is likely that the various survey teams (shipboard 

visual and acoustic, and aerial visual) counted different social units . Clearly, 

for the results to be comparable, each census team must use the appropriate 

group definition and number of animals per group. 

Based on association patterns between repeatedly identified animals within 

mixed age/sex groups in the Galapagos, Whitehead and coworkers described a 

hierarchical system for sperm whale social organization as including units, 

groups, and aggregations (Whitehead 1987, Whitehead and Arnbom 1987, 

Whitehead and Waters 1990, Whitehead et al. 1991, Whitehead et al. 1992a, 

Whitehead et al. 1992b, Whitehead and Kahn 1992). They found that each 

individual was a member of a "unit" consisting of approximately 13 whales. 

Individuals within units associated with one another for periods of several 

years. The associations may be permanent, lasting the entire lifetime of some 

animals. An average of about two of these units were together at any time, 

forming groups of roughly 20 individuals as noted in the earlier studies 

(Whitehead and Kahn 1992) . These associations integrated their behavior for a 

period of several days, splitting on average after 6.5 days. Aggregations were 

temporary associations of multiple groups, often consisting of two groups, 

although aggregations could also consist of a single group. 

The term "cluster" was used by Whitehead (1989) in describing an association 

of foraging sperm whales. His analysis did not include association patterns of 

identified animals. Foraging sperm whales, while at the surface, were seen in 

clusters, with one to four individuals coordinating their movements within 

100 m of each other. Clusters contained a mean of 1 .7 whales (Whitehead 1989) . 

Multiple clusters occurred in the same area, with a mean distance of 200 m 

145 
1 .2 

1 

C 0.8 

N 

O 

0.6 

O 

a 0.4 

0.2 

0 

. . . . . .. . . . . . . . . . . . . . . . . . . . . ... . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .f .. . . . . . . . . . . . . . . . . . . . . . . . .-rs~E .a . . .~r~ . .s . . . . .+~~,~ s s s 

s t S s 

l 

i 

Z 

i 

i 

i 

s 

1 

s 2 

i 

i 

0 2 4 6 8 10 12 

Distance in kilometers 

Figure 4.5 . The detection function for sperm whales using the acoustic array. 

146 
between clusters (Whitehead 1989). If a cluster contained on average 1 .7 

animals and groups contained roughly 20 animals, then the various members 

of a group may surface in 10-12 clusters over a period of 30-90 minutes. This 

organizational structure is complicated by the fact that, when socializing, 

entire groups may come together at the surface such that the entire group of 

10-20 animals may be visible at once. 

Geographic scale is a criterion in determining whether the observed animals 

are in a cluster, group, or aggregation. If animals are sighted together in a 

small area, then it is likely they are a cluster. If they are spread over a larger 

area, on the order of kilometers, they are probably a group. For example, a 

sighting of two to four animals in one location followed by another sighting of 

two to four animals within a few kilometers is likely to be one group of four to 

eight animals consisting of two clusters . 

The size of sperm whale social structures is relatively uniform throughout the 

world. Mean group sizes were 22 in the Galapagos (Whitehead 1989), 20 in Sri 

Lanka (Gordon 1987), and 25 in New Zealand (Gordon et al. 1992). Similarly, 

mean cluster sizes were consistent worldwide: 1 .7 in Galapagos (Whitehead 

1989), 2.1-3.5 in the Gulf of Mexico (Mullin et al. 1994, Collum and Fritts 1985), 

and 3 .1 in Sri Lanka (Gordon 1987). Mullin et al . (1994) noted that the small 

group size, 2.1, reported for the Gulf of Mexico may correspond to the clusters 

given by Whitehead and Arnbom (1987). All-male clusters off Nova Scotia 

averaged 1 .1 animals; these clusters rarely formed groups unless joining 

mixed age/sex groups. Whitehead (1989) found that mixed groups in the 

Galapagos Islands consisted of one to four clusters with an average of 3 .4 

animals per cluster and a mean inter-cluster distance of 213 m. 

The methodology of visual censusing requires that all sightings be 

independent events (Buckland et al. 1993) . In practice, if several animals 

surface at some distance to port, then five minutes later animals surface to 

starboard, they are counted as separate sightings. Using the nomenclature 

described above, what vas counted during visual surveys were almost 

certainly clusters. The geographic pattern of sperm whale acoustic contacts 

was analyzed, comparing visual and acoustic results to determine if acoustic 

surveys enumerated clusters or groups. 

Dolphin Group Sire 

The group size used in the dolphin abundance calculations was a weighted 

mean taken from the visual shipboard data. The weighting was calculated by 

taking the sum of the individual densities for each of the 14 species (see 

Chapter 3, Table 3.6) and dividing that value by the sum of the group densities 

for those same species. This takes into account the number of contacts for each 

species, such that those species that were seen more often were weighted more 

heavily than less frequently observed species. 

Abundance Estimates 

The estimated mean contact density (15) was calculated, using a detection 

function-modified strip transect method as follows: 

147 
k 

-1 D = f (d"X I L k 

L=1 `4 L 

where D = corrected mean contact density for transect lines 1 to k, 

f(d.) = correction factor resulting from detection function, 

k = number of transect lines, 

nL = number of contacts on transect line L, and 

AL = area of census effort on transect line L 

The estimated abundance, N, in the study area was calculated as follows: 

N=ASD 

where N = estimated sperm whale or dolphin abundance, 

A =total census area, 

S = sperm whale or dolphin group size, and 

D = mean density. 

The log-normal 95% confidence intervals for population estimates were 

calculated using the following equations: 

Lower 95% CI = 

ex pC 1 .96 ~ (ln(l + (CVN)2)) 1 

J 

Upper 95% CI = N - exp [V

1 .96 - (ln(l + (CVI~2)) 

where CVN = coefficient of variation of N. 

Statistical Methods for Cetacean Distribution Analysis 

A contour index (CI) was calculated to determine the effect of sea-floor 

topography on sperm whale distribution . The study area was divided into 

roughly equal blocks approximately 160 NM'- centered around CTD positions 

(see Chapter 6) . A CI reflects the percent change in depth of a given block and 

was calculated for each block using the following equation: 

CI = MM m ~ 100 

where M represents the maximum depth and m represents the minimum depth 

of a given block; however, m was restricted to depths >_ 100 m because blocks in 

the northern study area extended into very shallow areas (e.g., block 125 

encompasses part of the Mississippi River delta) . Following the conventions set 

forth in Mullin et al. (1991), each block was subsequently assigned to one of 

four CI categories : 20-39, 40-59, 60-79, or 80-99 . The null hypothesis tested was 

that sperm whale acoustic contacts were randomly distributed across all CI 

categories. The test statistic was a Chi-square comparing the expected sperm 

148 
whale contacts to the observed sperm whale contacts based on the level of 

effort (in km) for each block. The expected number of sperm whale contacts 

(Ei) was determined using the following equation: 

Ei = Ot (Li / Lt) 

where Ot represents the total number of sperm whale contacts, L.1 represents 

the kilometers of effort in CI-categoryi, and Lt represents the total kilometers 

of effort . 

Sperm whale and dolphin contacts by cruise and season were examined, as 

were sperm whale contacts by contact duration and water depths. The null 

hypothesis was that the observed distributions or values were randomly 

distributed relative to the expected value based on observer effort. Chi-square 

analyses were used to compare observed versus expected values. As stated 

above, the expected value was derived from effort (during the parameter of 

interest) divided by the total effort, with that ratio then multiplied by the total 

for all cruises for that parameter. In all cases, a < 0.05 or 0.1 as indicated. 

A Kruskall-Wallis test was used to compare lengths of sperm whale contacts by 

cruise and season, the water depths for sperm whale contacts, and both sperm 

whale and dolphin densities by season and region. Where statistical 

significance was indicated with the Kruskall-Wallis test, multiple pairedcomparisons were conducted using a Mann-Whitney U test. 

The above analyses describe the distribution and abundance of cetaceans in 

the study area using data similar to that available to the other survey 

techniques (i .e ., the number of contacts and their modal, or initial location). 

The acoustic data, however, permitted further analyses because acoustic 

contacts had a greater temporal and spatial extent than the visual methods. 

This was largely due to cetaceans being vocal animals that spend most of their 

time below the surface. Acoustic contact with the animals was longer, which 

permitted description of differences in the duration and length of contacts 

relative to seasonal and daily patterns. Time series analyses were used to 

determine if there were temporal patterns to the starting time of sperm whale 

contacts and to effort . Spectral analyses identified the correlation of different 

frequencies with the observed data. They indicated, for example, whether 

there was a cyclical pattern to when sperm whale contacts began. An 

autocorrelation analysis of the data was also conducted. This analysis showed 

whether a particular time lag was present in the data, such that, given that 

time lag the data were highly correlated. This tool was used to determine 

whether the pattern of sperm whale contacts correlated at a particular time 

lag (i.e., whether the pattern of contact initiation was similar at sunrise and 

sunset, indicating that the animals had a repetitive, crepuscular behavior 

pattern). 

149 
4.3 Results 

4.3.1 Acoustic Effort 

A total of 12,219 km and 1,055 hours of acoustic effort was completed. On-effort 

acoustic sampling occurred 95% of the available time for Pelican Cruises 2-7 

(Figures 4.6-4.12) . Gaps in effort during Cruises 3 and 7 occurred when the 

ship returned to port because of poor weather or illness. There were two 

cruises for each season, except winter, which had one (see Chapter 1, Table 

1 .3) . The four spring and summer cruises had approximately the same effort, 

whereas the two fall cruises were shorter. 

The mean hourly duration of effort was 42.08 hr (SD = 5.27, range 32.2-51 .7 hr, 

n = 24) (e.g., between 0000-0059 hr there were 45 .1 hours of effort over the 

seven cruises). The duration of effort by time of day was relatively constant 

(Figure 4.13) . A time series analysis of this data suggested that the only 

periodicity was a 24-hour cycle, about the time it took to transit one track-line 

and cross to the next in the western half of the study area. 

4.3.2 Species Recorded 

A total of 487 acoustic contacts were recorded (Table 4.1). Of that number, 124 

contacts were from 12 identified species (see Appendix A for the location and 

time of all acoustic contacts) . The sperm whale was the most commonly 

recorded species, accounting for 560 of identified contacts. The most 

commonly recorded small cetacean was the pantropical spotted dolphin, with 

22 contacts. A single recording of an unidentified baleen whale was made, 

probably a sei or Bryde's whale based on its spectral configuration. An 

additional 331 contacts were of unidentified dolphins. These were signals 

recorded during sightings of unidentified dolphins, or at times when there 

was no visual effort, such as during poor weather and at night. There were 30 

contacts with unidentified cetaceans. These were typically pulsed signals that 

did not sound like sperm whales or dolphins, and were possibly either 

dwarf/pygmy sperm whales or beaked whales . Also recorded were 19 

unidentified biological contacts, probably shrimp. Approximately half of the 

species expected to occur in the Gulf as determined by Jefferson et al. (1992) 

were recorded, including the rarely recorded clymene and rough-toothed 

dolphins as well as the first recording ever of Fraser's dolphin. 

Noteworthy in their absence were many baleen whale signals. The expectation 

had been to record at least minke, Bryde's, and humpback whales. No identified 

recordings were made of beaked whales, though they were seen on several 

occasions. Only a single identified recording was made from either pygmy or 

dwarf sperm whales. Of the expected dolphin species, only killer and pygmy 

killer whales were not recorded. 

150 
-95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

100-

1~ 

i 

12000 

-96 -95 -94 -93 -92 -91 -90 

,.. 

3 

2 

2 

2 

9 -88 -87 

9 

8 

7 

6 

S 

Figure 4.6. Distribution of sperm whale contacts on R/V Longhorn Cruise 1 

(first spring cruise) . Horizontal lines indicate the location of a 

single signal. 

-96 -95 -94 --93 -92 -91 -90 -89 -88 -87 

30 

29 - 9 

28 8 

27 - 

26 '6 

S 25 

Figure 4.7. Distribution of sperm whale contacts on R/V Pelican Cruise S 

(second spring cruise) . Horizontal lines indicate the location of a 

single signal . Effort is shown as the solid line along each trackline. 

151 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

!9 

.g 

!6 

S 

Figure 4.8. Distribution of sperm whale contacts on R/V Pelican Cruise 2 

(first summer cruise) . Horizontal lines indicate the location of a 

single signal. Effort is shown as the solid line along each trackline. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

~ 

A 30 

29 

28 

27 

26 

25 

!9 

rg 

!7 

!6 

!5 

Figure 4.9 . Distribution of sperm whale contacts on R/V Pelican Cruise 6 

(second summer cruise) . Horizontal lines indicate the location of 

a single signal . Effort is shown as the solid line along each 

track-line . 

152 
30 

29 

28 

29 

28 

27 27 

26 26 

25 

j 

25 

7 

Figure 4.10 . Distribution of sperm whale contacts on R/V Pelican Cruise 3 

(first fall cruise) . Horizontal lines indicate the location of a 

single signal. Effort is shown as the solid line along each trackline. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 v 30 

29 . t ^~ 29 

28 

27 -

26 ~ 

28 

27 

26 

25 ~- r -~ 2s 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 4.11 . Distribution of sperm whale contacts on R/V Pelican Cruise 7 

(second fall cruise) . Horizontal lines indicate the location of a 

single signal . Effort is shown as the solid line along each trackline . 

153 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 
7 

30 

29 

28 

27 

26 

25 

30 

29 

28 

27 

26 

25 

Figure 4.12 . Distribution of sperm whale contacts on R/V Pelican Cruise 4 (winter cruise) . Horizontal lines 

indicate the location of a single signal . Effort is shown as the solid line along each track-line . 
60 - 

50 

40 

w 

W 

0 30 

0 

x 

Zo 

i o ~~ 

0 2 4 6 8 10 12 14 16 18 20 22 

Hours of the Day 

Figure 4.13 . Hours of effort by time of day for Pelican/ Longhorn Cruises 1-7 . 
Table 4.1 . Number of on-effort acoustic contacts by cruise and 

species. 

Pelican/Longhorn Cruise 

Number 

Species 1 2 3 4 S 6 7 Total 

Unidentified cetacean 4 7 7 3 4 2 3 30 

Unidentified baleen whale 1 1 

Sperm whale 4 7 8 10 14 17 7 67 

Dwarf sperm whale 1 1 

Melon-headed whale 1 1 

False killer whale 2 1 3 

Short-finned pilot whale 1 2 3 

Rough-toothed dolphin 1 1 2 

Fraser's dolphin 1 1 

Bottlenose dolphin 7 3 2 0 1 2 0 15 

Risso's dolphin 2 1 3 

Atlantic spotted dolphin 1 1 

Pantropical dolphin 3 1 G 7 1 2 2 22 

Striped dolphin 1 1 2 

Clymene dolphin 1 3 4 

Unidentified dolphin 30 GS 26 60 64 47 39 331 

Total Contacts 49 85 50 80 95 75 53 487 

4.3.3 Distribution of Cetaceans by Species 

4.3 .3 .1 Sperm Whales 

Analysis By Cruise 

Sperm whales were encountered on all cruises and along all track-lines. The 

number of contacts ranged from five on Cruise 1 to 18 on Cruise G (Table 4.2). 

These data were analyzed to determine whether certain cruises had more 

contacts than expected by correcting for effort and conducting a Chi-squared 

test. The observed number of contacts by cruise was significantly different 

from expected (X'- = 16.1897, p = 0.01297, df = 6). There were fewer observed 

contacts during the early cruises and more than expected during the later 

cruises. These differences are examined in greater detail below. 

Sperm whales were detected throughout the study area (Figures 4.6 to 4.12) . 

There were, however, certain areas where sperm whales were encountered 

each time we surveyed that location, in particular, along track-lines 2, 6, 11, 

and 12 . For example, we encountered sperm whales six times within an average 

of 7.6 km (SD = 8.52) of a point 38 km south of the north end of track-line 12 . 

This point is 57 km SE of the Southwest Pass of the mouth of the Mississippi 

River. 

156 
Table 4.2 . Analysis of sperm whale contacts by cruise . The 

number of contacts includes two off-effort contacts. 

Pelican/ 

Longhorn Number Total length 

Cruise of Contacts of Contacts (lm) Effort (km) 

1 5 68.34 2105.6 

2 7 90.56 1867.8 

3 8 98.32 1727.5 

4 10 59.97 1888.1 

5 14 77.78 1733.5 

6 18 311 .88 1603.6 

7 7 130.71 1292.6 

Sperm whales not only were found repetitively at particular locations, but 

groups were also spread over differing amounts of area. The average length of 

a sperm whale contact was 12 .13 km (SD = 13 .26). For the first five cruises, 

there were few very long contacts. During the last two cruises, however, there 

were several long contacts, with the longest lasting 74.15 km. A comparison of 

the lengths of sperm whale contacts for all cruises indicated that they were 

significantly different (Kruskal-Wallis, H = 12 .669, p = 0.0486, n = 69) . A 

multiple paired comparison by cruises indicated that Cruises 4 and S were each 

statistically different from Cruises 1, 6, and 7 (p < 0.055) (Table 4.3) . In each 

case, contact lengths were less for Cruises 4 and 5 than their ordered pairs. 

There was a highly significant difference between the observed versus 

expected length of contacts (X'- = 496.915, p < 0.01, df = 6) (Figure 4.14) . This 

resulted from the long contacts during Cruise 6, when the observed contact 

length was 2.84 times the expected. Additionally, contact length was much less 

than expected on Cruises 4 and S . Since the distance censused on each cruise 

was not normally distributed, parametric regression methods were not 

applicable . A Kendall rank correlation between length of contact and level of 

effort resulted in a negative, though low correlation (r = - 0.7143, p = 0.0243), 

suggesting that only half of the variability in contact length was explained by 

level of effort. 

In May 1992, the R/V Gyre sailed south along Pelican/Longhorn track-line 4 

and then further south into Mexican waters in the western and west-central 

Gulf of Mexico. Marine mammals were surveyed using the same acoustic and 

visual methods as used on Pelican/Longhorn cruises. Of the six cetacean visual 

and acoustic contacts, no sperm whales were detected on that cruise . 

157 
Table 4.3 . Matrix of pair-wise comparisons of the lengths of all sperm whale contacts by cruise. 

The p-value is the two-tailed probability of equaling or exceeding the Mann Whitney 

U value for each comparison. * indicates significance at p < 0.055. 

Cruise # Season Cruise 1 Cruise 2 Cruise 3 Cruise 4 Cruise s Cruise 6 Cruise ? 

Cruise 1 spring - p = 0.167 p = 0.464 p = 0.0373* p = 0.012* p = 0.881 p = 0.570 

_ Cruise 2 summer - - p = 0.908 p = 0.558 

p 

0.371 

p 

0.304 

Cruise 3 fall - - - p ~~Z77 

p 

0.424 

p 

= 0.339 

p 

= 0.405 

p 

0.298 

Cruise 4 winter - - - - 

p 

- 0.815 0.055* 

Cruise s spring - - - - - p = 0.008* p -- 0.025* 

Cruise G summer - - - - - - p = 0.762 

Cruise 7 fall - - - - - - 
350 

300 

250 - 

U 

v 200 

4-1 0 

150 

0 

100 

50 -I 

. Observed Km of Contacts ~ Expected Km of Contacts I 

1 2 3 4 5 6 7 

Cruise Number 

Figure 4.14. Kilometers of sperm whale contacts for Pelican/ Longhorn Cruises 1-7 . 
vsis By Season 

There was no significant seasonal difference between observed and expected 

number of contacts (X2 = 2 .106, p = 0.507, df = 3) (Table 4.4). There was no 

apparent pattern to seasonal changes in sperm whale distribution. Similarly, 

there was also no apparent change in distribution between cruises during the 

same season. On many occasions, however, there were contacts on the same 

track-line during separate cruises. Occasionally, contacts were in virtually the 

same location during separate cruises : 

" Spring cruises: two sets of contacts near the same location on 

track-lines 12 and 6. 

" Summer cruises: three sets of contacts near the same location 

on track-lines 12, 6, and 5 . 

" Fall cruises: a single set of contacts near the same location on 

track-line 11 . 

" Summer and spring cruises combined: two sets of contacts near 

the same location on track-lines 12 and 6. 

Table 4.4. Number and length of sperm whale contacts by seasons. 

Season Number of contacts Length of contacts (km) Effort (km) 

Summer 25 402.44 3471.4 

Fall 15 229.04 3020.1 

Winter 10 59.97 1888.1 

Spring 19 140.12 3839.1 

There was a significant seasonal difference in the lengths of sperm whales 

contacts. A comparison of the observed versus expected lengths of contacts 

indicated there was a highly significant difference (X'- = 204.303, p < 0.0001, 

df = 3) . A Kruskal-Wallis test indicated a significant seasonal difference 

between the length of sperm whale contacts (H = 9.332, p = 0.0252, n = 69) . 

Table 4.5 presents a multiple paired comparison between seasons. Conclusions 

from statistical tests were: 

" Spring cruises: The contact lengths for the spring cruises were 

shorter than expected and significantly different from the 

summer cruises. The Spring contact lengths were less than the 

contact lengths noted on the summer cruises. 

160 
Table 4.5. Matrix of pair-wise comparisons of the lengths of all 

sperm whale contacts by season . The p-value is the 

two-tailed probability of equaling or exceeding the 

Mann-Whitney U value for each comparison . 

* indicates significance at a = 0.1 . 

Season Summer Fall Winter Spring 

Summer - p = 0.944 p = 0.093* p = 0.093* 

Fall - - p = 0.086* p = 0.140 

Winter - - - p < 0.0001* 

Spring - - - - 

" Summer cruises : The contacts lengths were longer than 

expected, and were significantly different from both winter 

and fall cruises. 

" Fall cruises : The contact lengths were greater than during 

winter. 

" Winter cruise : The contact lengths were shorter than expected 

and significantly different from both the summer and fall 

cruises. Winter contact lengths were less than either summer 

or fall . 

These differences indicated that within groups, animals were more widely 

dispersed in the summer than during other seasons . During the winter and 

spring, groups were concentrated into smaller areas. During fall, contacts 

were randomly distributed. 

There were no significant differences in the number of contacts by time of 

day (in four-hour intervals), or by track-line (p > 0.5, Kruskal-Wallis) . In 

addition, we found no significant differences between observed and expected 

duration of contacts analyzed at hourly intervals, corrected for effort 

(Xz = 7.46, p = 0.589) . The general pattern was for duration of contacts to 

increase through the day, with peaks at 1000, 1600, 1900, and 2100 hours 

(Figure 4.15). 

Contact duration was examined by using spectral analyses to determine 

whether there was a cycle to the daily pattern of hours of sperm whale 

contacts. The spectral analysis suggested a pattern to the contacts at periods of 

three and six hours. For example, there were long duration contacts at six hour 

161 
0.14 

0.12 

0 .1 

w 

0.08 

U 

N 

U 

0 0 .06 

0 x 

0.04 -- -- - -- - 

0.02 --

- -- -- 

2 I { 4 6 I 8 I 10 12 ~ ~ 14 I ~ 16 I I 18 I I 20 I f 22 I 

Hours of the Day 

Figure 4.15 . The daily pattern of hours of sperm whale contacts per hours of effort . 
intervals beginning two hours before sunrise (at 0400 hr) and 0-3 hours after 

sunset, depending on the time of the year. These peaks in contact duration 

were followed within three hours by periods of shorter contact duration. 

The analysis above pooled all contacts by time of day. A single long contact was 

spread over the hours in which it occurred. This blurred the difference 

between a single, long duration contact and multiple, short duration contacts. 

The pattern of long and short duration contacts became more evident when 

analyzed by their start time. In this data set, the number of minutes for each 

contact was allotted to the time in which it began . For example, for a 325 

minute contact beginning at 0800 hours, all 325 minutes were allotted to 0800 

hr. 

The diel pattern of contact duration by start time was highly variable relative 

to sunrise (Figure 4.16) . Sunrise was used as the coordinating point because of 

light's overriding influence on the vertical migration of organisms that may 

be important to sperm whales. Two families of squid, both found in sperm 

whale stomachs, migrate to the surface at night after spending the day at 

approximately 600 m (Clarke et al. 1993, Martin and Clarke 1986). No contacts 

started at sunrise, sunrise + 4 hr. (1100 hr), or sunrise + 7 hr (1400 hr). On the 

other hand, there were more than 500 minutes of contacts which started at 

sunrise + 1 hr (0800 hr) and sunrise + 12 hr (1900 hr) . When effort was 

factored in, the pattern remains variable (i .e ., high effort with no contact 

initiation at sunrise and low effort but many contacts at sunrise + 1 hr) . 

A spectral analysis of the contact duration pattern was conducted. The goal of 

this analysis was to locate diel patterns in the time series of contacts. The 

spectral analysis suggested several important patterns. There was a major peak 

at approximately three hours but also a secondary peak at 12 hours. The threehour pattern may be the result of the short-term behavior of sperm whales, in 

which animals have a cyclic pattern of deep dives during which they vocalize 

followed by silent periods at the surface, a pattern repeated every three hours. 

The longer pattern appears related to the pattern beginning at sunrise + 2 

hours and sunrise + 14 hours. Note that this is not the timing of the contact 

duration maxima at sunrise and sunset, when the longest duration contacts 

occurred. 

Figure 4.17 presents an autocorrelation of sperm whale contact duration by 

time of day. It shows that the highest positive correlation was at an 11 hour 

time lag. This corresponded to the pattern of contact maxima at sunrise + 1 hr 

(0800 hours) and sunrise + 12 hours (1900 hours) (e.g ., sunrise and sunset). 

Overall, there was a strong diel pattern to sperm whale contacts. In particular, 

there was a high correlation to their sunrise and sunset behavior as well as an 

overriding three hour cycle to their behavior. The crepuscular behavior 

pattern suggested that their behavior was associated with foraging, while the 

three-hour cycle may be related to the pattern of long dives. 

163 
goo 

600 

500 

U 

O 

V 

0 400 

a 

300 

r~l 

`V 

200 

100 

0 - 

Figure 4.16. Duration of contacts by hour of the day during which they started relative to sunrise. 

- / -b -J -4 -3 -L - i U 1 L .i 4 J 0 / 8 9 1 U 1 1 12 13 14 15 16 

Hours of the Day Relative to Sunrise 
Lag Corr 

1 - .069 

2 - .111 

3 + .158 

4 + .083 

5 - .155 

6 - .195 

7 + .052 

8 + .002 

9 - .063 

10 - .012 

11 + .296 

12 - .176 

13 - .068 

14 + .015 

15 - .169 

r 

1 

I 

-0.5 0 0.5 

Correlation Coefficients 

Figure 4.17. Autocorrelation of duration of sperm whale contacts by time of 

day. The autocorrelation is the correlation of the contact 

duration time series with itself, shifted successively by 1 hr 

increments out to 15 hr lags. The correlation coefficient and 

standard error for each time lag is also provided. 

S . E . 

.1920 

.1878 

.1834 

.1790 

.1745 

.1698 

.1651 

.1601 

.1550 

.1498 

.1443 

.1387 

.1328 

.1266 

.1201 

1 

165 
ative to Bottom Denth 

The GulfCet study area varied in depth from 100-2,000 m. Determining the 

average water depth of sperm whale contacts required defining the area in 

which each contact occurred. This was defined as a rectangle 22 km wide by 

the length of the contact. The depth of that rectangle was randomly sampled 

100 times to compute an average depth. 

The mean water depth for all contacts was 1,244 m (SD = 413, range: 407-2,011 

m) . Depth categories were selected to minimize the number of times a contact 

crossed from one depth category to the next, in order to have sufficient data to 

permit the chi-squared analysis. The resulting depth categories were 300-710 

m, 711-1,190 m, 1,191-1,800 m, and > 1,801 m. The water depth at first contact 

was then compared among these depth categories. There was a significant 

difference between observed and expected water depths, corrected for effort 

(X2 = 10.243, p = 0.017) (Figure 4.18) . In particular, there were many more 

observed contacts in the 711-1,190 m depth category than expected (24 versus 

16.3) . These data suggest that the sperm whales observed in the study area 

preferred intermediate water depths. 

An analysis of sperm whale contacts relative to the contour index showed that 

there was no significant difference between observed versus expected values 

(X2= 3.42, p = 0.33, df = 3) . Upon further examination of the eastern third of the 

study area which encompasses the Mississippi River Canyon (track-lines 11-

14), there was still no significant difference in the number of sperm contacts 

(n = 24) across CI categories (X' = 2.0, p = 0.53) . 

The contact water depths were not significantly different between seasons 

(Kruskal-Wallis, H = 2 .406, p = 0.925, n = 69) . There was no significant 

correlation between water depth and length of contacts (r = - 0.1244, 

rz = 0.0155, p = 0.3084) . Less than 2% of the variance in contact length was 

explained by variance in contact water depth. This showed that there was little 

relationship between the length of a contact and the water depth in which it 

occurred. In summary: 

" There was no significant difference between water depth at 

beginning and end of contact. 

" The mean water depth for acoustic contacts was 1,244 m. 

4.3 .3 .2 Distribution of Dolphins and Other Cetaceans 

Dolphins and other small cetaceans were detected on all cruises and on all 

track-lines (Table 4.1) . Combining all dolphin contacts together, including 

unidentified dolphins, there vas a significant difference between observed 

versus expected numbers of contacts by cruise (X'- = 44.25, p < 0.0001, df = 6) 

(Figure 4.19) . The significant difference was due to the greater number of 

observed versus expected contacts on Pelican Cruise S and fewer observed than 

expected on Longhorn Cruises 1 and Pelican Cruise 3. 

166 
50 

40 I 

30 - 

N 

Cr' 

O 

d 

N 

.G 

N 

o 

20 

10 I 

- Bottom Depth-Sperm Whale Contacts a Bottom Depth Area of Effort 

O { - 300-700 - {- - 711-1190 ~--_ 1191-1800 ~ -~ >1800 

Bottom Depth Categories (m) 

Figure 4.18 . Sperm whale contacts divided into four water depth categories and distribution of water depth in 

the study area. Depths are from the ETOPO-S dataset (Herring 1993) . 
100 

80 

60 

a 0 

U 

4-4 

o 

a 

40 

z 

20 

" Observed Number of Contacts EJ Expected Number of Contacts 

1 2 3 4 5 6 ~ 7 

Cruises 

Figure 4.19. Number of acoustic contacts for all species except sperm whales for Pelican/ Longhorn Cruises 1-7, 

including expected number of contacts based on effort . 
The acoustic contacts were fairly uniformly distributed throughout the study 

area in waters deeper than 200 m, where 95% (365/385) of all dolphin contacts 

were made (Figures 4.20 to 4.26). There was a significant difference (X2 = 6.842, 

p = 0.0771) between the observed and expected number of contacts by season 

(Figure 4.27). This was due to fewer than expected dolphin contacts in the fall 

and more than expected during the summer cruises. 

There was a strong diel pattern to dolphin acoustic contacts. Although acoustic 

effort was split evenly between day and night, 65% of all dolphin contacts 

occurred at night and 350 occurred during the day. This suggested that some 

dolphin species were more active at night. 

Four cetacean species were observed during the R/V Gyre cruise into Mexican 

waters, including the spinner dolphin, rough-toothed dolphin, bottlenose 

dolphin, and the first Gulf of Mexico sighting and tape recording of Fraser's 

dolphin. 

4.3.4 Dolphin Acoustics 

A total of 191 whistle samples were used in the analysis: 89 from bottlenose 

dolphins, 20 from clymene dolphins, 48 from pantropical spotted dolphins, and 

34 from striped dolphins . Analysis of these whistles, based on frequency and 

duration, using D'- canonical correlation tests, significantly separated all four 

species. The relationship, however, between the clymene dolphin and 

pantropical spotted dolphin was closer than for the other two species. Since 

this test is independent of sample size, the results are probably more reliable 

than the F-statistic which was also significant but is sample size dependent. 

All F-values for all pairwise comparisons were significant at p < 0. 0001 level 

except for the comparison between clymene dolphins and striped dolphins, for 

which p < 0.06 . This indicated that the multivariate mean vectors of the 

whistles were statistically different between different species . All 

Mahalanobis D' values were relatively large, and all percent correct 

classification scores (74-89) were relatively higher than the random chance 

level (50) . For the four-way comparison, the F-value was significant at 

p < 0.0001 level and the percent correct classification score (57) was also 

relatively higher than the random chance level of 25 . All frequency and 

duration comparisons between bottlenose dolphins and the three stenellid 

species showed significant differences . None of the three comparisons 

between the three species of stenellids showed great differences. Among 

stenellid species, there was only one frequency parameter for each pair-wise 

comparison that shoved a significant difference. For clymene dolphin versus 

pantropical spotted dolphin, it vas minimum frequency, while for clymene 

dolphin versus striped dolphin, it was maximum frequency. For pantropical 

spotted dolphin versus striped dolphin, it was beginning frequency. There was 

no significant difference between the whistle durations. The spatial plot of the 

first two canonical variables, which were computed from the linear 

combination of quantitative variables entered into the discriminant function, 

is shown in Figure 4.28. This figure shows that the whistles of the bottlenose 

dolphins, which are more coastal in their distribution, were significantly 

169 
30 

29 

28 

27 

26 

25 

Figure 4.20. Longhorn Cruise 1 acoustic contacts, except sperm whales. 

30 

29 

28 

27 

26 

25 

10 

!9 

!g 

r7 

!6 

!5 

t0 

!9 

.g 

!7 

!6 

S 

Figure 4.21 . Pelican Cruise 2 acoustic contacts, except sperm whales. 

170 
30 

29 

28 

27 

26 

25 

30 

29 

28 

27 

26 

25 

7 

30 

29 

28 

27 

26 

25 

7 

30 

>.9 

Zg 

!7 

!6 

!5 

Figure 4.22 . Pelican Cruise 3 acoustic contacts, except sperm whales. 

Figure 4.23 . Pelican Cruise 4 acoustic contacts, except sperm whales. 

171 
30 

29 

28 

27 

26 

25 

10 

!9 

!g 

!6 

r 

!5 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 4.24. Pelican Cruise S acoustic contacts, except sperm whales. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

!9 

!6 

!5 

Figure 4.25 . Pelican Cruise 6 acoustic contacts, except sperm whales. 

172 
30 

29 

i 28 w 

27 

26 

25 

Figure 4.26 . Pelican Cruise 7 acoustic contacts, except sperm whales. 

30 

29 

28 

27 

26 

25 

7 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 
- Observed Number of Contacts a Expected Number of Contacts 

0 -- -- ----~-------{---- -- _ _ _ ~ ._-- ---- -

Summer Fall Winter Spring 

Season 

Figure 4.27. Number of acoustic contacts of all cetaceans, except sperm whales, by season. 

160 

140 -

120 - 

v 100 -

a 0 

U 

0 80 

a 

z 60 -

40 

20 
0-8 -

0.6 -

0.4 -

02 -

z 

!V 

tZ 0.0-

V 

-02 -

-0.4-

-0.6 

0 1 2 

CAN 1 

Stonalla coerulaoalbi 

Stenolls clrmons 

StonN1s atttnusts 

Figure 4.28. Plot of the first two canonical variates as computed from the 

linear combination of the measured whistle variables for four 

species. 

175 
different from those of the more pelagic stenellids . It would be useful to 

compare the signals of the more coastal Atlantic spotted dolphins to both 

bottlenose and the more pelagic stenellid species. 

Although based on small sample sizes, it was apparent that these techniques 

can be used to discriminate pelagic from coastal dolphins. With a larger sample 

size, it may be possible to discriminate between at least three of the stenellids 

as well as bottlenose dolphins, pilot whales, false killer whales, and possibly 

Risso's dolphins. 

4.3 .5 Sperm Whale Group Size Estimates 

An examination of the physical spacing of the GulfCet cetacean visual and 

acoustic contacts illustrated which level of social organization was 

enumerated. In acoustic censuring, the exact location of detected animals is not 

known, although sperm whales can acoustically be detected up to 11 .1 km 

away. Any contact within 11 .1 km of another was, therefore, summed together 

as a single contact. The mean length of acoustic contacts was 12 .1 km (SD = 0.88, 

range 0.474 km, n = 69), with on average 32.5 lm between successive contacts 

on the same track-line (SD = ? .60 n = 22) . Given the length of these contacts, it 

is likely that the acoustic technique enumerated groups, not clusters. 

The visual survey team on the same vessel that towed the hydrophone array 

made 48 sperm whale sightings (both on- and off-effort) . Of those 48 contacts, 

43 were concurrent with acoustic contacts. On seven occasions there were 

multiple visual contacts within the space of a single acoustic contact; that is, 

on seven occasions the latitudinal extent of a single acoustic contact 

encompassed multiple visual contacts. The mean group size for the 48 visual 

sightings was 3 .8 animals/visual contact (Table 4.6). For the seven acoustic 

contacts that had multiple concurrent visual contacts, the mean number of 

animals per visual contact was 3 .9 sperm whales. When the multiple visual 

contacts within the space of the seven acoustic contacts were summed, the 

overall mean was 7.3 animals per acoustic contact (n = 7) . Alternatively stated, 

43 visual sightings averaging 3 .9 animals/contact can be reduced to 23 

contacts averaging 7.3 animals per contact. Based on this logic, the number of 

sperm whales in a group is 7.3 animals. Since clusters contain approximately 

1 .7-3 .4 individuals, sperm whale groups in the northern Gulf of Mexico consist 

of 2-4 clusters, fewer than in other areas of the world. 

An examination of the other GulfCet visual data sets using the same 11 .1 km 

clustering distance used with the acoustic data provides a useful comparison of 

the spatial distribution of sperm whales . An analysis of the Oregon II 

shipboard visual data indicated that on 21 occasions there were multiple visual 

sightings within 11 .1 km of one another. The mean distance between 

successive contacts on the same track-line was 15 .2 km (SD = 1 .56, n = 39, range 

= 0.22-92.7 km). Without using the 11 .1 km pooling distance, the mean contact 

size was 2 .12 animals/contact (Table 4.6) . Using the 11 .1 lm pooling distance, 

the mean group size was 5 .16 animals. In other words, 34 visual contacts with 

an average contact size of 2 .12 were reduced to 14 contacts averaging 5 .16 

animals. 

176 
Table 4.6 . Summary statistics of number of sperm whales per 

visual sighting for Pelican/Longhorn and Oregon II 

visual teams. 

Pelican/Longhorn Oregon II 

All Concurrent Summed over 

visually with acoustic concurrent acoustic All Summed 

sighted contact contact 

Mean 3.81 3.91 7.3 2.12 5.16 

SD 3.18 3.32 5.94 1.18 2.66 

Minimum 1 1 1 1 2 

Maximum 17 17 Z 6 12 

n 48 43 23 34 14 

For the aerial survey data, there was only one occurrence of multiple 

sightings within 11 .1 km of each other, with two whales in each group. Using 

the logic employed for pooling the acoustic contacts, the two contacts would 

have counted as one, with a total of four whales. The mean distance between 

same line aerial sightings of sperm whales was 57.3 km (SD = 10.22, n = S, 

range = 3.4 -105 .3 km). 

It is apparent from this analysis that the acoustic procedures censused a 

different level of sperm whale social organization than the visual survey, 

representing more animals per contact . The acoustic team enumerated groups 

of whales, whereas the visual team enumerated clusters of whales. Therefore, 

in the calculation of sperm whale abundance (below), 7.30 animals was used as 

the sperm whale group size. 

4.3 .5 .1 Sperm Whale Abundance Estimate 

A total of 67 sperm whale acoustic on-effort contacts was made along 85 tracklines. The mean sperm whale contact density was 2.8 x 10-4 contacts/km2 or 44 

groups in the study area. Using 7.3 individuals per group, the overall corrected 

mean sperm whale density was 2.041 individuals/1,000 km2 

(SD = 2 .38, n = 85) . The coefficient of variation was 12 .6%. The log-normal 

upper and lower confidence intervals were 1 .712 and 2.433 individuals/ 1,000 

km2. Within the 154,621 km'- study area, the total estimated population of sperm 

whales is 316 individuals (log-normal 95% CI = 265-377). For perspective, this 

means that one sperm whale group should be detected every 161 km. 

The five eastern-most track-lines had the highest densities (Figure 4.29) . The 

study area can be split into three sections, a western section containing track 

lines 2-6, a central region containing lines 7-10, and an eastern region 

containing lines 11-14. The four regions were significantly different at a =0.1 

177 
J 

00 

.5 -r . 0.4 I- 

~ 0.3 

a 

c 

y 

O 

O 

O 

A 

o, 0.2 

-O~ 

V 

0.1 - 

Z 3' 4 S 6 7 tf y 

Track-line 

Figure 4.29. Sperm whale group density by track line. 

IU 11 Il 13 14 
(H = 4.918, p = 0.0855, n = 85). The western region was significantly different 

from the eastern (Mann-Whitney U test, p = 0.033) . Track line 11 had the 

highest densities and was significantly different from the three legs with the 

lowest densities, lines, 3, 9, and S . There were no significant seasonal density 

differences (H = 2.421, p = 0.4898, n = 85). 

4.3 .5.2 Dolphin Abundance Estimate 

A total of 369 dolphin acoustic on-effort contacts were made along the same 85 

track-lines used to describe the sperm whale abundance. Using an acoustic 

detection range of 2 km and same correction factor derived from the sperm 

whale detection function, the mean dolphin contact density was 8 .08 x 103 

contacts/km2 or 1,298 groups in the entire study area. Using a weighted mean 

of 28.32 animals/group, the overall mean dolphin density was 2.29 x 10-1 

animals/km2 or 229 dolphins/1,000 km2 . Using the same coefficient of 

variation from the sperm whale estimate (12.6%) the log-normal upper and 

lower confidence intervals were 273 and 193 dolphins/1,000 km2 . The total 

estimated dolphin population within the study area was 36,760 animals (lognormal 959'o CI = 30,835-43,821) . This means that, on average, one dolphin 

group was detected every 31 km. 

The track-line with the highest density (12 .9 groups/1,000 km2) was line 14 

(the eastern-most) whereas the track-line with the lowest density 

(3 .2 groups/1,000 km2) was track-line 2 (the most westerly) (Figure 4.30) . 

Beyond these east-west extremes, there did not appear to be other geographic 

patterns . If the study area is divided into eastern, central, and western regions, 

as above, there were no significant regional differences ( H = 2.929, p = 0.2313, 

n = 85). Similarly, there were no seasonal differences (H = 3.369, p = 0.3382, 

n = 85). Groups density by season was summer (9.0 groups/1,000 km2), winter 

(8.5 groups/1,000 km-), spring (8 .35 groups/1,000 km2), and fall 

(6.75 groups/1,000 km2). An examination of the densities by cruise indicated 

wide interannual variations. Longhorn Cruise 1, the first spring cruise, had 

the second lowest density (5 .6 groups/1,000 km2), while Pelican Cruise 5, the 

second spring cruise, had the highest mean density (11 .3 groups/1,000 km2) . 

In most cases, when comparing the two seasonal cruises, the density for one 

cruise was approximately half of the other cruise . Even the annual pattern 

was mixed. For example, with the two spring cruises, the first cruise had the 

lower density, whereas for the two summer cruises, the first cruise had the 

higher density. Overall, there appeared to be little discernible seasonal or 

regional pattern to dolphin densities. 

There were no significant correlations between sperm whale and dolphin 

group densities by either track-line or season. There was a negative 

correlation between sperm whale and dolphin densities for spring, summer, 

and winter indicating that during those seasons high dolphin densities were 

related to low sperm whale densities or vice versa. Only during the fall was 

there a positive correlation. These patterns were made difficult to interpret by 

the lack of significant correlations. On average only 10.4% (SD = 8.10) of the 

variation in sperm whale densities was explained by variation in dolphin 

densities. 

179 
i a - 

12 -

10 

c 

N 

N O 

6 

0 

U 

4 -- 

2- 

-- - - -- -----------

------ -- 

2 3 4 5 6 7 8 9 10 11 12 13 14 

Track-line 

Figure 4.30. Density of dolphin contacts, per 1,000 square km, by track-line. 
4.4 Discussion 

4.4.1 Sperm Whale Distribution and Density Estimates 

Sperm whales were consistently located by the acoustic survey in several 

locations, in particular at specific spots on Pelican/Longhorn track-lines 2, 6, 

11, and 12. These locations were distributed across the study area. This suggests 

the possibility of site fidelity in sperm whales (the same individuals 

consistently located in the same area) . Alternatively, there may be large 

populations in those locales and the animals were simply contacted more often 

there. 

The manner in which sperm whales occupied their habitat, as measured by the 

dimensions of acoustic contacts, varied by season and cruise . Sperm whale 

groups occupied larger areas in the later cruises (Cruises 6 and 7) than the 

first five Pelican/Longhorn cruises. They occupied larger areas in the 

summer than spring . Similarly, contacts during both summer and fall 

occupied greater areas than during winter. In other words, sperm whale 

groups appeared to occupy larger areas in the warm weather seasons than 

during periods with colder weather. 

Estimates of sperm whale density were made from three sources, aerial visual, 

shipboard visual, and shipboard acoustic data (see Chapter 3, Table 3 .21). As 

expected, the aerial estimate was the lowest (1 .01 whales/1,000 km2) because 

sperm whales occur primarily beyond the 1,000 m isobath. The shipboard 

visual and shipboard acoustic density estimates were similar (2.02 and 1 .96, 

respectively) . The estimates were not significantly different using the criteria 

of non-overlap of 95% confidence interval . This indicated that the acoustic 

estimate may be as reliable as the visual estimates, and that there was little, if 

any, bias in the accuracy of the visual estimates due to lack of detection 

because of long-duration diving. However, this conclusion assumes accurate 

determination of sperm whale group size and requires further evaluation 

regarding the compatibility of the visual and acoustic methods. 

The distance between same-track-line sperm whale contacts for the three 

different techniques provides a measure of similarity . The acoustic method 

detected sperm whales when they occurred on the same track-line, on average 

every 12.1 km, which was very similar to the shipboard visual distance of 

15 .2 km. This is compared to 57.3 km for the aerial method. This suggests that 

the two shipboard methods are detecting animals at approximately the same 

rate, whereas the aerial method is detecting animals less often. This, in turn, 

suggests that slower moving ships may be a more appropriate censusing 

platform than planes for species that spend appreciable time underwater, such 

as sperm whales and beaked whales . 

4.4.2 Dolphin Density Estimates 

The acousic survey yielded higher dolph 

and aerial visual surveys. The most 

techniques was of density of groups, sine 

The estimated density of dolphin group 

acoustic survey, 4.39 groups/1,000 

in densities than the shipboard visual 

meaningful comparison among the 

e it avoided calculations of group size. 

s was 8 .08 groups/1,000 km2 for the 

km'- for shipboard visual, and 

181 
4.95 groups/1,000 km2 for aerial visual (see Chapter 3, Table 3 .21). The dolphin 

abundance estimates from the acoustic survey were 1 .98 times those of the 

shipboard visual survey (36,760 and 18,584, respectively) . Based on overlap of 

the 959'o confidence intervals, these estimates were not significantly different. 

The question arises, nevertheless, as to which estimate was the better 

representation of the true condition. In large part because there was a higher 

contact rate for the acoustic survey, it appears likely that the acoustic method 

was encountering more animals than the visual method. Given the similar 

detection ranges, it is also likely that hearing was a more sensitive detector 

than vision, perhaps because it was not so sensitively biased by adverse 

detection conditions . Subjectively, it is easier to detect sounds in a noisy 

environment than it is to visually detect animals when there is appreciable 

waves and wind. These differences, however, should not mask the great 

similarity between the two estimates. Assuming that the distribution of the 

sperm whales and dolphins was linked to their food resources, the lack of 

correlation between dolphin and sperm whale densities suggests that the 

distribution of the two groups were not motivated by the same resources . The 

most common dolphins is the study area are the various stenellid species. These 

species were largely piscivorous, whereas sperm whales are largely squid 

eaters . The data were insufficient to allow correlation sperm whale densities 

with those delphinids that also feed on squid, such as Risso's dolphins and pilot 

whales. 

4.4.3 Summary 

Acoustic survey methods are currently not a replacement for the shipboard 

visual survey methods. They do provide a significant and useful supplement 

and are a valuable and effective monitoring tool for future surveys. With the 

increased concern about the potential impacts of man-made noise on marine 

mammals and especially cetaceans, baseline data on the acoustic environment 

of various ocean areas including the Gulf of Mexico has taken on added 

significance . Studies by Ridgway (1983) and others point also to the use of data 

on vocal patterns of cetaceans as an indicator of individual physiological 

status and possibly the status of populations. Cetaceans along with many other 

vocal species do appear to cease or significantly change their vocal behavior 

in response to stress . 

During the GulfCet program, the acoustic survey has demonstrated: 

" Sperm whales and baleen whales can be detected and 

identified . 

Several species of dolphin can be detected and identified based 

on differences in onset frequency and other signal 

parameters. 

The ability to conduct acoustic surveys independent of poor 

weather and lack of daylight significantly increases the level 

of survey effort (acoustic effort occurred during 95% of the 

time versus 42% for visual concurrent surveys. 

182 
" 659% of dolphin acoustic contacts occurred at night while only 

35% occurred during daylight hours, even though effort 

during daylight and at night was equal. 

" The range of detection of cetaceans is significantly greater for 

acoustic versus visual methods. The acoustic encounter rate 

was 22% higher than the visual encounter rate. 

" Density estimates are comparable to those from the shipboard 

survey. 

" The increased duration of contacts permits a more synoptic 

view of the distribution providing more information on 

temporal and spatial changes in cetacean distribution and 

behavior. 

Current disadvantages of acoustic surveys include: 

" Inability to estimate group size precisely without extrapolation 

from visual sightings. 

" Identification of acoustic contacts. Currently, several species 

can be identified by signal characteristics such as frequency, 

duration, and is labor intensive although much effort is being 

placed on development of automated signal identification. 

This project has demonstrated that visual and acoustic survey methodologies 

complement each other and can be conducted simultaneously from the same 

platform. This provides a comprehensive and efficient survey technique that 

optimizes ship time by increasing percentage of time on-effort. Continued 

development of hydrophone array technology will permit higher towing 

speeds and improved signal-to-noise ratio which will increase detection range 

and contact rate. These array advancements, in conjunction with improved 

signal recognition methods, may eventually permit acoustic censuring to be a 

stand alone technology. 

4.5 Literature Cited 

Atkinson, R.C . 1988. Steven's handbook of experimental psychology, 2nd ed. 

Wiley and Sons, New York. 320 pp. 

Au, W.W.L 1993 . The sonar of dolphins. Springer-Verlag, New York. 277 pp. 

Berzin, A.A. 1971 . The sperm whale. Edited by A.V. Yablokov. Pishchevaya 

Promyshlennost Moscow. English Translation, 1972, Israel Program for 

Scientific Translations, Jerusalem. 394 pp. 

Buckland, S.T., D.R. Anderson, K.P. Burnham, and J .L . Laake. 1993 . Distance 

sampling: Estimating abundance of biological populations. Chapman 

and Hall, London. 446 pp. 

183 
Clark, C.W. 1995. Acoustic tracking of whales using hydrophone arrays: 

Implications for behavioral studies and population estimates. J. Acoust. 

Soc. Am. 97:3352. 

Clark, C.W., and W.T. Ellison. 1988. Numbers and distributions of bowhead 

whale, Balaena mysricerus, based on the 1985 acoustic study off 

Pt. Barrow, Alaska. Rep. Int. Whal. Comm. 38:365-370. 

Clark, C.W., W.T. IIlison, and K. Beeman. 1986. A preliminary account of the 

acoustic study conducted during the 1985 spring migration off Point 

Barrow, Alaska. Rep. Int. Whal. Comm. 36:311-316 . 

Clarke, M.R., H.R. Martins, and P. Pascoe. 1993. The diet of sperm whales 

(Physeter macrocephalus Linnaeus 1758) off the Azores. Phil. Trans. 

R. Soc. Lond. B:67-82. 

Collum, LA., and T.H. Fritts . 1985 . Sperm whales (Physeter catodon) in the Gulf 

of Me.%dco. Southwest. Nat. 30:101-104. 

Corcella, A.T., and M. Green. 1968. Investigations of impulsive deep-sea noise 

resembling sounds produced by a whale. J. Acoust. Soc. Am. 44:483-487. 

Cummings, W.C., P.O. Thompson, and S.J . Ha. 1986 . Sounds from Bryde's, 

Balaenoptera edeni, and finback, B. physalus, whales in the Gulf of 

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186 
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187 
V. BEHAVIOR OF CETACEANS RELATIVE TO SURVEY VESSELS 

B. Wiirsig, S.K. Lynn, and K.D. Mullin 

5 .1 Introduction 

There is great variability in morphology, group size, and behavior of the 

approximately 20 cetacean species that are commonly found in the northcentral and western Gulf of Mexico (Jefferson et al . 1993). The size of these 

animals ranges from small delphinids of 2.5 meters to sperm whales at over 15 

meters in length. Groups may be found containing single to hundreds of 

individuals, while behavior ranges from those animals that habitually 

approach boats and even bowride to those that ignore or avoid vessels 

(Leatherwood and Reeves 1983). Variation in these characteristics can result 

in differing abilities to detect, identify, and accurately count animals, but 

descriptive characteristics of detection variables, especially those stemming 

from different behaviors, have been reported only rarely (Barlow 1995). This 

chapter investigates the effect of behavior on sightability of cetaceans . 

There are often differences in the distances at which cetaceans are first seen, 

first identified, and at which they are most accurately counted. These 

differences in detection distance are determined by morphology and behavior, 

but also by variability in weather (i .e., sighting conditions) . For example, 

sperm whales can often be seen, identified, and counted over one kilometer 

from the survey vessel, often in rather inclement weather. Beaked whales 

(ziphiids) and pygmy/dwarf sperm whales (Kogia spp.), on the other hand, 

may or may not be seen at distance, and their often cryptic behavioral nature 

may preclude identification. A major difficulty in the present study was the 

inability to consistently deviate from the track-line once a distant marine 

mammal group was sighted. This restriction, due not only to the multipurpose 

nature of the survey design but also to ship time (of the Texas Institute of 

Oceanography part of the surveys) and cost limitations, precluded 

identification and enumeration of some observed animals, and thus decreased 

the overall sighting data base. 

Most marine mammal surveys have not taken into account variability in 

morphology and behavior related to sighting conditions. Weather has been 

considered and animals are usually only counted under certain weather 

conditions, such as Beaufort sea state 3 or less (Barlow and Lee 1994). However, 

personal experience (Wiirsig, Lynn, and Mullin, personal observation) has 

shown that there are strong differences in initial sightability of smaller 

cetaceans, even between Beaufort sea states 0-1, and 2-3 . A particularly 

dramatic example confronted GulfCet observers on 23 August 1994 (during TIO 

Cruise 8), when they experienced Beaufort sea state 0-1 for three hours in the 

afternoon. During that time, at least five small cetacean groups were seen 

more than S km from the vessel, with some of these groups being probable 

resightings. Before and after these excellent sighting conditions, when sea 

state was greater than 2, no small odontocetes were seen in the same area, 

despite the same vigilance of observation. It seems convincing, although it is 

difficult to put into a numerical basis, that the change in sea state caused a 

change in sightability of at least some of these more distant cetaceans . 

189 
The present analysis does not attempt to augment discussion of the complex 

issues of weather-related (as well as observer-related) differences in 

sightability (see Breese and Tershy 1993). However, the differences in 

sightability and identifiability of species or species categories as a result of 

their differing morphology and behavioral reactions are addressed. 

Observations of responses to survey aircraft provide an understanding of 

sensitivity to disturbance of different species and species categories. 

5 .2 Methods 

5 .2 .1 Shipboard Observations 

Visual surveys for cetaceans were performed from the upper decks of the 

following vessels: NOAH Ship Oregon II (52 m long) where observations were 

made 10.1 m above sea level; R/V Longhorn (32 m) with sightings made at 

7.7 m above sea level; and the R/V Pelican (32 m) where the sighting platform 

was 8.9 m above sea level. Details of observation and survey protocol are given 

in Chapter 3 . The equation used to determine the distance to cetaceans from the 

vessel (based on binocular reticles) is given in Appendix A. Behavioral 

reactions could not be determined for many sightings, but for those for which 

adequate behavioral notes were made, categories of avoidance (-), no reaction 

(0), approach to vessel (+), and bowriding (b) were given. It was often very 

difficult to categorize potential reaction to disturbance, and the analyses only 

included clear reactions, for which the authors could unequivocally assign a 

behavioral response. A (-) assignation was earned when an individual or 

group moved away from the vessel or appeared to dive in response to the 

vessel, in either case making it more difficult to identify the animal(s) than 

with a neutral or positive response. A (0) designation means that the animal(s) 

showed no apparent response relative to the approach or pass-by of the vessel . 

This is to be distinguished from the many cases during which it could not be 

determined whether there was a response; none of these cases are presented. A 

(+) indicates that the animal(s) approached the vessel during at least some 

time of their observation, and it is likely that a positive mark would have 

enhanced identifiability if the animals were seen at distance. However, a (+) 

response was earned even if the animals were not identified because of their 

positive response ; only their actions were of importance. Finally, a (b) for 

bowriding is an extreme case of a (+) response; all such animals encountered 

during regular daylight on-effort watches were identified . 

Pooling of some species was accomplished in a manner similar to the criteria 

and justification for pooling for estimates of abundance, as given in Chapter 3. 

It was felt that a similar pooling regime should be followed for heuristic 

comparisons of the estimates as well as this behavioral data base. Briefly, the 

following cetaceans were pooled into categories: (1) pygmy/dwarf sperm 

whales and beaked whales; (2) small delphinids; (3) Atlantic spotted dolphins 

(4) bottlenose dolphins; (5) larger delphinids ; and (6) sperm whales. However, 

separate species or species groups within pooled categories are also discussed, 

as these relate to differences in morphology, behavior, and hence potential 

sightability . Due to a presently small data base of behavioral reactions, no 

formal attempts were made to separate reactions relative to the three 

observation vessels. The differences among vessels, if they existed, were 

190 
judged to be much smaller than the differences among species and species 

categories. Differences in initial sighting distance were analyzed with a 

Kruskal-Wallis test, followed by Fisher's LSD post hoc. 

5 .2.2 Aerial Observations 

As required by Marine Mammal Research Permit Number 738 issued by the 

NMFS to the Southeast Fisheries Science Center, a subjective assessment of the 

response of each cetacean group to the survey aircraft was made and recorded. 

For each cetacean group encountered, four questions were addressed: 

" What was the behavior of the group when first sighted? 

" Did the survey aircraft approach within 305 m (1,000 feet), straight-line 

distance, of the group? 

" Did the behavior change during the observation period? 

" If so, what was the new behavior? 

When a cetacean group was encountered, the data recorder would fill out a 

standardized sighting sheet with lists of these four questions and possible 

behaviors. The data recorder would prompt the other observers to make 

behavior determinations and would record the responses. Details of how each 

question was answered follow . 

" What was the behavior of the group when first sighted? 

When a group was sighted, the observer started making an assessment of the 

behavior of the group immediately and continued while the aircraft 

approached the group. As the aircraft approached the group, the other 

observer and data recorder would attempt to get into a position to observe the 

group and participate in determinations of behavior and changes in behavior. 

The group behavior was defined as the most frequently displayed behavior of 

the majority of the animals in a group . Groups were usually circled for the 

minimum time necessary to make species identification and to estimate the 

number of animals in the group. This time was usually about 10 minutes but in 

the extreme case was up to 50 minutes. The time spent circling was dependent 

on size of the group, how the group was aggregated, and the ease of 

identification . The observers' ability to make an identification was affected by 

the species, weather conditions, and behavior of the animals. In some cases, 

additional time was spent circling in order to take photographs and to teach 

new observers the distinguishing characteristics of a species. Behaviors were 

categorized as: resting, feeding, complex social, milling, spyhopping, traveling 

(north, south, east or west), traveling fast, diving, breaching, and other 

(described on sighting sheet) . 

At least one observer in the aircraft, usually two, had in excess of 1,000 hours 

conducting aerial surveys of cetaceans in the northern Gulf of Mexico and 

Atlantic Ocean. All the observers had additional experience observing 

191 
cetaceans during ship surveys . For continuity, the most experienced observer 

was almost always involved in determinations of behavior. 

" Did the survey aircraft approach within 305 m (1,000 feet), straight-line 

distance, of the group? 

The data recorder also determined if the aircraft approached within 305 m of 

the group. The data recorder recorded the sighting angle or interval of the 

sighting and monitored the altitude of the aircraft by communicating with the 

pilots . 

Since the survey altitude was 229 m (750 feet), the aircraft was within 305 m if 

the Perpendicular Sighting Distance (PSD) was less than or equal to 202 m 

(661 feet) (sighting angle of 41 degrees or less) . In some cases, the pilots were 

asked to increase the altitude while approaching and before circling in an 

attempt to increase the observation period of species that were known to be 

cryptic (e.g ., pygmy/dwarf sperm whales and beaked whales). In some cases, 

for photographic purposes or identification, pilots were asked to make several 

fly-bys at less than 229 m. 

" Did the behavior change during the observation period? 

If the behavior of the majority of the animals in the group changed and 

remained changed during the remainder of the observation period, an 

affirmative answer was recorded. Often, while circling a group, dives occurred 

at the same time the aircraft passed overhead or closest to the group. This was 

considered a response to the aircraft . A change in direction of travel was 

considered a behavioral response. 

" What was the new behavior? 

The new behavior was assessed in the same manner as the initial behavior. 

Data on the reaction of cetaceans to GulfCet aerial surveys provided a second 

behavioral database, independent of shipboard observations. Frequencies of 

change in behavior provide an initial evaluation of sensitivities of species and 

behaviors to disturbance. 

5 .3 Results 

A list of the data presented here can be found in Appendix A. 

5 .3 .1 Shipboard Observations 

Distance at initial sighting of cetaceans from the survey ships are described 

for 655 sightings (Oregon II and Pelican/Longhorn combined) for which 

cetaceans were identified to species or species category (Figure S.1) . Mean 

initial sighting distance was 2 .3 ± 1 .77 km (SD) (n = 655), with mean sightings 

as close as 1 .6 ± 1 .50 (SD) km (n=110) for bottlenose dolphins, 1 .6 ± 1 .33 (SD) km 

(n=46) for beaked whales, and as far as 4.2 ± 1 .46 (SD) km (n=6) for killer 

whales. Summaries for all species and categories are in Figures 5 .2 and 5 .3, 

respectively . Behavioral reactions are in Table 5 .1 . 

192 
45 n = 655 sightings 

40- 

35 

30 

y 

r 

.0 25 

cn 

w 

20 

c 

" 15 

a 

10 

5 

0 

0.0 2.0 4.0 6 .0 8.0 10.0 

Initial Sighting Distance (km) 

Figure 5.1. Distance from ship of cetaceans when initially sighted. 

5.3 .1 .1 Category 1 : Pygmy/Dwarf Sperm Whales and Beaked Whales 

This pooled category of cetaceans had an initial mean sighting distance of 

1 .7 ± 1 .28 (SD) km (n = 86) . Eleven of 15 sightings (739'0) involved animals 

reacting negatively to the vessel by orienting away; in no instance did 

members of this category appear to react positively to the vessel. None 

bowrode. The designation of "cryptic" is certainly appropriate, and it is 

unknown how many pygmy/dwarf sperm whales and beaked whales were 

unseen, unidentified, and uncounted because of their cryptic nature. 

5 .3 .1 .2 Category 2: Small Delphinids 

This category of delphinids consisted of pantropical spotted dolphins, clymene 

dolphins, striped dolphins, spinner dolphins, melon-headed whales, rough 

toothed dolphins, and Fraser's dolphins. 

The initial sighting distance of this category was 2.4 ± 1 .83 (SD) km (n = 264). 

With the exception of striped dolphins, all stenellids habitually approached the 

vessel and rode the bow: 83% of pantropical spotted dolphins (137 of 165 

sightings), 100% of spinner dolphins (14 of 14 sightings), and 920 of clymene 

dolphins (22 of 24 sightings) bowrode. The overall reaction for striped 

dolphins was different, however, with only 14 of 27 sighted groups (52%) 

riding the bow. For the entire small delphinid category, nine of the 13 

negative (-) reactions to the survey vessel (where the animal oriented away 

from the vessel or abruptly dove) were exhibited by striped dolphins. Positive 

reactions (+) combined with bowriding occurred for 90% of Category 2 

behaviors. 

193 
6.0 - 

5 .0 87 

80 264 

c 4.0 655 

`° 4f 28 

110 

3 0 . 

n =86 

t 

N 

iC 2.0 

1 .0 

0.0 

N~ r'0 ^ Tfl 

-2 t v ~ ~ fl ~ O' 0 C . O L H 

~ O 00 

- 

O ~ 

pIL 4) t 2 Q a~r~ 

a) 0 

~ Q 

U E Y ~ U ~ U d U ~ U ~ U `y 

Q d 

N 0 

N O N 

. 

= 

M 

E 

N 

U 

v 

A v 

fd v ~ 

~ 

u 

fp \ y., 

.. 

.. 

Figure 5.2 . Initial sighting distances from ship for species categories. Data 

are represented with the mean as the central horizontal line, 

standard deviation as the outer two lines, and 95g'o confidence 

interval as the boa. Sample sizes are shown at the top of each 

category. 

5.3 .1 .3 Categories 3 and 4: Atlantic Spotted Dolphins and Bottlenose Dolphins 

The two species of dolphins that occur over the broad continental shelf of the 

present study area have similar sighting characteristics and at times school 

together. They were first seen at 1 .6 ± 1 .58 (SD) km (n = 138) distance; and, 

overall, 799'0 of the members (a similar 77% for bottlenose dolphins and 85% 

for Atlantic spotted dolphins) of sighted groups came to the ship and rode the 

bow wave for at least one minute, and at times rode for over one-half hour. 

Fourteen of 88 behavioral descriptions of bottlenose dolphins and two of 22 of 

Atlantic spotted dolphins were classed as no (0) reaction, and throughout the 

study area no reactions were classed as negative. Overall, positive reactions 

combined with bowriding occurred for 86% of these groups. 

194 
y 

r 

X 

fD 

~ QQ 

~ °° 

rp n d 

N

r, 

-1 r) 

V F+" 

y 

b C. n 

O ¢+ rp "y O' C/~ 

n 

f-+ V: 

~ 

Xn 

v 

C 

p' C 

\° F" 

0 

r n 

o rD 

a " o 

:+ . (~A 

n 

r-D (D 

.-- r. 

r 

N 

sperm whale 

Beaked whale (ziphiids) 

Fraser's dolphin 

Melon-headed whale 

Pantropical spotted dolphin 

N 

Clymene dolphin 

Striped dolphin 

Spinner dolphin 

Rough-toothed dolphin 

Atlantic spotted dolphin 

Bottlenose dolphin 

Pygmy killer whale 

Short-finned pilot whale 

v, 

(M I 

Risso's dolphin 

Killer whale 

False killer whale 

Sperm whale 

N 

' W ' 

r f1 i ~ 

N 

m 

0 

~N 

0 

co 

I L^ 00 

a) 

I co 

v 
Table 5 .1 . Behavioral reactions* of cetaceans relative to the survey ship . 

Reaction 

Species (-) (0) (+) (b) %(b) 'Cotal 

Category 1 Pygmy/dwarf sperm whales 6 l 0 0 0 7 

Beaked whales S 3 0 0 0 8 

Subtotal 11 4 0 0 15 

Category 2 Fraser's dolphin 1 n 0 1 SO% 2 

(smaller delpllinicis) Melon-headed whale 1 1 2 3 43% 7 

Pantropical spotted dolphin 1 9 18 137 83% 165 

Striped dolphin 9 Z 2 14 52% 27 

Spinner dolphin 0 0 0 14 l00°/v 14 

Clymene dolphin 1 0 1 22 92°/v 24 

Rough-toothed dolphin 0 0 2 G 75% 8 

Subtotal 13 12 25 197 247 

Category 3 Atlantic spotted dolphin 0 2 2 22 85% 26 

Category 4 Bottlenose dolphin 0 14 6 68 77% 88 

Subtotal 0 16 8 90 114 

Category S Pygmy killer whale 1 n 0 U 1 

(larger dE'_1pI11111CjS) Short-finned pilot whale 1 4 0 1 17% G 

Risso's dolphin 5 13 7 S 17% 30 

Killer whale v 2 0 4 67% G 

False killer whale 0 1 1 3 GU% S 

Subtotal 7 20 8 13 48 

(-) indicates animal orienting away from vessel or abrupt diving . 

(0) indicates no reaction. 

(+) indicates approach. 

(b) indicates bowriding. 

%(b) indicates percentage of bowriding per total reactions . 
5 .3 .1 .4 Category 5: Large Delphinids and Small Whales 

This category consisted of short-finned pilot whales, Risso's dolphins, false 

killer whales, killer whales, and the smallest member of the category, the 

pygmy killer whale. The mean distance of first sighting was 2.7 ± 1 .65 (SD) km 

(n = 80), with killer whales sighted at greater distance, 4.1 ± 1 .46 (SD) km 

(n = 6), than any other species. Pilot whales and Risso's dolphins exhibited the 

least attraction to the vessel, with only one of six in the former and 12 of 30 in 

the latter species moving towards the vessel or bowriding. 

5 .3.1 .5 Category 6: Sperm Whales 

Sperm whales were sighted at a mean distance of 3.0 ± 1 .86 (SD) km (n = 87). 

Generally, sperm whale reaction was not described in the sighting notes, but 

the overall impression was that reactions tended to be non-existent for all but 

approaches to within several hundred meters. Eleven of 15 sightings with 

behavioral notes were labeled as (0) reaction, none as (+) reaction, and 4 as (-), 

consisting of the whales diving abruptly in apparent response to the vessel, 

all within 200 m. 

Sperm whales were unlikely to be misidentified even at several kilometers 

distance, due to their characteristic bushy, single-spouted blow at the very 

front of the head, and due to their unique head-to-dorsal-hump surface 

profile. However, sperm whales dive for long times, often greater than onehalf hour, and accurate counts of animals as a survey vessel passes through an 

area are therefore difficult. 

5.3 .1 .6 General 

Overall, killer whales were initially sighted at the greatest distance from the 

survey vessels (n = 6, Figure 5 .3). This distance was significantly greater than 

for all other animals (p < 0.0001 for all comparisons, Fisher's LSD), except for 

melon-headed whales . Results for paired comparisons between species are 

given in Appendix A. The cryptic pygmy/dwarf sperm whales and beaked 

whales, Category l, were not sighted at distances significantly different from 

Atlantic spotted dolphins or bottlenose dolphins categories (categories 3 and 4, 

respectively, Table 5.2) . These three categories were sighted closer to the ship 

than the small delphinids, large delphinids, and sperm whales (Categories 2, S, 

and 6, respectively) . There was a significant difference between initial 

sighting distances of small and large delphinids (p = 0.005), but not between 

the small delphinids and sperm whales (p = 0.09), and the large delphinids and 

sperm whales (p = 0.41). 

Overall, the pygmy/dwarf sperm whales and beaked whales showed the 

greatest percentage (73%) of negative (-) reactions, with the large delphinids 

at 15%, the small delphinids at 6%, and the Atlantic spotted and bottlenose 

dolphins at 0% each. Risso's dolphins, of the large delphinid category, reacted 

negatively 17g'o of the time (five of 30 sightings), and striped dolphins, of the 

small delphinid category, reacted negatively 33% of the time (nine of 27 

sightings). Spinner dolphins, rough-toothed dolphins, bottlenose dolphins, 

197 
Table 5 .2 . Paired comparisons (Fisher's LSD) of initial sighting 

distances for the 6 species categories . "S" denotes 

significant comparisons (p <_ 0.05) . 

Comparison Mean Critical p value 

difference difference 

Category 1 vs. Category 2 -724.4 414.6 0.0006 S 

Category 1 vs. Category 6 -1,312 .5 507.7 <0.0001 S 

Category 1 vs. Category S -1,096.6 518.6 <0.0001 S 

Category 1 vs. Categories 3 & 4 58.1 458.7 0.8036 

Category 2 vs. Category 6 -588.1 412.8 0.0053 S 

Category 2 vs. Category S -372.2 426.1 0.0868 

Category 2 vs. Categories 3 & 4 782.6 350.7 < 0.0001 S 

Category 6 vs. Category S 215.9 517.2 0.4127 

Category 6 vs. Categories 3 & 4 1,370.6 457.1 < 0.0001 S 

Category S vs. Categories 3 & 4 1,154.7 469.2 < 0.0001 S 

killer whales, and false killer whales never appeared to react negatively 

towards the vessel. On the other hand, pygmy/dwarf sperm whales, beaked 

whales, and the pygmy killer whale (with only one sample for it, however) 

never showed positive reactions, including bowriding. 

5 .3 .2 Aerial Observations 

Aerial surveys encountered many of the same species as the ship-based 

surveys. Additionally, aerial observers saw one sei or Bryde's whale. Melonheaded whales and pygmy killer whales were grouped together for aerial 

surveys, rather than separately, as in the preceding analyses of shipboard 

observations. 

Species which were found to respond to the ship (either positively or 

negatively) were also found to change behavior in response to the survey 

aircraft's activities (Figure 5 .4, Table 5 .3). Pygmy/dwarf sperm whales 

changed their behavior in response to the survey airplane during 40% of 

their sightings (and beaked whales during 899%, though with only nine 

sightings). Several of the smaller delphinids also showed sensitivity to 

disturbance by the airplane. Pantropical spotted, clymene, striped, and 

spinner dolphins all were judged to have changed their behavior in response 

to the airplane during over 40% of their sightings. 

As with reactions to the boat, species' reactions to the airplane differed (Table 

5 .4) . Over 85g'o of the responses of pygmy/dwarf sperm whales and beaked 

whales, were to dive. Many of the small delphinids dove approximately SO% of 

the time, or exhibited an undefined "other" behavior. These differences reflect 

the findings from shipboard observations; pygmy/dwarf sperm whales and 

beaked whales usually exhibited negative reactions, while small delphinids 

often come to ride the bow (Table S .1) . From the air, responses of striped 

dolphins were not distinguishable from those of other stenellids, while toward 

the boat, striped dolphins displayed negative reactions more frequently than 

198 
100 - 

80 - 

C 

t 

60 

w 

30 42 

= 40 

11 72 25 

a 

20 37 

0 

C C C C N 

e0 CL 

O p CL ~ O. L 

-°0 3 

~ u ~O O ~ to N 

(C 

a 

fA Q 

N 4.0 

V O 

.Q v '+'r m c 

o 

N c Q 

n. 

Figure 5 .4. Percentage of sightings during which cetaceans (species with >_ 

10 sightings) exhibited a change in behavior relative to aerial 

surveys. Numbers above columns indicate the total number of 

sightings for a species. 

other small delphinids. While "diving" and "other" were the most common 

responses by cetaceans to disturbance from the airplane, 339'0 of bottlenose 

dolphin responses were to begin traveling or to change direction of travel . 

As a generalization (over all cetacean species), the behavioral states "milling" 

and "resting" appeared to be sensitive to disturbance; over 399% of initial 

observations of these behavioral states were followed by observations of a new 

behavior (Table 5 .3) . Cetaceans changed from these behaviors to new 

behaviors from 40-100% of the time, except for melon-headed and pygmy 

killer whales, rough-toothed dolphins, and Risso's dolphins, which never 

responded while in these behavioral states . Some species were also sensitive to 

disturbance while traveling. 

5 .4 Discussion 

Sperm whales are easy to sight and identify, and reactions relative to the 

vessel may be unimportant in estimating abundance. However, a caveat must 

be made: it is possible that sperm whales that hear an approaching or passing 

vessel change their dive times. They may remain submerged longer than they 

199 
Table 5 .3 . Sensitivity of cetaceans and initial behaviors to disturbance by survey aircraft. The 

g 

proportion of times an initial behavior changed to the total number of times that 

behavior was seen is expressed. The observation frequencies from which these data 

are derived are given in Appendix A. 

Diving -Feeding- Mil ling RestingSocial _ . Travel Unknown Other Overall 

Pygmy/dwarf 

sperm whale 0.00 0.50 0.00 0.00 1 .00 0.40 

Beaked whale (ziphiids) 1 .00 0.50 0.89 

Fraser's dolphin 0.00 0.00 

Melon-headed whale and 

pygmy killer whale* 0.00 0.17 1 .00 0.25 

Pantropical spotted dolphin 0.00 0.00 0.48 0.67 0.00 0.43 

Clymene dolphin 0.71 0.71 

Striped dolphin 0.71 1 .00 0.75 

Spinner dolphin 1.00 1 .00 

Rough-toothed dolphin 0.00 0.50 0.00 0.00 0.00 0.00 0.13 

Atlantic spotted dolphin 0.50 1 .00 0.13 0.27 

Bottlenose dolphin 0.00 0.63 0.57 0.00 0.20 0.25 0.28 

Short-finned pilot whale 1 .00 0.25 0.00 0.29 

Risso's dolphin 0.00 0.00 0.00 0.19 0.40 0.16 

False killer whale 0.00 0.00 

Sperm whale 0.40 0.00 () .Sn 0.28 

Sei and Bryde's whale 1 .00 1 .00 

Mean 

(over all cetaceans) 0.00 0.17 0.43 0.39 0.00 0.31 0.38 0.67 

n 1 3 S 9 3 14 In 3 

* Note : Melon-headed whale and pygmy killer whale sightings were pooled for aerial surveys. 
Table 5 .4. Responses of cetaceans, grouped by species categories, to the survey aircraft . The 

number of times a species responded with a particular behavior is expressed as a 

proportion of its total number of responses . 

Diving Feeding Milling Resting Social Travel Unknown Other Total number 

of responses 

Pygmy/dwarf 

sperm whales 1 0 0 0 0 0 0 0 12 

Beaked whales (ziphiids) 0.87 0 n 0 0 0 0.12 0 8 

Fraser's dolphin 0 0 U 0 0 U U 0 0 

Melon-headed whale and 

pygmy killer whales* 0 n 0.50 0 0 0 0.50 0 2 

Pantropical spotted dolphin 0.28 0 0.11 0 0 0.11 0 0.50 18 

Clymene dolphin 0.40 0 U 0 0 0 0 0.60 S 

Striped dolphin 0.50 0 0 0 0 U 0 0.50 G 

o Spinner dolphin 0.50 0 0 0 n 0 0 0.50 4 

Rough-toothed dolphin 0 0 0 0 0 0 1 0 1 

Atlantic spotted dolphin 0.33 U 0 0 0 0.33 0.33 0 3 

Bottlenose dolphin 0.48 n 0 0 0 014 0 0.33 20 

Short-finned pilot whale 0 U U 0 0 U U 1 2 

Risso's dolphin 0.17 0 U U 0 0.17 0 0.67 6 

False killer whale 0 0 n , 0 0 U 0 0 0 

Sperm whale 0.86 0 0 0 0 0 0 0.14 7 

Sei and 13i-yde's whale 1 () 0 0 0 0 0 0 1 

Overall proportion 0.53 0 0.03 0 n 0.07 0.04 0.33 95 

* Note: Melon-headed whale and pygmy killer whale sightings were pooled for aerial surveys . 
would otherwise or shorten their surface time in response to the vessel . 

Neither of these possibilities is indicated by present information however. It is 

assumed that behavioral reactions do not consistently cause over- or 

undercounts of sperm whales. As a matter of fact, the consistency of density 

estimates between visual and acoustic surveys (Chapter 10) indicates that 

overall sperm whale numbers were probably well represented in the study. It 

is, however, possible that the number of sperm whales within sighting 

distance is at times undercounted simply because of their normally long 

submergences, and this surface/dive ratio cannot be assessed without further 

work . 

Bottlenose dolphins and Atlantic spotted dolphins are relatively easy to sight 

and identify, in large part because of their habit of approaching vessels. The 

behavior patterns relative to vessels are quite similar for the two species, and 

population estimates of the two species relative to one another are not likely to 

be skewed due to behavioral reactions, as long as animals are sighted before 

they respond to the vessel . However, their proclivity for bow-riding may 

result in overcounts relative to other less attracted species. 

The small delphinids are also rather similar in reaction, except for the striped 

dolphin and perhaps the lesser seen and known Fraser's dolphins and melonheaded whales. For the striped dolphin, there was a strong indication of 

avoidance reaction and apparent leaping away from the vessel at distances as 

far as three kilometers . All nine negative reactions that were observed 

nevertheless resulted in species identifications, simply because the dolphins 

were leaping at distance, and therefore allowed their clearly marked flanks to 

be seen. There are no data on how many times striped dolphins avoided the 

vessels and were thereby not seen or, if seen, not identified to species category 

or species. It is probable that striped dolphins are undercounted relative to 

other stenellids . 

A similar problem of potential undercounting may exist for Fraser's dolphins 

and melon-headed whales, which were only identified two and seven times, 

respectively. Fraser's dolphins bow-rode one of two times (50%) and melonheaded whales showed positive reactions or bow-rode five of seven times 

(719'0) . These data would indicate that positive and other reactions might 

balance out, but the numbers of identified sightings are simply too small, and 

the possibility exists that these delphinids are more cryptic than indicated. 

Further work, including detailed comparisons with aerial surveys, may shed 

light on this question . 

The mid-sized blackfish of the large delphinid category are probably seen 

with approximately equal frequency with varied reactions to the vessel . They 

are also large and identifiable enough to be seen and counted from well over 

one kilometer. The killer whales are probably overcounted relative to others, 

with killer whales in approximately the same category as the easily seen 

sperm whales. On the other side of this scale are beaked whales and especially 

pygmy/dwarf sperm whales. Beaked whales are often not identifiable to 

species, but at least can be placed into the beaked whale category much of the 

time. Pygmy/dwarf sperm whales are smaller and generally behaviorally 

cryptic, and so the assumption can be made that these two species of 

physeterids are undercounted to a relatively higher but unknown degree. 

202 
The noise of the survey vessel, both from the engines and propeller cavitation, 

alert cetaceans to the vessel's presence (Richardson et al. 1995) . Cetaceans that 

react positively are probably either curious or are gauging the possibility of 

riding bow or stern waves of the vessel . The forward leaps of spotted dolphins, 

for example, as they race towards the bow, can be described as a play activity 

which makes these animals very easy to see indeed. Cetaceans also become 

habituated to vessels, and much "ignoring" of the survey vessels is probably 

due to habituation in a propeller-noisy environment such as the northern 

Gulf of Mexico. It is unclear why some cetaceans, even those not known to 

have been harassed or killed by humans on any large scale, are habitually or 

at times evasive. Perhaps the noise of the vessel is disruptive to feeding or 

resting or other activities. 

Distances at which ship noises are heard are variable by ship, weather (sea 

state and rain) conditions, oceanography, depth of dive of the target species, 

frequencies of sensitivity, general ambient noise conditions, and the angle at 

which the species is seen from the bow (Greene 1991, Malme 1991). These 

variable factors make it very difficult to summarize distances of potential 

noise influence. However, supply vessels of the approximate sizes of our 

survey vessels have sound levels in the 20 Hz to 1000 Hz range of about 120 to 

150 dB re. 1 gPa at a distance of 0.2 km, and about 105 to as high as 125 dB re. 1 

gPa at a distance of 9-10 km while underway (Greene and Moore 1991). The 

hearing sensitivity of the toothed whales in the present study are well above 

100 Hz, with the smaller delphinids doing almost all communicating and 

echolocating well above 1000 Hz (Au 1993) . 

Aerial survey data concerning the sensitivity to disturbance support 

conclusions from the shipboard observations. Cryptic species, such as 

pygmy/dwarf sperm whales and beaked whales, which were seen resting on 

most occasions, responded to the airplane a high proportion of the time, and 

responded by diving over 85% of the time. Less cryptic species, such as the 

small delphinids, may respond as often, but their response does not necessarily 

make them harder to identify. Additionally, certain behavioral states, such as 

resting or milling, appear to be more sensitive to disturbance than others, and 

this also varies by species. 

These data indicate that the sightability and identification of cetaceans may 

change with the variable behavior of species or species categories. However, 

behavior is even more variable than summarized in this section, with 

potential differences by group size, age and sea, time of day, season, weather, 

and other factors. For example, spinner dolphins of Hawaii are known to be 

more shy and cryptic while resting in early morning, and more aerially 

demonstrative in the afternoon (Wursig et al. 1994). Dusky dolphins, 

Lagenorhynchus obscures, of the southern hemisphere attune their human 

interactions, including approaches to boats, closely to group size and whether 

or not they have fed in the previous several hours (Wiirsig and Wiirsig 1980). 

They show marked differences in human interaction responses relative to age 

and sex, and seasonality. Sperm whales and several species of baleen whales 

may increase their aerial activity prior to and in the initial stages of a drastic 

weather change (Wursig and Lynn, personal observation). It is likely that 

similar differences exist for the cetaceans of the north-central and western 

203 
Gulf of Mexico, but our behavioral data base, gleaned literally while transiting 

past the animals, is at present too meager for more definitive statements . 

5 .5 Literature Cited 

Au, W.W.L 1993 . The sonar of dolphins. Springer-Verlag, New York. 277 pp. 

Barlow, J. 1995. The abundance of cetaceans in California waters. Part I: Ship 

surveys in summer and fall of 1991 . U.S . Fish. Bull. 93:1-14 

Barlow, J., and T. Lee. 1994. The estimation of perpendicular sighting distance 

on SWFSC research vessel surveys for cetaceans: 1974 to 1991 . NOAH 

Tech. Mem. NOAA-TM-NMFS-SWFSC-207. National Marine Fisheries 

Service, La Jolla, CA. 46 pp. 

Breese, D., and B. Tershy. 1993. Relative abundance of cetacea in the Canal de 

Ballenas, Gulf of California . Mar. Mamm. Sci. 9:319-324. 

Greene, C.R., Jr. 1991 . Ambient noise, pp 54-69. In W.J . Richardson, 

C.R. Greene, Jr., C.I . Malme, and D.H . Thomson, with S.E. Moore and B. 

Wursig . Effects of noise on marine mammals. OCS Study MMS-90-0093. 

Minerals Management Service, Atlantic OCS Regional Office, Herndon, 

VA. 

Greene, C.R., Jr., and S.E. Moore. 1991 . Man-made noise, pp 70-140. In 

W.J . Richardson, C.R. Greene, Jr., C.I . Malme, and D.H. Thomson, with S.E 

Moore and B. Wursig. Effects of noise on marine mammals. OCS Study 

MMS-90-0093. Minerals Management Service, Atlantic OCS Regional 

Office, Herndon, VA. 

Jefferson, T.A., S. Leatherwood, and M.A. Webber. 1993. Marine mammals of 

the world - FAO species identification guide. Food and Agriculture 

Organization of the United Nations, Rome. 320 pp. 

Leatherwood, S., and R.R. Reeves. 1983 . The Sierra Club handbook of whales 

and dolphins. Sierra Club Books, San Francisco. 302 pp. 

Malme, C.I . 1991 . Sound propagation, pp 22-53 . In W.J . Richardson, 

C.R. Greene, Jr., C.I. Malme, and D.H . Thomson, with S.E. Moore and 

B. Wursig. Effects of noise on marine mammals. OCS Study MMS-90-0093. 

Minerals Management Service, Atlantic OCS Regional Office, Herndon, 

VA. 

Richardson, W.J ., C.R. Greene, Jr., C.I . Malme, and D.H. Thomson, with 

contributions by S.E. r7oore and B. Wursig. 1995. Marine mammals and 

noise. Academic Press, New York. 576 pp. 

Wiirsig, B., and M. Wiirsig. 1980 . Behavior and ecology of the dusky dolphin, 

Lagenorhynchus obscurus, in the South Atlantic. U.S . Fish. Bull . 77:871-

890. 

204 
Wiirsig, B., R.S . Wells, K.S . Norris, and M. Wiirsig. 1994. A spinner dolphin's 

day, pp 65-102 . In K. S. Norris, B. Wiirsig, R S. Wells, and M. Wiirsig, eds. 

The Hawaiian spinner dolphin. University of California Press, 

Berkeley . 

205 
VI. OCEANOGRAPHIC SURVEYS 

G.S . Fargion, N. May, T.D. Leming, and C. Schroeder 

6.1 Introduction 

The physical and biological characteristics of the Gulf of Mexico are 

remarkable in their variability and intensity. Due to these characteristics, 

oceanographic features may have very important implications for cetacean 

habitat, possibly affecting cetacean distribution patterns. Therefore, one goal 

of the GulfCet program was to develop an understanding of mesoscale 

oceanographic features and their potential effect on the spatial and temporal 

distribution of cetacean species in the north-central and western Gulf of 

Mexico. 

Recent research indicates that cetaceans may aggregate in areas where 

upwelling and eddies dominate the circulation (Brown and Winn 1989) . This is 

probably due to the increased primary productivity and subsequent increased 

density of prey species which characterize these areas. Fresh water influx and 

its associated higher nutrient concentration can have a similar effect on 

productivity levels . Further, Biggs (1992) has shown that anticyclonic (warm) 

eddies in the western Gulf are biologically impoverished, while cyclonic (cold) 

eddies located peripherally to anticyclonic features have higher nutrient 

levels and a higher level of primary productivity. 

Hydrographic data were collected during 11 shipboard surveys and 

synoptically by satellite remote sensing . Sea surface temperature satellite 

images were generated from data collected by NOAA's Advanced Very High 

Resolution Radiometer (AVHRR) polar orbiting satellites . 

6.2 Hydrographic Data Collection and Analysis 

6.2.1 Background 

This section presents an overview of the extensive hydrographic data set 

collected during the GulfCet program. Its objective is to provide the 

information needed to understand methods of data acquisition and processing . 

Pre-analysis corrections or adjustments are identified and discussed. 

The variability in certain environmental parameters was used to delineate the 

mesoscale features in the north-central and western Gulf. Differences in 

temperature and salinity (T-S) were used to characterize water masses in the 

Gulf. Gulf Common Water (GCW) and Caribbean Subtropical Underwater (SUW) 

can both be found within the top 250 m of water depth, while Antarctic 

Intermediate Water (AAIW) is located deeper, at a depth of 600 to 1,000 m. In 

addition, temperature and salinity changes were used to detect warm and cold 

water rings (eddies) as well as fresh water input. Dynamic height, as an 

indicator of geostrophic flow, was employed to detect general circulation 

patterns, including eddies . Chlorophyll & concentrations were used as an 

indicator of primary productivity. Standard hydrographic techniques were 

used to measure these parameters. 

207 
Table 6.1 . TIO and SEFSC hydrographic cruise chronology. 

Date Hydrographic Date Hydrographic 

Survey Survey 

15 April- TIO Spring 3 May- SEFSC Spring 

1 May 1992 Cruise 1 15 June 1993 Cruise 204 

R/V Longhorn NOAH Ship 

Oregon II 

17 April- SEFSC Spring 23 May- TIO Spring 

8 June 1992 Cruise 199 S June 1993 Cruise 5 

NOAH Ship RN Pelican 

Oregon II 

10 August- TIO Summer 28 August- TIO Summer 

24 August 1992 Cruise 2 5 September 1993 Cruise 6 

R/V Pelican R/V Pelican 

8 November- TIO Fall 3 December- TIO Fall 

22 November 1992 Cruise 3 14 December 1993 Cruise 7 

R/V Pelican R/V Pelican 

S January- SEFSC Winter 15 April- SEFSC Spring 

13 February 1993 Cruise 203 10 June 1994 Cruise 209 

NOAH Ship NOAH Ship 

Oregon II Oregon II 

12 February- TIO Winter 

27 February 1993 Cruise 4 

R/V Pelican 

Hydrographic data collected during GulfCet cruises have being submitted to 

the National Oceanographic Data Center (NODC) and are available to the public 

from that source. Table 6.1 details the chronology of shipboard hydrographic 

surveys completed for the GulfCet program. 

6.2 .2 Transect and Cruise Design 

Shipboard hydrographic data were collected using two distinct cruise track 

designs, each of which is described in detail in section 3 .2.1 .1 of this report. 

Four SEFSC Oregon II surveys were completed in the spring and winter and 

sampled the entire northern Gulf (Figures 6.1-6 .3) . The Oregon II cetacean 

surveys occurred simultaneously with an ichthyoplankton survey and were 

divided into three legs. Hydrographic stations were located every 30-40 

minutes of latitude or longitude along the cruise track. 

208 
-96 -94 -92 -90 -88 -86 -84 -82 -- 

3( 

2. 

21 

30 

28 

26 

24 

Figure 6.1 . Representative Oregon II hydrographic survey station plan for leg 1 . 

-96 -94 -92 -90 -88 -86 -84 -8: 

T --1 I 

30 30 

,- - o 

. _ r~.. 

vx~ 28- .lhM" ~"~ 28 

:.t :.. .fi x'' O 

26 o " ~ 26 

D CTD Station 

" XBT Station 

24 X98 -96 -94 -92 -90 -88 -86 -84 -82 

Figure 6.2 . Representative Oregon II hydrographic survey station plan for leg 2 . 

209 
-98 -96 -94 -92 -90 -88 -86 -84 -82 

e 

D 

>,,~ 

?~~`>`?8 . . n{. ?.% ::: QZ 

1 

A C.TD SfehOII 

0 XBT scaaon a ~ a ° 

98 -96 -94 -92 -90 -88 -86 -84 -82 

30 

28 

26 

24 

Figure 6.3 . Representative Oregon II hydrographic survey station plan for 

leg 3 . 

Conductivity-Temperature-Depth (CTD) hydrocasts were conducted at every 

other ichthyoplankton station at night, with three casts made during daylight 

hours (dawn, midday, and sunset) . CTD hydrocasts ordinarily were lowered to a 

maximum depth of 200 m, but were deepened to 500 m for the GulfCet project. 

An expendable bathythermograph (XBT) was dropped midway between CTD 

stations. In general, the cruise track remained relatively consistent for the 

four surveys, but the frequency of CTD and XBT casts and the station positions 

varied among the four cruises. The Oregon II data shown in the following 

sections represent all stations that were sampled during each cruise and are 

not limited to just those in the GulfCet study area 

Since 1990, similar Oregon II vessel surveys have been conducted annually 

during the spring, summer, and winter in the northern Gulf of Mexico. Data 

from these hydrographic surveys are stored in the Southeast Area Monitoring 

and Assessment Program (SEAMAP) data base maintained by the National 

Oceanic and Atmospheric Administration (NOAH). 

The second cruise track design was implemented for the Longhorn and Pelican 

surveys and covered the GulfCet study area once per season, for a total of seven 

hydrographic surveys. A transect consisting of 14 north-south track-lines was 

followed during the cruises. The hydrographic survey was designed to sample 

the mesa to large-scale features in the Gulf. The choice of location and 

spacing of the 50 CTD hydrographic stations for this study was based on the 

following: 

a) spatial scale estimates of oceanographic features in the study area (e.g ., 

slope eddy radii of 5o-100 km) from bibliographic references; 

b) acoustic and visual survey constraints; 

c) ship time constraints; 

30 

28 

26 

24 

210 
d) similar survey patterns in other MMS Programs: Louisiana and Texas 

Shelf Circulation and Transport Process Study (LATEX A), Louisiana and 

Texas Mississippi River Plume Study (LATEX B), and Louisiana and Texas 

Eddy Circulation Study (LATEX C); 

e) C'TD time estimates; and 

previous historical data. 

As a result, CTD stations were located at the 100 and 2,000 m isobaths (except at 

the Mexican border), and at 74 km (40 nautical mile) intervals on each trackline. The location and spacing of the 84 XBT hydrographic stations was based 

on the 200, 350, 500, 800, 1,000, and 1,500 m isobaths at each of the 14 northsouth track-lines (Figure 6.4). 

6.2.3 Shipboard Measurements and Procedures 

6.2.3 .1 Conductivity, Temperature, Depth (CTD) Hydrocasts 

Data collected during each Oregon II cruise were obtained following standard 

SEFSC protocol, and further details are available in individual Oregon II cruise 

reports (U.S. Dept. of Commerce 1992, 1993a, 1993b, 1990 . Few modifications 

were made to the original cruise design, with the exception of deepening the 

CTD hydrocasts from 200 to 500 m, and changing the data acquisition rate of the 

CTD instrument. The CTD data for spring Cruise 199 were acquired at 8 Hz and 

averaged at 1 second intervals, while the CTD data for the remaining three 

Oregon II cruises were also acquired at 8 Hz, but not averaged. 

Vertical salinity and temperature profiles were measured each day on Oregon 

II cruises. A Sea-Bird Electronics, Inc. (SBE) SeaCatTI'l or SealoggerT'" CTD and 

rosette were lowered to the sea bottom or to a maximum depth of 500 m. Niskin 

bottles were closed on the upcast, and data from these samples were later used 

to verify CTD data. While the actual depths sampled were variable, the standard 

cruise plan called for water samples at the surface, mid-depth, and the bottom. 

Surface chlorophyll for Oregon II cruises were derived from sea water samples 

taken at regular intervals along the ship transect. Up to nine liters of water 

were collected using either a surface bucket or a Niskin bottle during a 

hydrocast. One milliliter of a 1% suspension of MgC03 vas added to each of 

three three-liter replicate seawater samples as each was filtered through GF/C 

filters. The replicate samples were frozen until they were analyzed for 

chlorophyll .a using Strickland and Parsons (1972) spectrophotometric method. 

Vertical profiles of salinity, temperature, oxygen, and beam attenuation 

coefficient (transmissometry) were measured at every Longhorn and Pelican 

CTD station. A rosette with 12 5-liter Niskin bottles was lowered with the CTD. A 

SBE 9 Plus' CTD was used on every cruise except summer Cruise 2, where a SBE 

9 was used. During the downcast, temperature, salinity, and beam attenuation 

coefficient were graphically displayed in real-time as a function of depth. CTD 

data were acquired at 24 Hz. While the CTD/rosette equilibrated at the bottom 

depth for five minutes, the sampling depths for the upcast were selected. The 

upcast was identical to the downcast except the instrument was stopped at the 

selected sampling depths and the Niskin bottles were tripped. The CTD/rosette 

was lowered to the sea floor or to a maximum depth of 1,000 m. At stations less 

211 
N 

N 

Figure 6.4. Longhorn and Pelican hydrographic survey station plan. 

31 

30 

29 

28 

27 

26 

25 

24 
than 500 m deep, in situ fluorescence was also measured. Secchi depths and 

environmental data were also gathered using World Meteorological 

Organization (WNIO) codes. 

The Pelican/Longhorn water sample depth selection was based on chlorophyll 

sample criteria and followed these general guidelines: 

" At 100 m stations, water samples were taken at depths of 0, 5, 15, 20, 30, 40, 

50, 60,70, 80, 90, and 100 m. 

At all other stations, water sampling depths were 0, 10, 20, 30, 40, 55, 70, 85, 

100, 125, 150, and 1,000 m (at stations where that sampling depth was 

appropriate) . 

Nutrient samples were only collected on Pelican summer Cruise 6. 

A salinity sample was always taken from the shallowest and deepest bottle of 

Pelican hydrocast. Salinity samples were analyzed at TAMU Department of 

Oceanography using a Guideline Connectively Coupled SalinometerT' . One liter 

water samples for chlorophyll analysis were filtered at sea using GF/F filters 

after adding MgC03 to stabilize the pigments. The sample filters were stored in 

liquid nitrogen at sea and transferred to a -76°C freezer at TAMUG for storage 

until they were analyzed. The samples were analyzed for chlorophyll a and 

phaeopigments using a Turner Designs Fluorometer and following a modified 

Strickland and Parsons (1972) procedure . Precision of chlorophyll and 

phaeopigment analysis vas ± 0.01 gg 1-1 . Replicates of Pelican track-line 4 

chlorophyll samples were given to the MMS LATEX A program for highperformance liquid chromatography (HPLC) pigment analysis. Data from these 

samples were used as quality control for Pelican's chlorophyll data. 

6.2.3 .2 Expendable Bathythermograph (XBT) Surveys 

During Oregon II cruises, XBTs were launched between CTD stations (refer to 

Figures 6 .1-6 .3), and on some legs in place of CTDs due to CTD instrument 

failure. XBTs were launched at depths of 200, 350, 500, 800, 1,000, and 1,500 

meters along each Pelican/Longhorn cruise track-line . Additional XBTs were 

launched at some marine mammal sightings, for acoustic array calibration, 

and when unusual hydrographic features were detected. The probes were 

deployed while underway, with the ship speed not exceeding seven knots. T-7, 

T-10, or T-20 XBT probes (depending upon the depth) were the primary types of 

probes used on all surveys (Sparton of Canada, Ltd. or Sippican XBT probes) . 

6.2.3 .3 Continuous Flow-through Systems 

A continuous flow SBE thereto-salinograph and fluorometer were operated 

throughout each Oregon II cruise. Navigation, surface water salinity, and 

temperature were recorded at 60 seconds intervals. The SBE sensor components 

were sent for factory recalibration once per year. 

The Pelican cruises used the Multiple Interface Data Acquisition System 

(MIDAS) (Walser et al. 1992) to continuously record navigation data, surface 

hydrographic data (salinity, temperature, fluorescence, light transmission, 

213 
and sea water flow rate), and meteorological data (wind speed, wind direction, 

air temperature, barometric pressure, and solar irradiance) . The MIDAS was 

configured to sample these data at an average of every fifteen seconds. This 

system used a SBE conductivity-temperature sensor and a Sea Tech, Inc. 

fluorometer and transmissometer. The conductivity-temperature sensor was 

calibrated annually by the manufacturer. 

6.2.4. Data Analysis 

The analyses that are accepted as routine within the oceanographic 

community are not described in detail . 

6.2.4.1 XBT and CTD Data Processing 

Raw XBT frequency data for Pelican/ Longhorn cruises 1 and 2 and all Oregon 

II surveys were processed with an in-house conversion program using 

Sparton of Canada, Ltd. drop rates. Pelican Cruises 3 through 7 used Sparton's 

software (Sparton of Canada, Ltd. 1992) . All processed XBT data were 

interpolated at 1 m intervals using a program developed at Scripps Institution 

of Oceanography (La Jolla, CA). Pelican/Longhorn XBT data were listed in 10 m 

steps (Appendix C), while Oregon II XBT data are listed in standard depths 

(Appendix B) . 

All XBT data were calibrated against CTD temperature data according to Singer's 

(1990) procedure . XBT depths were adjusted using the following first order 

empirical fit : 

New XBT depth = 0.07 . (old XBT depth) - 3. 

Data corrected by this method are not found in the XBT data listings 

(Appendices B and C), but were used to generate any plots that required XBT 

temperature data. 

All CTD data were processed using SeasoftT' software (Sea-Bird Electronics, Inc. 

1992) . The following CTD data processing steps were used: 

1 . DATCNV: Converts raw data to binary engineering units and stores data in 

CNV files. 

2. SPLIT: Splits the CNV (converted) files into upcast and downcast files. 

3 . WILDEDIT: Checks for and marks `wild' data points. 

4. FILTER: Filters data columns to produce zero phase time shifts. 

S .ALIGNCTD: Aligns specific temperature, conductivity, and oxygen 

measurements with their corresponding pressure measurements. 

6. In-house program: Converts temperature to ITS-90 scale (UNFSCO/JPOTS 

1991). 

7. CELLTM: Removes conductivity cell thermal mass effects from conductivity 

data. 

8. LOOPEDIT: Marks the scan where CTD is moving less than the minimum 

velocity or traveling backwards due to ship roll . 

9. DERIVE: Computes dissolved oxygen and depth. 

10. BINAVG: Averages the data into 1 m. depth bins. 

214 
11 . DERIVE:- Computes salinity (PSS-78), density (EOS80), potential temperature 

(Pot.Temp), specific volume anomaly (SVA), and sound velocity (ChenMillero) using Fofonoff and Millard's (1983) formulas. 

All CTD salinity data were calibrated against bottle data. SEFSC used the PVWAVE'1' program to interrogate and verify salinity values. TIO CTD salinity data 

were also verified against bottle values. Any differences were found to be 

within the accuracy range of the instrument. Corrections from temperature 

and salinity sensor calibrations were also made. These sensors were sent to 

Sea-Bird Electronics, Inc. for calibration after 100 hydrocasts . 

All XBT temperature data were corrected and integrated with CTD temperature 

data to compute isotherm depths. The 20°C, 15°C, and 8°C isotherm depths were 

used to show shallow, mid-water, and deep features, respectively. 

The fresh water fraction was computed for 0 to 3 m water depths for each 

Pelican CTD station. The salinity values for these depths were averaged, and a 

reference salinity value of 36.560 psu was used. This value was obtained from 

the LATEX A program and was the highest salinity value obtained from their 

1992 LATEX HO1 cruise. The following equation developed by Dinnel and 

Wiseman (1986) was used to determine the fresh water fractions for the study 

area: 

Sb-S Where: Sb = Reference salinity 

F - Sb S = Salinity average 

F = Fresh water fraction. 

6.2.4.2 Chlorophyll Data 

Chlorophyll a and phaeopigment concentration values for Pelican/ Longhorn 

CTD stations are listed in Appendix C of Volume III. The chlorophyll 

concentrations for the upper 100 m at each CTD station were used to define a 

chlorophyll function, c = f (d), where c represents chlorophyll 

concentrations and d equals depth (up to 100 m) . The integral of this function 

was then calculated numerically using the trapezoidal approximation: 

I99 ICd+L-Cdl + Cd 

d=O 

The individual integral values were treated statistically by season. 

The surface chlorophyll data from Oregon II cruises were gridded and an 

interpolated surface was computed for each cruise leg. The interpolated values 

were then extracted for each transect and marine mammal position. 

6.2 .4.3 Continuous Flow-through Systems 

The MIDAS continuously recorded data (Pelican cruises) were processed with 

an in-house program that cut cruise track-lines from the continuously 

recorded file and plotted raw data with no corrections. 

215 
The Oregon II continuous flow-through salinity and temperature data were 

processed and integrated into the overall transect database. Due to spiking and 

high frequency noise, both temperature and salinity data were low-pass 

filtered after collection . The low-pass filter used a spatial frequency cutoff of 1 

cycle per 2 grid units (i .e ., all spatial frequency signals higher than 1 cycle 

per 2 .2 km were eliminated) . 

6.2.4.4 Dynamic Height 

Height differences in the ocean surface would normally be expressed in 

relation to sea level, but since these differences are reflective of variations in 

pressure, oceanographers have devised dynamic height to relate these 

differences . Dynamic height is calculated from a distribution of water 

densities and actually shows a water column's ability to do work due to the 

differences in geopotential. In short, it is the potential for gravity to do work 

because of the height of the water in relation to some reference level (Pickard 

and Emery 1990) . Differences in dynamic height (topography or geopotential 

height) provide oceanographers with a measure of the horizontal pressure 

gradient from which geostrophic flow or current velocities may be derived. 

Corrected XBT data were combined with corrected CTD data to compute dynamic 

heights. A micro VAX 3600 computer was used for the calculations of dynamic 

height and mass transport/geostrophic velocity between station pairs, as 

described by Biggs et al. (1991) . The dynamic height computations for the 

Pelican/Longhorn Cruises 1-4, and 6-7 were referenced to the 800 db surface. 

Pelican Cruise S and all the Oregon II cruise calculations were referenced to 

the 500 db surface (as 500 m was the maximum depth sampled). Hofmann and 

Worley (1986) have shown empirically that the optimum reference level of no 

motion in the central and western Gulf is near the bottom boundary of the 

Antarctic Intermediate Water (AAIW) at a depth of 850 to 950 m. Their model is 

supported by transport calculations for anticyclonic eddies (Biggs 1992). 

6.3 Hydrographic Results 

6.3 .1 Cruise Summaries 

From April 1992 through June 1994, the GulfCet program gathered 

hydrographic data from seven Pelican/Longhorn surveys (one cruise per 

season) and four Oregon 17 surveys (three spring and one winter) in the 

north-central and western Gulf of Mexico (refer to Table 6.1) . The results from 

these hydrographic surveys are presented in this section. 

The first survey (Cruise 1), was a spring cruise aboard the University of Texas 

at Austin's ship, R/V Longhorn. This cruise was divided into three legs because 

of personnel transfers and inclement weather. The following are the dates for 

each leg of the cruise : Leg 1: 15-17 April 1992, Leg 2: 20-21 April 1992, and Leg 

3: 23 April-1 May 1992 . No navigation or meteorological system was available 

for this cruise . Technical difficulties with the CTD hydrocasts resulted in fewer 

CTD stations being sampled than had been planned, but more XBTs were 

deployed to compensate for this (Table 6.2) . A complete description of all 

216 
Table 6.2 . The type and number of stations and samples taken on 

the Oregon II and Pelican/ Longhorn hydrographic 

cruises. 

CID XBT Salinity Chlorophyll 

Stations Stations Samples Samples 

Oregon u 

Cruise 199- Spring 1992 111 114 232 85 

Cruise 203- Winter 1993 106 76 79 107 

Cruise 204 Spring 1993 128 136 108 151 

Cruise 209- Spring 1994 117 184 100 - 

Pelican/Longhorn 

Cruise 1- Spring 1992 (Longhorn) 17 % 157 171 

Cruise 2- Summer 1992 44 78 84 273 

Cruise 3- Fall 1992 39 77 75 436 

Cruise 4 Winter 1993 44 85 80 476 

Cruise 5- Spring 1993 42 75 84 111 

Cruise 6- Summer 1993 38 95 146 341 

Cruise 7- Fall 1993 32 74 75 216 

Totals 

Oregon 11 462 510 519 343 

Pelican/Longhorn 256 580 701 2024 

Combined 718 1090 1220 2,367 

Pelican/ Longhorn hydrographic cruises. may be found in TAMUG's 

hydrographic data technical reports (Fargion and Davis 1993a, 1993b, 1993c, 

1993d, 1994x, 1994b, and 1994c). 

Following Cruise 1, all Teas A&M University cruises were conducted aboard 

the R/V Pelican. This vessel presented several advantages over the R/V 

Longhorn: increased platform stability for the marine mammal visual survey, 

increased laboratory space, and a continuously recording navigation and 

meteorological system. 

All of the National Marine Fisheries Service surveys were aboard the NOAH 

Ship Oregon II. The first Oregon II survey (Cruise 199) was a spring cruise. 

This cruise was divided into three legs; Leg 1 : 17 April-4 May 1992, Leg 2: 6 

May- 25 May 1992, and Leg 3: 26 May- 8 June 1992 . The first two legs covered 

the off-shelf waters of the northern Gulf between 83°-96°W longitude. These 

legs were part of the SEAMAP ichthyoplankton survey . The third leg 

concentrated on the GuIfCet study area between 87°-96°W longitude . Further 

information concerning this cruise's hydrographic data may be found in 

Cruise Results: NOAH Ship Oregon II Cruise 92-02 (199) (U.S . Dept. of Commerce 

1992). 

Track-line 1 was dropped from the station plan for the first Pelican summer 

cruise (Cruise 2) and in all succeeding cruises due to schedule constraints. 

217 
Refer to the technical report for Cruise 2 for complete details (Fargion and 

Davis 1993b) . Pelican Cruise 3 was a fall survey that did not sample track-line 

10 or a portion of line 11 due to inclement weather. 

The second Oregon II survey (Cruise 203) occurred in the winter of 1993 . This 

survey also consisted of three legs, all essentially within the GulfCet study area 

between 87°-96°W longitude. This cruise covered north-south transects 

between the 90 m to 1830 m contours of this area. The chronological 

breakdown of the cruise is as follows: Leg 1 : 5-17 January 1993, Leg 2 : 18-30 

January 1993, Leg 3 : 1-14 February 1993 . A more detailed summary of this 

cruise's hydrographic data may be found in Cruise Results: NOAH Ship Oregon 

II Cruise 93-01 (203) (U.S . Dept. of Commerce 1993a) . 

Fargion and Davis (1993d) summarize the winter Pelican survey (Cruise 4) in 

their fourth hydrographic technical report. Pelican Cruise S was a spring 

survey that dropped track-line 2 from the station plan due to ship time 

scheduling constraints. In addition, CTDs were cast to a maximum of 500 m. 

Cruise 204 was the second Oregon II spring cruise that surveyed the SEAMAP 

and GulfCet study areas. It was separated into three parts: Leg 1 : 3-17 May 1993, 

Leg 2: 18 May- 2 June 1993, and Leg 3 : 415 June 1993 . Cruise Results: NOAH Ship 

Oregon II Cruise 93-02 (204) (U.S . Dept. of Commerce 1993b) details the 

hydrographic collections made during this survey. 

The second summer Pelican survey (Cruise 6) dropped track-lines 2 and 3 from 

the station plan because of ship schedule restrictions . A maximum depth of 800 

m was used for the CTD to save time. The last Pelican cruise (Cruise 7) was 

completed in late fall of 1993 . Track-lines 2, 3, 4, and half of 5 were dropped 

from the station plan due to crew member illness that required the ship to 

return to Galveston. The cruise was aborted at that point. Again, 800 m was the 

maximum depth to which the CTD was lowered to maximize available time. 

Following the recommendation of the GulfCet Scientific Review Board (SRB), a 

spring Oregon II cruise was made rather than a second winter survey. This 

decision was based in the better sighting conditions that exist in the spring. 

This fourth survey was Oregon II Cruise 209, and was the last survey to be 

completed for the GulfCet project . This cruise was divided into four 

legs: Leg 1: 16-23 April 1994, Leg 2 : 27 April-7 May 1994, Leg 3: 8-13 May 1994, 

and Leg 4: 1418 May 1994. Cruise Results: NOAH Ship Oregon II Cruise 209 

presents a summary of this survey (U.S . Dept. of Commerce 1994) . 

Hydrographic data for the four Oregon II cruises are included in Appendix B, 

and Data for Pelican/ Longhorn Cruises 1-7 are included in Appendix C. The 

combined total of hydrographic stations sampled by the GulfCet program were 

1,808. 

6.3 .2 Temperature-Salinity Relationships 

Temperature versus salinity (T-S) plots were computed for all the Pelican and 

Oregon II surveys (Figures 6 .5 and 6.6). These plots show that for temperatures 

colder than 18°C there is a close T-S relationship with little scatter. This 

indicates that waters in the study area constitute essentially a single system. 

218 
35 . 1 

30 .000 

U 25 .000 

9 20 .000 

15 .000 

10 .000 

5

.26 

o° o° o° o° 1313 1313 o13 011 o° 

%Q" ,1:1

. 

1P, 

Zti . 22 . 

.000 28 .000 30 . 00C 

LD 

O 

0 G 

O 

2 . 000 34 . 000 36 . 000 3B Oi )0 

Salinity, PSS-7g (+ vU) 

Figure 6.5 . Temperature versus salinity relationship for all TIO CTD stations 

from Pelican and Longhorn cruises (1-7) . 

219 
35 .000 

o° o° o° o° o° o° o° o° 

30 .000 

. . . 

+> 

.} .: . ~ : 

V 0 25 .000 

Y 

W 

20 .000 

15 .000 

o° 22 

i 

O 

G0 

OP 

10 .000 

OP 

5 .00o L26 .000 

Figure 6.6. 

28 .000 30 .000 32 .000 34 .000 36 .000 38 .000 

Salinity, PSS-78 (PSLn 

Temperature versus salinity relationship for all SEFSC CTD 

stations from Oregon II cruises (199-209) . 

220 
Data from the combined GulfCet hydrographic stations reveals a distinct 

salinity maximum greater than 36 .6 practical salinity units (psu) with an 

accompanying temperature of approximately 22-23° C. The minimum salinity 

of less than 34.9 psu excludes the surface fresh water found near the 

Mississippi River plume (which was as low as 12 .8 psu) . These salinity 

signatures are characteristic of SUW and AAIW, respectively. Usually the SUW 

salinity maximum is centered at about 200 m and the AAIW salinity minimum 

in the eastern Gulf occurs between the depths of 800 to 1,000 m (shallower in 

the western Gulf) (Nowlin and McLellan 1967). The intense salinity maximum 

of the SUW was found in the region of the Loop Current and in warm 

anticyclonic eddies derived from this current. 

Pelican/ Longhorn cruises detected several warm anticyclonic eddies in the 

GulfCet study area that were characterized by a salinity greater than 36.6 psu. 

These eddies were: Eddy Triton (T) (Cruise 2), Eddy Unchained (U) (Cruise 2), 

(Eddy Vazquez (V) (Cruises 3 and 4), Eddy Whopper (W) (Cruise 6), and Eddy 

Extra (X) (Cruises 6 and 7) . Figure 6 .7 presents an example of the T-S 

characteristics of one of these eddies, in this case, Eddy V. The T-S plots 

characterizing the other eddies are found in Appendix C. The T-S plots can be 

used to describe the spin down of an eddy. While an eddy is spinning down, the 

salinity maximum will gradually decrease as more GCW (salinity 36.4 to 

36.5 psu) mixes with the core water. 

XBT temperature versus depth data have been plotted to show the temperature 

range, the depth range of the mixed layer, and the interannual variability of 

temperature profiles during all GulfCet surveys in the period 1992-1994. XBT 

temperature data have been plotted by probe type. Figures 6.8 and 6.9 present 

T-7 probe data for all Pelican and Oregon II surveys, respectively . 

Surface water temperatures throughout the study area ranged from 16.8°C to 

30.4°C. The mixed layer was seasonally deepest in the winter, ranging from the 

shallower spring-summer depth range of 0-SO m and a fall-winter depth range 

of 35-110 m. A good deal of the scattering observed in the temperature profiles 

may possibly be due to the presence of warm or cold eddies in the Gulf. 

6.3 .3 20°C, 15°C, and 8°C Isotherm Depths 

The 20°C, 15°C, and 8°C isotherm depths were used to show shallow, mid-water, 

and deep features, respectively. For this study the 20°C, 15°C, and 8°C isotherm 

contour maps were always compared with the dynamic height topography 

maps (over 800 db) to assure that the features were accurately characterized. A 

comparison between the 15° and 8° C isotherms can reveal different sizes and 

areas of eddy location and whether the vertical axis of the eddy core is tilted . 

The phenomenon of eddies shedding from the Loop Current in the northern 

Gulf is a major feature of the meso- to large-scale Gulf circulation. Eddies are 

shed at a rate of one to three per year (Berger 1995). These warm-core 

anticyclonic rings have a diameter of 300-400 km and a possible depth 

signature of about 600 m. Topographical gradients in the isothermal surface 

indicate the position of the eddies . In particular, doming isotherms may 

represent the initial stages of development of a cyclonic eddy feature. Cyclonic 

221 
35 . 

30 .000 

0 25 .Q00 

G 

H 20 .000 

15 .000 

10 .00 

5

.026 

o° o° o° o° o° o° o° o° o° 

D 

i 

i 

.000 e8 . 000 30 . 000 32 .000 34 .000 

O O 

ro 

?: 

. F-' . . . 

v 

v 

00 38 OI 

Salinity, PSS-78 (PSin 

)0 

Figure 6 .7 . Pelican fall Cruise 3 temperature versus salinity relationship 

for CTD stations within Eddy V. 

222 
0 

100 

200 

300 

F 

400 

u 

A 

500 

600 

goo 

soo 

. . . 

. . .~ ;'' 

sfr r~~ 't~ ._ . 

. '~ 

'r . . 

A`l 

-=3 P 

J r? . 

5 10 15 20 25 30 35 

Temperature (deg. Q 

Figure 6.8 . T-7 XBT temperature data for Pelican and Longhorn Cruises 1-7 

(every sixth data point was plotted). 

223 
IIX 

20( 

300 

400 

d 

A 

500 

600 

1 .,. . 

~ S- 'i' ?} 

.,~ . : . 

".s:.~i '~ Z ff 

~ i, I j.: 

: ( ~~(f. t 

- , ~~ ,i - ti J 7 . r t 

` r , 

700 

OW 

5 10 15 20 25 30 

Temperature (deg. Q 

Figure 6.9 . T-7 XBT temperature data for Oregon II Cruises 199-209 (every 

sixth data point was plotted) . 

224 
eddies are peripherally linked to a primary anticyclonic eddy and evolve in 

strength during subsequent stages of eddy-slope interaction. This 

intensification of the anticyclonic-cyclonic eddy pair (oppositely rotating 

vortices) has been observed in the western Gulf in the past (Merrell and 

Morrison 1981, Brooks and Legeckis 1982, Merrell and Vazquez 1983, Brooks 

1984). 

Within eddies, there is spatial variability in all isotherm depths. This feature 

can be useful as an eddy detecting tool . Within the upper temperature profile 

of a warm-core (anticyclonic) eddy, the isotherm depths are found to be below 

those of the surrounding water column (i .e ., the 15°C isotherm depth may be 

depressed as much as 100 m lower than its average depth outside the eddy) . 

Regions where the temperature surface is deep or depressed correspond to 

anticyclonic (clockwise or warm) eddies. Conversely, shallow (or doming) 

temperature surfaces correspond to cyclonic (counterclockwise or cold) 

eddies. When the western Gulf surface waters are warmer than 15°C, the 

temperature isobaths appear to be relatively flat . Therefore, the 15°C and 20°C 

isotherms do not always detect cyclonic (cold) eddies. By comparison, the 8°C 

isotherm exhibits the greatest depth difference, often being depressed more 

than 150 m in an anticyclonic eddy. Therefore, the 8°C isotherm depth was the 

best contour level to use for detecting anticyclonic and cyclonic features. 

Figures 6.10 through 6.12 are examples of the three isotherm depth 

topographies used to analyze the temperature data from Pelican winter Cruise 

4. Of these, the 8°C isotherm, when referenced with the dynamic height 

topography from the same cruise, proved to be the most useful tool to detect 

warm and cold water eddies at depths greater than 800 m. In shallower water, 

the 15° C isotherm is the only usable isotherm with which to detect warm or 

cold eddies. A NOAH-AVHRR SST (°C) satellite image of the western Gulf (Figure 

6 .13) from the same period (February 1993) verified that the position of 

anticyclonic Eddy V in the western Gulf corresponded to that which the 8°C 

isotherm detected. The presence of the cold/cyclonic eddy paired with Eddy V 

could not be verified by this satellite image even though the 8°C isotherm had 

detected it . The warm-core eddy also visible near the Mississippi delta was 

apparent from the isotherm data, but disappeared rapidly from AVHRR images. 

The three isotherm contours were prepared for all the GulfCet hydrographic 

surveys and are located by cruise in Appendices B and C. 

6.3 .4 Dynamic Height 

Dynamic topography highs and lows were used to describe anticyclonic and 

cyclonic eddies. An estimate of an eddy's life span is nine months to one year, 

and as an eddy ages it spins down. Spinning down means that an eddy loses 

vorticity, and as a consequence, external water begins to mix with the discrete 

inner core water. The changes in dynamic height can be an indicator of the 

life span of a particular eddy. 

Prominent anticyclonic eddies in the western Gulf were detected during the 

GulfCet 1992-199 hydrographic surveys . Although dynamic heights were 

computed for each of the Oregon II CTD stations, due to the spatial resolution of 

225 
N 

N 

-94.5 -93.5 -92.5 -91.5 -90.5 

-60m~ 

27. 

O 

80 m 

26. ~~ ~ ^ ~~ l 

zs 

-92.5 

-89.5 -88.5 

-87.5 

.5 

5 

5 

Figure 6.10. Topography of the 20 °C isotherm based on all XBT and CTD data from winter Pelican Cruise 4, 12-27 

February 1993 . 
N 

N 

v 

-95.5 

29 

2001 

27.5- 

26.t 

25 

-88.5 -87.5 

i 

v 

-92.5 -91.5 -90.5 -89.5 -88.5 -87.5 

.5 

.5 

.5 

5 

Figure 6.11 . Topography of the 15 °C isotherm based on all XBT and CTD data from winter Pelican Cruise 4, 12-27 

February 1993 . 
-95.5 -94.5 -93.5 -92.5 -91 .5 -90.5 -89.5 -88.5 -87.5 

N 

N 

00 # 

_6 

26.T 

9.5 

8.5 

7.5 

5 

-95.5 -94.5 -93.5 -92.5 -91 .5 -90.5 -89.5 -88.5 -87.5 

Figure 6.12. Topography of the 8 °C isotherm based on all XBT end C'fD data from winter Pelican Cruise 4, 12-27 

February 1993. 
N 

N 

Figure 6.13. NOAH-AVHRR SST (°C) analysis of the western Gulf of Mexico for 12 February 1993, coinciding with 

the beginning of Pelican winter cruise 4 (image courtesy of Coastal Studies Institute, Louisiana State 

University) . 
the station grid and the shallowness of the CTD casts (i .e ., 200 m), dynamic 

heights could not be assigned to any individual oceanographic feature nor 

could a dynamic height contour map be produced for the Oregon II surveys. 

6 .3 .5 1992-1994 Oceanographic Features 

The following summary describes the major hydrographic features found in 

the GulfCet study area by season during the 1992-1994 period. Table 6.3 lists the 

cruise, the feature identified during that cruise, the depth of the 8°C isotherm, 

and the dynamic height used to identify these features. The locations of these 

features are referenced by Pelican survey track-lines (see Figure 6.4 for 

orientation of track-lines) and general direction: 

Spring 1992: Eddy Triton (T) was present in the northwestern Gulf during 

spring 1992 as determined by Oregon II Cruise 199 and by 

Longhorn Cruise 1 . The Longhorn located the following 

features: anticyclonic Eddy T on track-line 1 ; a cyclonic eddy 

on track-line 7 ; and the north edge of the Loop Current on 

track-lines 12-13 . An overview of the Gulf was given by Oregon 

II survey 199 . The Loop Current strongly intruded into the 

north-central Gulf and the cyclonic eddy associated with Eddy T 

moved to the northwestern corner of the Gulf (see isotherm 

figures in Appendix B and C). 

Summer 1992: Eddy T was also seen on Pelican Cruise 2 (August 1992) on 

track-lines 2 and 3 with an associated strong cyclonic eddy on 

track-line 5 . During this cruise, Eddy U, a new Loop Current 

eddy, was seen in the central area of the study on track-line 8 

with an associated cyclonic eddy on track-line 11 . Figure 6.14 is 

a composite figure of dynamic heights and the track of LATEX C 

drifter buoy number 447 for the month of August 1992. The 

dynamic height contours show the position of Eddy T in the 

western part of the map and just the northern tip of Eddy U in 

the center. The drifter is within Eddy U itself, and its track 

implies both the size and location of this eddy. 

Fall 1992: The third Pelican cruise detected anticyclonic Eddy V on tracklines 2 to 4. Eddy V was completely surveyed by this cruise and 

a drifter buoy, and it had a diameter of 100 km. Figure 6.15 is 

also a composite of dynamic heights and LATEX C drifter 

buoy #247 track for November 1992 . This figure clearly shows 

the location of the eddy as well as the circulation pattern 

within the eddy. 

Winter 1993: Data from both Pelican Cruise 4 and Oregon II Cruise 203 

showed that anticyclonic Eddy V had moved northward onto the 

continental slope and retained the same diameter as noted on 

the previous cruise . The Pelican located Eddy V on track-lines 

2 and 3 with an associated cyclonic eddy. Also detected was a 

second strong cyclonic eddy on track-lines 9 and 10 associated 

with an anticyclonic eddy on track-line 12 (see Figure 6.12) . 

230 
Table 6 .3 . Oceanographic features located during 1992-1994 and 

their properties. 

Survey Date Oceano- 8°C Iso- Dyn. 

graphic therm Height 

Feature Depth (m) (800 db) 

Longhorn spring 15 May 1992- Eddy T > 625 115 

Cruise 1 1 Apr 1992 cyclonic Eddy < 475 

Loop Current > 600 

Oregon II spring 15 May 1992- Loop > 700 

Cruise 199 8 tun 1992 Current 

Pelican summer 10 Aug 24 1992- NW corner of 

Cruise 2 24 Aug 1992 Eddy U & > 600 > 135 

cyclonic Eddy < 475 

Eddy T& > 625 > 125 

cyclonic Eddy < 425 

Pelican fall 8 Nov 1992- Eddy V 650 140 

Cruise 3 22 Nov 1992 

Oregon II winter 4 Jan 1993- Eddy V & >650 

Cruise 203 14 Feb 1993 cyclonic eddy 

Pelican winter 12 Feb 1993- Eddy V & 625 125 

Cruise 4 27 Feb 1993 cyclonic eddy, 

"no name" Eddy & 600 110 

cycionic eddy < 400 

Oregon II spring 6 May 1993- Loop Current & 

Cruise 204 13 Jun 1993 ps. Eddy V > 750 

Pelican spring 23 May 1993- Eddy V 

Cruise 5 5 June 1993 not present, 

cyclonic eddies < 450 

Pelican summer 28 Aug 1993- North side Eddy 575 125 

Cruise 6 S Sep 1993 W & cyclonic < 450 

eddy, & Eddy X > 675 > 145 

Pelican fall 3 Dec 1993- North side 

Cruise 7 14 Dec 1993 Eddy X & > 625 >125 

cyclonic eddies < 475 

Oregon II spring 12 Apr 1994- 

Cruise 209 10 Jun 1994 

This anticyclonic eddy was not named, but was also evident in 

satellite images (see Figure 6.13). Cruise 203 data showed the 

development of a strong cyclonic eddy and confirmed the 

presence of Eddy V in the NW corner of the study area in 

January. A contour of the dynamic heights obtained from 

Cruise 4 is represented in Figure 6.16. 

Spring 1993 : Data from the Pelican's second spring survey (Cruise 5) showed 

that a very complex topography existed. There were small, 

strong cyclonic eddies and possibly a small anticyclonic eddy 

in the northeast corner of the Gulf (Figure 6.17) . An overview 

231 
30.5 

29.5 

28.5 

27.$ 

N 

W 

N 

26.5 

25.5 

24.5 

-95 .5 -14.5 -y3 .5 

10 Aug 

0 -

31 Adf 

i 

-95 .5 -94.5 

-tS`J .S -tStS .J -2S/ .J 

1,5 

1 

Aug 

25 A 

.5 -90.5 -89.5 -88.5 -87.5 

30.5 

29.5 

28.5 

27.5 

26.5 

25.5 

2,4.5 

Figure 6.14. Pelican summer Cruise 2 surface dynamic topography (dyn crn) with respect to 800 m, with LATEX-C 

drifter #2447 track from August 1992 superimposed (drifter track data courtesy of LATEX-C 

program) . 
30.5 

29.5 

28.5 

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24.5 1 i I I . . . . ' 24.5 -95.5 -94.5 -93.5 -92.5 -91 .5 -90.5 -89.5 -88.5 -87.5 

Figure 6.15. Pelican fall Cruise 3 surface dynamic topography (dyn cm) with respect to 800 m, with LATEX-C 

drifter #2447 track from November 1992 superimposed (drifter track data courtesy of LA'T'EX-C 

program) . 
30.5 

29.5 

28.5 

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Figure 6.16 . Pelican winter Cruise 4 surface dynamic topography (dyn cm) with respect to 800 m. 
30.5 

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28.5 

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24.5 " ' 24.5 -95.5 -94.5 -93.5 -92.5 -91 .5 -90.5 -89.5 -88.5 -87.5 

Figure 6.17. Pelican spring Cruise 5 surface dynamic topography (dyn cm) with respect to 500 m. 
of the Gulf was provided by Oregon II Cruise 204. The Loop 

Current showed a strong intrusion with cyclonic eddies on its 

western side, while Eddy V spun down (see Appendix C isotherm 

figures) . Eddy V was no longer present in the northwest Gulf 

by early June. Eddy V showed a very complex cyclonicanticyclonic-cyclonic triad system that interacted, merged, and 

separated over the period of late March through May 1993 

(Jockens et al . 1994) . The detection, spin down, and fate of Eddy 

V were all possible to determine by merging all ship and 

satellite data. 

Summer 1993: Pelican Cruise G located anticyclonic Eddy W on track-lines 47. 

Eddy W was elongated and squashed with an associated cyclonic 

eddy on track-line 7. A second very strong, anticyclonic Eddy X 

was found on track-line 12, and was possibly associated with a 

cyclonic eddy (Figure 6 .18) . Eddy X had a diameter of about 300 

km. 

Fall 1993 : The last Pelican survey, Cruise 7, detected a cyclonicanticyclonic-cyclonic ring triad. Eddy X was found on trackline 9, with cyclonic eddies on either side. Eddy X moved east 

along the 2,000 m isobath (Figure 6.19) . Eddy X interacted with 

an isolated cyclonic eddy on the lower continental slope 

between 93° and 92°W causing the cyclonic eddy to move 

eastward towards the Loop Current. 

Spring 1994: The last Oregon II survey, Cruise 209, covered most of the 

northern Gulf, but no major features were observed. 

6.3.6 Chlorophyll _a Concentrations 

Chlorophyll concentrations can be used as an estimate of primary 

productivity. Oceanographic features such as upwelling, eddies, and fresh 

water inflow are associated with increased nutrient levels and, therefore, with 

increased chlorophyll concentration . High chlorophyll concentrations 

indicate an area that may also have an accompanying increase in densities of 

higher trophic level species upon which marine mammals feed. In such cases, 

chlorophyll concentration may affect marine mammal distribution. 

Literature on primary productivity indicates that most oceanic regions of the 

Gulf are oligotrophic . Data from ship surveys of the 1960s and 1970s showed 

that the surface mixed layer of the Gulf seldom had a chlorophyll 

concentration of more than a few tenths of a milligram per cubic meter, and 

that these waters were also depleted of nitrates and were low in zooplankton 

biomass (El-Sayed 1972, Biggs 1992). However, within the Mississippi River 

plume, at the cooler periphery of the Loop Current where the nutracline was 

shallower, and in other local regions where higher nutrient concentrations 

are present, chlorophyll concentration is dramatically increased (Biggs 1992) . 

Vertical profiles of chlorophyll a_ concentrations from the Pelican inshore 

(100 m CTD stations), offshore (all CTD stations except 100 m), and the 

236 
30.5 

29.5 

28.5 

27.5 

v 

26.5 

25.5 

-y5.5 -J4 .5 

24.5 1 

-95.5 -94.5 -93.5 -92.5 -91 .5 -90.5 -89.5 -88.5 -87.5 

Figure 6.18 . Pelican summer Cruise 6 surface dynamic topography (dyn cm) with respect to 800 m. 

-Y1.5 -y1 .5 -JU.S -tSy .S -2ftS .5 

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Figure 6 .19 . Pelican fall Cruise 7 surface dynamic topography (ciyn cm) with respect to 800 m. 

30.5 

29.5 

28.5 

27.5 

26.5 

25.5 

24.5 
Mississippi River plume stations in the Gulf were plotted (Figures 6.20-6 .22). 

Chlorophyll values presented a gradient inshore to offshore (high to low) with 

no clear secondary chlorophyll peak. Chlorophyll A concentrations from 

inshore stations had a mean of 0.4 mg/m3 and a maximum of 28.15 mg/m3, 

while offshore stations had a mean of 0.2 mg/m3 and a maximum of 2.74 

mg/m3 . 

To examine seasonality, chlorophyll a concentrations for all Pelican CTD 

stations were integrated (up to 100 m) by season, and these integrals were then 

plotted. Figures 6.23 and 6 .24 are the resulting contour maps for Pelican 

Cruises 3 and 4. Surface chlorophyll a concentration values from all Pelican 

CTD stations were plotted by quarter. The winter quarter consisted of the 

months December, January, and February; the spring quarter included March, 

April, and May; and so on. These divisions made it possible to determine if a 

spring or fall bloom occurred, which would have been the expected result . The 

data indicate that neither a spring nor fall bloom occurred during the survey 

period. Even when the Mississippi River influence was removed as a possible 

bias, the data show that no spring or fall phytoplankton bloom occurred. While 

seasonal chlorophyll signals were detected in the surface chlorophyll a_ values 

(Table 6.4), they seemed to be a poor estimate of integral water column 

chlorophyll values. 

No integral values were obtained for Pelican Cruise 5 ; it was the only survey 

used to obtain the spring values, and was a shorter survey with fewer CTD 

stations. Longhorn Cruise 1 was the other spring survey and this cruise, due to 

equipment failures, resulted in fewer CTD stations than had been desired. 

Along with fewer samples to begin with, the chlorophyll data from this cruise 

were available for only half the CTD stations. "Hot spots" of chlorophyll were 

detected offshore in Pelican Cruises 3 and 4. The higher values on Cruise 3 

(integral = 40 mg/m-' ) were probably due to fresh water that was pushed 

seaward to at least the 1,000 m isobath by northeasterly winds (refer to Figure 

6 .23 and wind figures later in this section) . The high values 

(integral >_ 65 mg/m') seen in Pelican Cruise 4 were located at the edge of a 

warm anticyclonic eddy ("no name") off the Mississippi delta (see Figure 6.24) . 

The other area showing a high offshore value (integral = SO mg/m2) was 

probably related to fresh water from the Mississippi River extending offshore 

from wind forcing. 

Replicates of chlorophyll samples for Pelican track-line 4, totaling 117 

samples, were analyzed by the LATEX A program. These HPLC data were used as 

quality control for TIO analyzed samples from the Pelican. 

6.3 .7 Mississippi River Discharge 

The Mississippi and other rivers, with their associated nutrient and sediment 

loads and pollutants, have a great impact on all aspects of continental margin 

oceanography in the northern Gulf. The discharge/plume relationship 

reveals that wind forcing is a critical factor in determining plume size and 

orientation (Walker and Rouse 1993) . The Mississippi River plume was detected 

239 
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Integrals (mg/m2) of chlorophyll il concentrations versus depth (up to 100 m) for Pelican fall 

Cruise 3 . 
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24.5 

Figure 6.24. Integrals (Mg/M2) of chlorophyll a concentrations versus depth (up to 100 m) for Pelican winter 

Cruise 4. 
Table 6.4 . Chlorophyll 1 concentrations (mg/m3 ) and integral 

values (mg/m2) by quarter for all seven Pelican 

cruises (100 m integration level) . 

Minimum Maximum Mean Standard N 

Deviation 

Surface Chl a Conc. (All) 0.001 10.884 0.416 1.023 203 

Chl I Integrals (All) 5.198 79.624 24.260 11.510 155 

Surface Chl a Conc. (Spring) 0.061 2.646 0.310 0.495 29 

Chl .a Integrals (Spring)1 

Surface Chl a Conc. (Summer) 0.025 10.884 0.512 1.622 49 

Chl .a Integrals (Summer) 5.198 40.536 20.240 8.160 35 

Surface Chl a Conc. (Fall) 0.037 0.715 0.243 0.160 59 

Chl a Integrals (Fall) 9.310 46.765 21 .894 8.174 52 

Surface Chl g Conc. (Winter) 0 6.201 0.551 1.054 66 

Chl .a Integrals (Winter) 8.670 79.624 26.740 12.654 68 

1 Insufficient data to generate integral values. 

using shipboard data, fresh water fraction maps, salinity maps, sea surface 

temperature (from continuous flow-through data), and AVHRR satellite 

images. 

Two major events related to the Mississippi River plume and fresh water input 

into the Gulf occurred during the study period 1992-1993 . The first occurred in 

the fall of 1992 when Mississippi River fresh water extended outward into the 

Gulf to the 1,000 m isobath. The second event was the "great" flood during 

summer 1993 . Colder coastal waters were trackable as a distinct plume using 

sea surface temperature AVHRR (4, 11, and 13 October 1992) images. Figure 6.25 

shows the mushroom-shaped plume, with a maximum observed extrusion to 

27°N and 91°30'W. This plume was advected between two eddies, Eddy V and 

Eddy U, surveyed by LATEX C, Survey F04 (P. Hamilton personal 

communication). This fresh water extrusion was still detectable during the 

Pelican November 1992 Cruise 3. It is clear from Pelican Cruise 3 data that the 

fresh water seen in late October traveled further seaward, extending as far as 

the 2,000 m isobath. Figure 6.26 is a plot of the Pelican Cruise 3 wind data that 

was extracted from the MIDAS database. This diagram clearly showed the 

northeasterly to easterly wind forcing responsible for the southerly intrusion 

of the Mississippi River plume fresh water into the Gulf. 

During the summer of 1993, anomalously high rainfall vas experienced over 

the midwestern U.S.A. During the subsequent flood, the Mississippi River 

discharge was described as streaming to the east (Walker et al. 1994). This was a 

rare occurrence as ordinarily the flow of fresh water is to the west. This event 

was shown in an August 1993 satellite image (Figure 6.27) and confirmed by 

Pelican salinity data (from Cruise 6). Salinity contours representing the usual 

flow of fresh water discharge (August 1992) as well as the anomalous August 

1993 discharge may be found in Appendix C. Wind was also thought to be a 

significant factor in the eastward flow of the river water during this interval. 

245 
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associated primary SST fronts on (a) 4 October 1992, (b) 11 October 

1992, and (c) 13 October 1992 . The mushroom-shaped plume can 

be seen by 13 October (courtesy Coastal Studies Institute, 

Louisiana State University) . 

246 
30 

29 

28 

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Figure 6 .26. Pelican fall 1992 Cruise 3 MIDAS wind data plotted to show the northeasterly to easterly wind forcing 

in area of freshwater extrusion near 27°N, 91°30'W. 
N 

Figure 6.27. NOAH-AVHRR reflectance analysis of the central-western Gulf of Mexico for 10 August 1993 (image 

courtesy of the Coastal Studies Institute, Louisiana State University). 
Figure 6.28 shows the wind data taken on Pelican Cruise 6 and the 

northwesterly to westerly wind patterns that prevailed during late August 

1993 in the region east of the Mississippi River delta. 

Figures 6 .29 and 6 .30 shows the fresh water content (or fraction) for August 

1992 and August 1993, respectively. A maximum of 56% of fresh water was 

found east of the Mississippi delta during the summer cruise in 1993 . The 

Mississippi River water was moving eastward, and the fresh water fraction 

remained above 20% to 87°30'W and seaward to the 2,000 meter contour. The 

values east of the delta were approximately double those obtained in August 

1992 (see Figure 6.29). The fresh water fraction southwest of the delta was 35% 

in 1993, considerably lower than the fresh water contribution east of the delta, 

but slightly higher than that encountered in August 1992 (22g'o). 

6.4 Discussion of the Major Oceanographic Features of the 

North-Central and Western Gulf of Mexico 

The GulfCet hydrographic surveys have focused on sampling the meso- to 

large-scale features of the northwestern (NW) and central (C) Gulf of Mexico. 

GulfCet data often showed eddy progression to the edge of the Texas-Louisiana 

shelf. The GulfCet hydrographic program obtained valuable information from 

other MMS sponsored hydrographic studies concurrently sampling the 

northern Gulf. These studies included the three program units of the 

Louisiana-Texas Shelf Physical Oceanography Program (LATEX); LATEX A 

studied the shelf circulation, LATEX B the Mississippi river plume, and LATEX C 

the eddy system. The Ship of Opportunity Program (SOOP) also examined the 

eddy system over the continental slope of the NW Gulf. Therefore, a total of 

five recent studies have investigated the hydrographic features found in the 

northwestern to central Gulf, and together provide a nearly comprehensive 

hydrographic data set for this area for the period 1992-93 . 

The primary physical oceanographic components of the Gulf are the Loop 

Current, eddies derived from this feature, and the Mississippi River plume. 

Eddies are important physically and biologically because they function as 

pumps, mixing water masses and their constituent organic and inorganic 

compounds. An eddy's capability for moving great distances coupled with its 

vorticity and ability to diverge circulation all have an affect on the location of 

upwelling and downwelling regions in the Gulf. Biggs and Muller-Karger 

(1994) suggested that the co-occurrence of cyclonic circulation cells in 

association with anticyclonic eddies may enhance primary productivity by 

increasing nutrient resources in the upper 200 m. Cyclonic eddies lead to 

higher production because of the increased upward nutrient flux at their 

periphery. Furthermore, this cyclonic eddy-anticyclonic eddy pairing 

transports high-chlorophyll shelf water seaward at least 100-200 km. 

Over the period 1992-1993, ship surveys, aerial surveys and satellite coverage 

allowed continuous monitoring of the Loop Current, eddy shedding, and eddy 

propagation at unprecedented spatial and temporal resolution. At least three 

anticyclonic eddies (U, W, and X), each with a diameter of at least 300 lm, were 

shed from the Loop Current during this period, and moved with their 

249 
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28 

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26 

25 

.0 

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Figure 6 .28. Pelican summer Cruise 6 MIDAS wind data plot to show the northwesterly to westerly wind forcing 

in the Mississippi River plume region. 
N 

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Figure 6.29 . Fresh water fraction (%) for 0-3 m depths during the August 1992 Pelican summer Cruise 2 . 
30.5 

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v, 

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Figure 6.30. Fresh water fraction (9'0) for 0-3 m depths during the August 1993 Pelican summer Cruise 6. 
associated cyclonic eddies into the western Gulf. Table 6.5 is a chronological 

listing of the eddies or oceanographic feature found by each of the 

aforementioned programs. This table summarizes the survey dates, identity of 

the study program, and the eddy characteristics (i .e ., the depth of the 8 °C 

isotherm and the dynamic height computed at a 800 db reference level) . The 

integration of hydrographic data with satellite data paints a picture of a 

complex region where cyclonic-anticyclonic pairs and cyclonic-anticycloniccyclonic triad systems interact, merge, and separate several times before 

disintegrating, or "spinning down" (Jockens et al. 1994). 

6.4.1 Eddy Histories 

Eddy Triton (T) detached from the Loop Current in late summer 1991 with a 

diameter over 300 km (Biggs et al. 1992). This warm anticyclonic eddy was 

located in the central Gulf for several months and drifted slowly to the westsouthwest (WSW) . Biggs et al. (1992) reported a probable interaction between 

the Loop Current and Eddy Triton in early December 1991 . By January 1992, 

Triton was nearly circular in shape with a diameter of 270 km and exhibited a 

dynamic height higher than 125 dyn cm at 800 db (Biggs and Muller-Karger 

1994, Biggs et al. 1995a) . Eddy Triton was located near 25 .7°N and 91°W and 

moved west of 94°W after April 1992. Pelican Cruise 1 located the northwestern 

perimeter of Triton (refer to Appendix C isotherm and dynamic height 

figures) . By late summer 1992, Eddy Triton interacted with the continental 

margin of the NW Gulf and had a dynamic height of 125 dyn cm. During this 

interaction with the continental margin, the anticyclonic eddy may have shed 

vorticity as local regions of cyclonic circulation (Biggs et al . 1992). 

Eddy Unchained (U) was shed in mid-summer 1992. This eddy was detected by 

satellite altimeter data as an anticyclonic geopotential anomaly of +SO dyn cm 

(Hamilton et al. 1994, Hamilton et al . 1995, Biggs et al. 1995a) and captured an 

Argos drifter buoy 02447 from the Louisiana shelf in August 1992 . During 

Pelican Cruise 2 (August 1992), new Eddy U, as well as old Eddy T, were present 

in the central and NW corner of the Gulf. Eddy Triton was spinning down in 

the NW corner with a dynamic height of 125 dyn cm, while Eddy U had a 

dynamic height greater than 140 dyn cm. Eddy U was vigorous and large, about 

300 km diameter, and centered at 25°N and 90°W in September 1992 (Hamilton 

et al. 1995) . 

In early fall 1992, drifter trajectory and satellite altimeter data showed that 

this vigorous eddy (U) soon cleaved into two eddies; a minor eddy, Eddy V, with 

a dynamic anomaly of +27 dyn cm, and a major eddy, still referred to as Eddy U, 

with a dynamic anomaly of +40 dyn cm portions (Biggs et al . 1994) . A NOAAAVHRR image of 12 October 1992 captured these two eddies after the split 

(Figure 6.31). Two hydrographic surveys by SOOP and LATEX C in late October 

confirmed the separation of V from U (Table 6.5). Dynamic heights at this time 

were 142 dyn cm and 157 dyn cm for Eddies V and U, respectively (Biggs et al . 

1994, Hamilton et al . 1995). 

During this period, the smaller Eddy V was centered at the base of the 

northwestern continental slope, NW of Eddy U. Between September and 

253 
Table 6.5 . Oceanographic features located during GulfCet, LATEX, 

and SOOP programs during 1992-1993, and their 

properties. 

Survey Date Oceanographic 8°C Iso- Dyn. 

Feature therm Height 

Depth (m) (800 db) 

GulfCet 15 May 1992- Eddy T > 625 m 115 cm 

Longhorn Cruise 1 1 Apr 1992 

GulfCet 15 May 1992- Loop 

Oregon 11 Cruise 199 81un 1992 Current 

LATEX C 7 Aug 1992 Eddy U SSO m 115 cm 

Surveys F02 & F03 9 Aug 1992 

GulfCet 10 Aug 24 1992- NW corner 

Pelican Cruise 2 24 Aug 1992 of Eddy U, > 600 m > 135 cm 

Eddy T 625 m > 125 cm 

LATEX C 11 Oct 1992 Eddy U both 

Survey F04 & Eddy V > 675 m 

SOOP 28 Oct 1992- Eddy U 718 m 157 cm 

Cruise 92G-13 31 Oct 1992 & Eddy V 656 m 142 cm 

GulfCet 8 Nov 1992- Eddy V 650 m 140 cm 

Pelican Cruise 3 22 Nov 1992 

LATEX C 19 Dec 1992 Eddy V 650 m 

Survey FOS 

LATEX C 4 Jan 1993 Eddy V >650m 135 cm 

Surveys F06 & F07 6 Jan 1993 

SOOP 9 Jan 1993- Eddy V 678 m 133 cm 

Cruise 93G-01 12 Jan 1993 

GulfCet 4 Jan 1993- Eddy V >650 m 

Oregon II Cruise 203 14 Feb 1993 

GulfCet 12 Feb 1993- Eddy V & 625 m 125 cm 

Pelican Cruise 4 27 Feb 1993 "no name" Eddy 600 m 110 cm 

LATEX C 12 May 1993 Eddy V 600 m 

Surveys F08 & F09 16 May 1993 (moving inshore) 

GulfCet 6 May 1993- Loop Current & >750m 

Oregon II Cruise 204 13 June 1993 ps. Eddy V 

GulfCet 23 May 1993- Eddy V 

Pelican Cruise S S June 1993 not present 

SOOP 1 June 1993- Eddy W 788 m 170 cm 

Cruise 93G-07 4 June 1993 

LATEX C 28 Aug 1993- North side Eddy W 575 m 125 cm 

Surveys FO 10 & FO 11 S Sept 1993 & Eddy X > 675 m > 145 cm 

GulfCet 28 Aug 1993- North side Eddy W 575 m 125 cm 

Pelican Cruise 6 5 Sept 1993 & Eddy X > 675 m > 145 cm 

LATEX C 28 Oct 1993 

Survey F012 31 Oct 1993 Eddy W 650 m 

1 Nov 1993 

GulfCet 3 Dec 1993- North side 

Pelican Cruise 7 14 Dec 1993 Eddv X > 625 m >125 cm 

LATEX C 16-18 Dec 1993 Eddy X > 625 m 130 cm 

Surveys F013 & F014 23 Dec 1993 

254 
N 

C!t 

Figure 6.31 . NOAH-AVHRR SST (°C) analysis of the western Gulf of Mexico for 12 October 1992, showing Eddy V 

(northwest) and Eddy U (southeast) (image courtesy of the Coastal Studies Institute, Louisiana State 

University) . 
December 1992, Eddy V moved westward along the 2000 m isobath. The 

remainder of Eddy U advected WSW until it eventually collided with the 

continental margin of the western Gulf in spring 1993 (Biggs et al . 1994) . 

Pelican Cruise 3 surveyed Eddy V at the base of the NW corner of the Gulf in 

late November 1992. The dynamic height of Eddy V was greater than 140 dyn 

cm. At the same time, Eddy V was entirely surveyed by LATEX A drifter 02447. 

In the later half of December 1992, Eddy V abruptly moved northward onto the 

continental slope in the northwest corner of the Gulf. It remained about the 

same size (100 km diameter) and in approximately the same position through 

May 1993 (Fargion et al . 1994a). In January 1993, a LATEX C survey showed Eddy 

V to have a dynamic height greater than 135 dyn cm (Fargion et al. 1994a) . In 

the ensuing months, Eddy V continued to spin down while remaining in the 

same region (NW corner) . In late February 1993, Pelican Cruise 4 found that 

Eddy V continued to spin down to a dynamic height of -125 dyn cm. A NOAAAVHRR image in mid-February confirmed the location of both Eddies U and V 

in the Gulf, with Eddy U in the southwest and Eddy V spinning down in the 

northwest. This image also indicated an interaction between the two eddies 

with a subsequent water exchange. 

Altimetry data suggested that Eddies U and V began to coalesce by mid-to lateMarch 1993 (Jockens et al. 1990 . In early April, LATEX C drifter buoy 02449, 

which had been circulating in Eddy U, shot north-northeast into the region 

occupied by Eddy V (Figure G .32) . Throughout the remainder of April, the 

joined eddies were centered about 24.5°N and 96°W with an arm of Eddy U 

extending to the Northeast into the region formerly occupied by Eddy V. 

Thereafter, Eddy V existed primarily as an "arm" extension of Eddy U. Satellite 

altimetry data showed the presence of cyclonic rings on the northwest and 

southeast flanks of Eddy V (Jockens et al. 1994) . As April progressed, the arm 

strengthened and extended further north. A NOAA-AVHRR image in mid-April 

confirmed the altimeter data (Figure 6.33) . 

In late April to early May 1993, Eddy V was beginning to pinch off Eddy U 

again, and had its center located at -27°N x 94.5°W. A LATEX A hydrographic 

cruise confirmed the presence on the shelf of the anticyclonic Eddy V with 

associated cyclonic circulation to its northwest (Jockens et al. 1994) . Altimetry 

and NOAH-AVHRR data confirmed that Eddy U and Eddy V separated again by 

the second week of May (Jockens et al. 1994) . The thermal structure obtained 

from a LATEX C aerial survey in mid-May also showed a full separation of Eddy 

V from Eddy U, and a weakening of Eddy V. LATEX A drifter buoy 6938, deployed 

in early-May, circulated in Eddy V throughout May. Satellite altimeter data also 

indicated cyclonic circulation to the South of Eddy V (Jockens et al. 1994) . All 

that remained of Eddy V by the end of May to the beginning of June was a 

small region adjacent to the shelf with a weak, generally anticyclonic 

circulation, and a broad region of cyclonic circulation to the Southeast. 

Pelican Cruise S in early June showed, at the most, a weak anticyclonic or 

absent anticyclonic circulation in the former region of Eddy V. At the same 

time, drifter buoy 6938 changed course to move in a cyclonic loop to the 

southeast of Eddy V (Jockens et al. 1994). Eddy V appears to have dissipated 

completely in the summer of 1993 . There is evidence from June and July 

drifter tracks that a cyclonic eddy may have formed in that region. 

256 
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separately and after their coalescing (drifter buoy data courtesy of the LATEX-C program). 
~ 

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258 
Eddy W was formed in June 1993. It was a large eddy that formed a subsidiary 

warm anticyclonic eddy, similar in size to Eddy V, at the base of the 

continental slope at about 93°W. Unlike Eddy V, however, this northern 

portion of Eddy W apparently interacted with a cyclonic eddy on the lower 

slope and moved rapidly south by southeast, away from the slope in August 

1993 (Hamilton et al. 1994, Hamilton et al. 1995, Biggs et al. 1994). Eddy W had 

an anticyclonic geopotential anomaly of +SS dyn cm. Similar to Eddy U, Eddy W 

also split into two portions shortly after its formation. A combination of 

drifter, altimeter, and SST data showed that the northern (minor) portion 

collided with the continental margin of the NW Gulf in summer 1993, while 

the southern (major) portion advected WSW and collided with the western 

margin of the Gulf in December 1993 (Biggs et al. 1994, Fargion et al. 1995). 

Eddy W was surveyed in August of 1993 by GulfCet Pelican Cruise 6 while the 

eddy moved in the NW corner of the Gulf. At that time, the eddy had a "bone" 

shape with a dynamic height of 120 dyn cm. 

By late August 1993, Eddy X, a large vigorous Loop Current eddy, was located by 

ship (Pelican Cruise 6) on the continental slope at 89.5°W. It was shown to 

have a dynamic height of 145 dyn cm . Eddy X interacted with an isolated 

cyclonic eddy on the lower slope between 93° and 92°W, causing the cyclonic 

eddy to move eastward towards the Loop Current (Hamilton et al. 1995) . Eddy X 

subsequently moved westward along the base of the slope as tracked by 

AVHRR satellite data in late November (Figure 6.34) . In the satellite images 

from that time, Eddy W appears to be in the NW corner with the cyclonic eddy 

on the eastern side. In early December 1993, Eddy X was surveyed by ship 

(Pelican Cruise 7) and had a dynamic height greater than 125 dyn cm. 

Analysis of NOAH-AVHRR satellite data suggests that Eddies W and X began to 

coalesce by mid-to-late December, and by early January 1994 they appear to 

have formed one large eddy with elongated arms (Figure 6.35) . Unfortunately, 

altimeter data were not available for this period. Intense cloud cover over the 

Gulf resulted in poor NOAH-AVHRR satellite coverage for the first quarter of 

1994. The first cloud-free NOAH-AVHRR image was in early May 1994, and no 

evidence of Eddy W or X was found in the NW corner of the Gulf (Figure 6.36) . 

Clarification of the fate of these eddies will probably be resolved by the LATEX 

C and SOOP programs, which are to be conducted through 1995. 

The life span of the minor portions of Eddies U and W averaged 9 months, while 

the longevity of the major portions of these eddies averaged 12 months before 

being lost into the background altimeter sea surface height (SSH) signal 

(Biggs et al. 1994) . A possible four-stage scenario for dissipation of these eddies 

has been suggested by Dietrich and Lin (1994): 

1 . The dissipation first occurs at the outer edges, probably because of lateral 

mixing, so that the eddy decreases in size while maintaining its intensity. 

2. This mining reduces the swirl near the outer edges of the surface eddy, so 

the outward pressure gradient causes an outward secondary flow. 

259 
N~1 

O 

Figure 6.34. NOAH-AVHRR SST (°C) analysis of the western Gulf of Mexico for 28 November 1993, indicating Eddy 

X (north-central) and Eddy W (northwest) (image courtesy of the Coastal Studies Institute, Louisiana 

State University). 
N 

rr 

Figure 6.35 . NOAH-AVHRR SST (°C) analysis of the western Gulf of Mexico, showing the Loop Current (L.C.) and 

Eddies X and W merging together to form one large eddy in the SW corner of the Gulf. Drifter buoy 

tracks from 15 December 1993 to 4 January 1994 have been superimposed over this image to show 

the coalescence (image courtesy of the Coastal Studies Institute, Louisiana State University, and 

drifter buoy data courtesy of the LATEX program) . 
N 

N 

Figure 6.36. NOAH-AVHRR SST (°C) analysis of the western Gulf of Mexico for 7 May 1994, showing the Loop 

Current (L.C.) with no eddies visible in the central or NW Gulf (image courtesy of the Coastal Studies 

Institute, Louisiana State University). 
3. Mass conservation requires upwelling of cooler water in the central region 

of the surface eddy, which is trapped above the thermocline because of the 

strong stratification . 

4. The upwelling requires an inward flow toward the eddy core just above the 

thermocline. The cooling between the surface and the thermocline lowers 

the surface pressure and increases the pressure near and below the 

thermocline, resulting in a downward intensification of the eddy. 

In terms of water properties, these eddies appear as warm salty bodies. Salinity 

plots, referenced at 25 and 26 sigma-theta, show that the vertical structure of a 

recent Loop Current eddy is strongly concentrated above the thermocline, 

while old anticyclonic eddies in the western Gulf may be strongly 

concentrated below the surface (200-300 m) . Temperature-salinity (T-S) plots 

of CTD hydrocasts made near ring centers show that four eddies, T, U, V, and X 

can be distinguished by the high salinity signature of their SCU (> 36.5 PSU in 

the range 24.5-26.6 sigma theta) (Appendix C Figures) . The T-S relationships 

show the salinity signature of the GCW as the eddies spin down. 

6.4.2 Freshwater Influx 

Two major events related to the Mississippi River occurred during 1992-1993 . 

The first occurred in the fall of 1992 when Mississippi River fresh water 

extended outward into the Gulf to the 1,000 m isobath. The second event was 

the "great" flood of 1993, occurring during the summer of 1993 . Walker and 

Rouse (1993) reported an unusual Mississippi River plume feature which 

occurred in October 1992 . Under maximum discharge and under strong 

northeasterly winds, shelf water was rapidly forced away from the Mississippi 

River delta and over the continental slope, extending from 88°20'W to 90°SO'W 

and offshore farther than the 1,000 m isobath. This observation was confirmed 

the following month by Pelican Cruise 3, but the fresh water intrusion into 

the Gulf had extended to the 2,000 m isobath. This nutrient laden fresh water 

resulted in higher values of chlorophyll at those stations touched by the 

plume. 

A combination of natural variability and global-scale circulation anomalies 

during the 1992-1993 period resulted in severe and persistent precipitation 

over the central United States and brought the total flow of the Mississippi 

River to new records. Monthly mean Mississippi River discharges during 

April and May were 50% higher than the long-term mean (1930-1992), and 

August discharge was higher than the long-term peak monthly mean 

discharge which usually occurs in April (see Chapter 2, Figure 2 .4) . 

Undoubtedly, the 1993 El Nino/Southern Oscillation (ENSO) event contributed 

to the flooding of the Mississippi and Missouri River valleys (Richards et al. 

1994). The "Great Flood" of the Mississippi River caused significant changes to 

the landscape and ultimately, to the coastal ocean. This flooding event was 

exceptional for the season (summer), duration (weeks to months) and 

magnitude, all of which created unusual hydrographic features in the Gulf of 

Mexico. The effects of fresh water were detected not only in the northern 

Gulf, but also in the Florida Keys and along the U.S . east coast (Walker et al . 

1994). The most obvious effect of the increased Mississippi River flow was 

increased nutrient influx with correspondingly increased phytoplankton 

263 
concentrations . Dortch (1994) reported that total phytoplankton 

concentration during the period of flooding was more than an order of 

magnitude greater than normal . High concentrations of phytoplankton result 

in an increased carbon flux to the bottom, either as a result of dead plankton 

sinking or as zooplankton fecal pellets. This higher flux consequently creates 

large areas of hypoxia. 

Wind measurements from Louisiana coastal stations and continental shelf 

buoys suggested that the eastward flow of the Mississippi plume in the summer 

1993 was at least partially wind-driven (Walker et al. 1994). From mid-July 

through August, surface winds along the Louisiana coast were predominantly 

westerly and southwesterly. Another concurrent factor was the presence of 

Eddy X at the delta of Mississippi. Eddy X could have enhanced the eastward 

direction of the plume of fresh water. 

Previous studies have shown a positive correlation between the Mississippi 

River flow and the interannual variations in chlorophyll concentration, 

which in turn influences the development of primary productivity in the 

Gulf. Higher chlorophyll values have been found in association with the fresh 

water influx from the Mississippi river and cold cyclonic eddies (Fargion et al. 

1994b). 

6.4.3 Conclusion 

The Pelican and Oregon II sampling grid has proved to be useful in sampling 

the meso- to large-scale features of the Gulf of Mexico . GulfCet was able to 

detect all the major eddies and events present in the northern Gulf from 1992 

to 1993 . The hydrographic sampling program was able to detect all the major 

warm-core eddies as well as their affiliated cold-core eddies. The detection of 

these cold or cyclonic eddies is particularly significant as upwelled water, with 

its subsequently higher nutrient and oxygen content, is the result of these 

oceanographic features. 

The study area in 1992-1993 presented a complex hydrographic scenario. The 

following features were seen: a) new warm anticyclonic eddies with associated 

cyclonic eddies moved in and out the northern Gulf; b) recently formed warm 

anticyclonic eddies interacted with older eddies in the northwestern corner of 

Gulf; and c) unusual fresh water outflow extended offshore as far as the 2,000 

m isobath in fall of 1992 and in the summer of 1993 fresh water discharge 

streamed to the east of the delta. As a result of eddy movement, each of the 

GulfCet surveys had a unique opportunity to view meso- to large-scale 

hydrographic features. No generalizations can be made regarding eddy path, 

residence time, or frequency of occurrence in the study area. Generally, 

however, after separation from the Loop Current, anticyclonic eddies drift 

westward until their progress is halted by the northwestern continental slope, 

in the "eddy graveyard". Some eddies reached the western margin in just a few 

months, while others took longer and cleaved into secondary eddies during the 

westward transit. Recent altimeter studies indicate that the interaction of these 

Loop Current eddies with the western margin of the Gulf of Mexico could drive 

a Western Boundary Current there (Biggs et al . 1995b) . 

264 
In the north-central and western Gulf, anticyclonic warm eddies with their 

affiliated cold cyclonic eddies and the fresh water influx from the Mississippi 

River are the primary features which can enhance primary productivity and 

subsequently increase production at higher trophic levels . Biggs and MiillerKarger (1994) reported that the continental slope of the NW Gulf is a region 

where pelagic predators are abundant. Since these predators (such as skipjack, 

blackfin tuna, blue marlin, swordfish, and shark) require consistent food 

sources, they are not likely to be sustained by low primary productivity or 

infrequent episodes of enhanced primary productivity. Primary productivity, 

therefore, must be maintained relatively consistently . Particular areas where 

this level of production are likely to remain relatively consistent are the 

Mississippi River plume vicinity and the area just peripheral to the eddy 

pathway from the Loop Current. It is suspected that cetacean food sources, as 

well, would most likely be concentrated in these areas of consistently higher 

primary productivity. Cetacean foraging efficiency would be maximized when 

effort was concentrated in these areas . Therefore, it would seem likely that 

these areas would be preferred habitats for many marine mammals present in 

the Gulf. 

6 .5 Literature Cited 

Berger, T. 1995 . Loop Current eddy shedding cycle, pp. 385-388. In 

Proceedings of the Thirteenth Annual Gulf of Mexico Information 

Transfer Meeting, 46 December 1993, New Orleans, LA. OCS Study MMS 

940061 . U. S. Dept. of the Interior, Minerals Management Service, Gulf 

of Mexico OCS Region, New Orleans, LA. 

Biggs, D.C. 1992. Nutrients, plankton, and productivity in a warm-core ring in 

the western Gulf of Mexico. J . Geophys. Res. 97:2143-2154. 

Biggs, D.C., and F. Miiller-Karger . 1994. Ship and satellite observations of 

chlorophyll stocks in interacting cyclone-anticyclone eddy pair in the 

western Gulf of Mexico. J. Geophys. Res. 99:7371-7384. 

Biggs, D.C., M.M . Crawford, D. Salas de Leon, O. Salas Flores, and S. Escoto 

Hidalgo. 1991 . A US-Mexico cooperative study of a cold-core ring in the 

western Gulf of Mexico, pp. 374-380. In Proceedings of the 11th Annual 

Gulf of Mexico Information Transfer Meeting, 13-15 November 1990, 

New Orleans, LA. OCS Study MMS 91-000. U.S. Dept. of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

LA. 

Biggs, D.C., D. Lopez-Veneroni, A. Vazquez de la Cerda, and R. Kozak. 1992. 

Repeated observation of the Gulf Loop Current Eddy "Triton" in 1992 

from Ship-of-Opportunity oceanographic cruises and Argos-tracked 

surface drifters . Presented at The IX Congreso Nacional de Oceanografia, 

23-25 November 1992, Veracruz, Mexico. 

Biggs, D.C., G.S . Fargion, P. Hamilton, K. Schaudt, J . Feeney, R. Leben, L Rouse 

and N. Walker. 1994. Ship, aircraft, and satellite observations of the 

Loop Current Eddies U and W and their interaction with the continental 

margin of the Western Gulf of Mexico. EOS 75 :212. 

265 
Biggs, D.C., G.S . Fargion, and P. Hamilton. 1995a. Repeated observations of an 

anticyclonic Loop Current eddy in 1992, pp 388-392. In Proceedings of 

the Thirteenth Annual Gulf of Mexico Information Transfer Meeting, 4-

6 December 1993, New Orleans, LA. U.S. Dept. of the Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 

Biggs, D.C., R.R. Leben, G. S. Fargion, and P. Hamilton. 1995b. The mesoscale 

Eddy Zoo and Western Boundary Current in the Gulf of Mexico, 10/92-

12/93. Presented at IUGG, XXI General Assembly, 2-14 July 1995, Boulder, 

Oa. 

Brooks, D.A. 1984. Current and hydrographic variability in the northwestern 

Gulf of Mexico. J. Geophys. Res. 89 :8022-8032 . 

Brooks, D.A., and R.V. Legeckis . 1982 . A ship and satellite view of 

hydrographic features in the western Gulf of Mexico . J. Geophys. Res. 

87:4195-4206. 

Brown, C.W., and H.E. Winn . 1989 . Relationship between the distribution 

pattern of right whales, Euba]aena glacialis, and satellite-derived sea 

surface thermal structure in the Great South Channel. Cont. Shelf Res. 

9:247-2G0. 

Dietrich, D.E., and C. Lin. 1994. Numerical studies of eddies shedding in the 

Gulf of Mexico. J. Geophys. Res. 99:7599-7615. 

Dinnel, S.P., and W.J . Wiseman, Jr. 1986 . Fresh water on the west Louisiana and 

Texas shelf. Cont. Shelf Res. 6 :765-784. 

Dortch, Q N.N. Rabalais, R.E. Turner, and G.T. Rowe. 1994. Respirations rates 

and hypoxia on the Louisiana shelf. Estuaries 17:862-872 . 

El-Sayed, S.Z. 1972 . Primary productivity and standing crop of phytoplankton 

in the Gulf of Mexico, pp. 8-13. In V.C. Bushnell, ed. Chemistry, primary 

productivity and benthic algae of the Gulf of Mexico. Ser. Atlas Mar. 

Environ., Folio 22. American Geographical Society, New York. 

Fargion, G.S ., and R.W. Davis. 1993a. GulfCet Cruise 01 hydrographic data. 

Tech. Rept. 93-O1-T. Dept. of Marine Biology, Texas A&M University, 

Galveston. 76 pp. 

Fargion, G.S ., and R.W. Davis. 1993b. GulfCet Cruise 02 hydrographic data. 

Tech. Rept. 93-02-T. Dept. of Marine Biology, Texas A&M University, 

Galveston. 127 pp. 

Fargion, G.S ., and R.W. Davis. 1993c. GulfCet Cruise 03 hydrographic data. 

Tech. Rept. 93-03-T. Dept. of Marine Biology, Texas A&M University, 

Galveston. 141 pp. 

266 
Fargion, G.S ., and R.W. Davis. 1993d. GulfCet Cruise 04 hydrographic data. 

Tech. Rept. 93-04-T. Dept. of Marine Biology, Texas A&M University, 

Galveston. 151 pp. 

Fargion, G.S ., and R.W. Davis . 1994a. GulfCet Cruise 05 hydrographic data. 

Tech. Rept. 94-O1-T. Dept . of Marine Biology, Texas A&M University, 

Galveston. 144 pp. 

Fargion, G.S ., and R.W. Davis. 1994b. GulfCet Cruise 06 hydrographic data. 

Tech. Rept. 94-02-T. Dept. of Marine Biology, Texas A&M University, 

Galveston. 144 pp. 

Fargion, G.S ., and R.W. Davis. 1994c. GulfCet Cruise 07 hydrographic data. Tech. 

Rept. 94-03-T. Dept. of Marine Biology, Texas A&M University, Galveston. 

124 pp. 

Fargion, G.S., D. C. Biggs, P. Hamilton, R. Leben, L. Rouse, and N. Walker. 1994a. 

Ship, aircraft, and satellite observations of the Loop Current Eddies V 

and X and their interaction with the continental margin of the 

Northern Gulf of Mexico. EOS 75 :212. 

Fargion, G.S., C. I. . Schroeder, and D. Brandon. 1994b. Hot spots of chlorophyll 

pigments in the northern Gulf of Mexico associated with cyclonic 

eddies. EOS 75 :51 . 

Fargion, G.S., D.C. Biggs, and P. Hamilton. 1995 . Dynamic topography of eddies 

U, V, and W in 1992-93 from GulfCet, aircraft, and Ship-Of-Opportunity 

surveys, pp. 399-408. In Proceeedings of the Thirteenth Annual Gulf of 

Mexico Information Transfer Meeting, 4-6 December 1993, New Orleans, 

LA. OCS Study MMS 94-0061 . U.S . Dept. of the Interior, Minerals 

Management Service, New Orleans, LA. 

Fofonoff, N.P., and R.C . Millard, Jr. 1983 . Algorithms for computation of 

fundamental properties of seawater. UNESCO Technical Papers in Marine 

Science, No. 44. UNESCO, Vendrome, France. 

Hamilton, P., G. S. Fargion, and D. C. Biggs. 1994. Survey of eddies U, V, W, and X 

in the central Gulf of Mexico with XBT's and ARGOS drifters. EOS 75 :212. 

Hamilton, P., G. S. Fargion, and D. C. Biggs. 1995. Survey of eddy U, V, and W 

with expendable probes and drifters, pp. 392-399. In Proceedings of the 

Thirteenth Annual Gulf of Mexico Information Transfer Meeting, 4-6 

December 1993, New Orleans, LA. OCS Study MMS 940061 . U.S . Dept. of 

the Interior, Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, LA. 

Hofmann, E.E, and S.J . Worley. 1986. An investigation of the circulation of the 

Gulf of Mexico. J. Geophys. Res. 91 :14221-14236. 

Jockens, A., Fargion G.S . and R. Leben. 1994. Observation of the Fate of Eddy V: 

April-June 1993. EOS 7 :212. 

267 
Merrell, W.J ., and J.M. Morrison. 1981 . On the circulation of the western Gulf 

of Mexico with observations from April 1978. J. Geophys. Res . 86:4181-

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Merrell, W.J ., and A.M. Vazquez. 1983 . Observations of changing mesoscale 

circulation patterns in the western Gulf of Mexico . J. Geophys. Res. 

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Nowlin, W.D., Jr., and H.J . McLellan . 1967. A characterization of the Gulf of 

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Pickard, G.L, and W.J . Emery. 1990. Descriptive physical oceanography: An 

introduction. Pergammon Press, New York. 320 pp. 

Richards, F., D. Miskus, and S. Changnon. 1994. Hydrometeorological setting, 

pp. 3-26. In M.J . Dowgiallo, ed. Coastal oceanographic effects of 1993 

Mississippi River flooding. Special NOAH Report. NOAH Coastal Ocean 

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Sea-Bird Electronics, Inc. 1992 . CTD data acquisition software, Seasoft version 

4.015 . Sea-bird Electronics, Inc., Bellevue, WA. 30 pp. 

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Survey, NOAH Ship Oregon II Cruise 92-02 (199) . National Marine 

Fisheries Service, Southeast Fisheries Science Center, Pascagoula, MS. 

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268 
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269 
VII. BIRD SURVEYS 

D. E. Peake 

7.1 Introduction 

The GulfCet project greatly expanded the knowledge of birdlife in the Gulf of 

Mexico. Although much information exists concerning the coastal seabirds, 

very little is known regarding the species composition, distribution, and 

abundance of pelagic birds in the north-central and western Gulf (Clapp et al. 

1982, Clapp et al. 1983) . Duncan and Harvard (1980) conducted the only other 

ship-based pelagic bird census in the Gulf. The Duncan and Harvard study was, 

however, narrowly focused on the waters offshore of Alabama. Fritts et al. 

(1983) conducted an aerial census in the Gulf, but it was primarily limited to 

the continental shelf. 

7.2 Methods 

Detailed bird observations were made on five Pelican cruises (numbers 3, 4, 5, 

6, and 7) and the four Oregon II cruises (see Chapter 3 for descriptions of 

transect lines, dates, etc.) . During a 30-month period, the survey effort 

consisted of 160 days at sea in the GulfCet study area. Fifty-five percent of the 

observations occurred during spring months, 28% during winter, 11% during 

fall, and 6% during summer. 

On Pelican Cruises 3-6, a bird observer collected data continuously during 

daylight hours. On-effort observations were defined in the same manner as for 

the shipboard marine mammal data collection (Chapter 3). Sampling occurred 

on the north-south track-lines and on transit between the track-lines. Bird 

observations were made with hand held 8X or lOX binoculars and were possible 

when marine mammal observations had been discontinued (up to Beaufort 6 

sea state and in light rain and fog). 

The methods described by Tasker et al. (1984) were used as a guide for the 

300 ni strip transect protocol followed for bird observations on Pelican Cruises. 

Sighting effort was concentrated on the area from the bow to the beam of the 

ship, out to a distance of 300 m on one side of the ship only. All birds seen 

within 300 m of the ship in this 90° arc were recorded. These data were used 

for density, distribution, and sighting rate analyses. Birds observed outside the 

strip area were also recorded, and these data were used for distribution and 

sighting rates. With the exception of Pelican Cruise 3, the distance from the 

bird to the observer was estimated using the Heineman range finder 

(Heineman 1981). On Cruise 3, the distance was visually estimated. Sighting 

data included time, latitude, longitude, species, number of individuals, distance 

of bird from the ship, behavior of the bird, association with other birds, age 

and sex of the bird (if this data could be determined), and other comments of 

the observer concerning the sighting. Weather and sea state were recorded at 

the beginning and end of each track-line . The line transect method was not 

used on Pelican Cruise 7, but bird species and the numbers of birds sighted 

were recorded. 

271 
For the Oregon II cruises, bird observations were done by the marine mammal 

observers during their normal observing duties . The observations were made 

along the same track-lines as the marine mammal census, but a 300 meter strip 

transect was not done. Observations were made using hand-held binoculars 

and 25X deck-mounted binoculars. Information regarding species, sighting 

time, location, and number of individuals was recorded and the observations 

were supervised by an experienced seabird observer. Detailed information 

regarding time spent on-effort looking for birds was not recorded. 

The American Ornithologists' Union (1983) "Check-list of North American 

Birds" and supplements to that list (1985, 1987, and 1989) were the basis for the 

bird nomenclature used. Group size for individual species was calculated by 

dividing the number of individuals seen by the total number of sightings. The 

latitude and longitude of each sighting was used to extract the estimated depth 

from the ETOPO-S data set (Herring 1993) . The Two-Sample Wilcoxin Rank-Sum 

Test was used to compare water depth at sighting location for several species 

pairs. The species were chosen because they were similar species or because 

they were often found in the same areas. 

7.3 Results 

Of the 3,276 total bird sightings, 2,692 were seabirds. Seabirds are defined as 

coastal, offshore, or pelagic species of birds which have their usual habitats 

and food sources in the sea (Harrison 1983). Non-seabird species accounted for 

584 sightings. Thirty-two species representing nine families and three orders 

of seabirds were observed (Table 7 .1) . The scientific and common names of the 

bird species sighted, a complete listing of the bird sightings, and distribution 

maps for species with only a few sightings are provided in Appendix A. 

7.3 .1 Seabirds 

Table 7.2 shows the seabird species sighted on Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209. The table includes information regarding 

species, the number of sightings, mean group size, group size range, and mean 

water depth and depth range at sighting locations. Table 7.3 summarizes the 

contribution of the most frequently sighted birds to the percent of the total 

seabird sightings . Overall sighting rates are listed by season and survey for 

each species in Table 7 .4. For Pelican Cruises 3-6, the sightings per hour of oneffort observation and number of individuals seen per hour of on-effort 

observation are listed in Tables 7.5 and 7.6, respectively . 

7.3 .1.1 Distribution 

Order Procellariiformes 

The Order Procellariiformes is represented by sightings of one unidentified 

Prerodroma sp. petrel, two shearwater species, and three storm-petrel species. 

Although only one previous record exists for the Gulf, the most likely 

Pterodroma species is the Black-capped Petrel which breeds on Hispaniola 

(Clapp et al. 1982). Cory's Shearwater was sighted five times at scattered 

locations throughout the study area on Pelican Cruises 3, S, and 6 (see 

?7? 
Table 7.1 . Orders, families, and species of seabirds identified on 

the GulfCet cruises. 

Order Procellariiformes 

Family Procellariidae (gadfly petrels and shearwaters) 

gadfly-petrel (Pterodroma sp.) 

Cory's Shearwater (Calonectris diomedea) 

Audubon's Shearwater (Pufflinus Iherminieri) 

Family Hydrobatidae (storm-petrels) 

Wilson's Storm-Petrel (Oceanites oceanicus) 

Band-romped Storm-Petrel (Oceanodroma castro) 

Leach's Storm-Petrel (Oceanodroma leucorhoa) 

Order Pelecaniformes 

Family Phaethontidae (tropicbirds) 

Red-billed Tropicbird (Phaethon aethereus) 

White-tailed Tropic-bird (Phaerhon lepturus) 

Family Pelecanidae 

Brown Pelican (Pelecanus occidentalis) 

Family Sulidae (gannets and boobies) 

Northern Gannet (Sula bassanus) 

Masked Booby (Sula dactylatra) 

Family Phalacrocoracidae (cormorants) 

Unidentified Cormorant (Phalacrocorax sp.) 

Family Fregatidae (frigatebirds) 

Magnificient Frigatebird (Fregata magnificens) 

Order Charadriiformes 

Family Scolopacidae (sandpipers and allies) 

Subfamily Phalaropidinae 

Red Phalarope (Phaloropus fulicaria) 

Family Iaridae (jaegers, gulls, terns, and skimmers) 

Subfamily Stercorariinae (jaegers) 

Pomarine Jaeger (Stercorarius pomarinus) 

Parasitic Jaeger (Srercorarius parasidcus) 

Subfamily Larinae (gulls) 

Laughing Gull (Larus atricilla) 

Franklin's Gull (Larus pipixcan) 

Bonaparte's Gull (Larus Philadelphia) 

Ring-billed Gull (Larus de]awarensis) 

Herring Gull (Larus argentatus) 

Subfamily Sterninae (terns) 

Gull-billed Tern (Sterna nilotica) 

Royal Tern (Sterna maxima) 

Sandwich Tern (Sterna sandvicensis) 

Common Tern (Sterna hirundo) 

Forster's Tern (Sterna forsreri) 

Least Tern (Sterna anrillarum) 

Bridled Tern (Sterna anaethetus) 

Sooty Tern (Sterna fuscata) 

Black Tern (Chilidonias niger) 

Brown Noddy (Anous stolidus) 

Subfamily Rynchopinae (skimmers) 

Black Skimmer (Rynchops niger) 

273 
Table 7.2. Seabird sighting results for Pelican Cruises 3-7, with 

range of water depth at sighting location, mean 

water depth at sighting location, and mean group size 

of seabirds sighted. 

Species 

Number 

Sightings 

Mean 

Depth 

(m) 

Depth 

Range 

(m) 

Mean 

Group 

Size 

Group 

Size 

Range 

Unidentified gadfly-petrel 1 1314 1314 1.0 1 

Cory's Shearwater S 1004 81809 1.4 1-3 

Audubon's Shearwater 127 1209 106-2184 1.7 1-32 

Wilson's Storm-Petrel 54 747 111-1998 1.4 1-12 

Band-tamped Storm-Petrel 41 1332 216-2714 1.3 1-5 

Leach's Storm-Petrel 8 719 405-113 1.1 1-2 

Unidentified storm-petrels 572 1044 19Cr1997 1.3 1-18 

Red-billed Tropicbird 1 1837 1837 1.0 1 

White-tailed Tropicbird 3 1805 1677-1982 1.3 1-2 

Magnificent Frigatebird 15 723 75-1772 1.5 1-4 

Unidentified frigatebird 15 828 192-1399 1 .8 1-11 

Unidentified cormorant 1 1335 1335 1.0 1 

Northern Gannet 49 792 103-2341 1 .9 1-20 

Masked Booby 77 1142 111-2306 1.2 1-4 

Brown Pelican 1 100 100 1.0 1 

Red Phalarope 1 1995 1995 1.0 1 

Unidentified phalarope 2 1132 85x1415 5.5 5-6 

Pomarine Jaeger 292 1335 109-2587 1.9 1-20 

Parasitic Jaeger 3 656 338-815 1.0 1 

Unidentified jaegers 247 1228 102-2380 2.0 1-100 

Laughing Gull 369 805 100-2313 2.1 1-50 

Franklin's Gull 1 1406 1406 1 .0 1 

Bonaparte Gull 1 219 219 1 .0 1 

Ring-billed Gull 16 982 131-1733 1 .8 1-5 

Herring Gull 102 699 9-2584 2.0 1-20 

Gull-billed Tern 1 471 471 1.0 1 

Royal Tern 17 674 9-2584 1 .3 1-3 

Sandwich Tern 19 795 102-1825 1.8 1-4 

Common Tern 7 1196 106-1410 1.7 1-2 

Forster's Tern 2 490 312-669 1.0 1 

Least Tern 3 592 106-1880 1 .7 1-2 

Bridled Tern 113 886 1042176 1 .8 1-25 

Sooty Tern 40 1212 125-1993 5.8 1-48 

Black Tern 412 752 75-2104 9.8 1-400 

Brown Noddy ? 1493 1396-1591 1.0 1 

Unidentified tern 251 1097 136-2006 1.2 1-2 

Black Skimmer 4 849 252-1431 2.2 1-3 

274 
Table 7.3 . Summary of seabird sightings by percentage of total 

seabird sightings . 

Percentage of total 

Species or species category seabird sightngs* 

Total terns (32.4) 

Total storm-petrels (25.1) 

Unidentified storm-petrels 21 .2 

Total jaegers (20.0) 

Black Tern 15.3 

Laughing Gull 13.7 

Pomarine Jaeger 10.8 

Unidentified tern 9.3 

Unidentified jaegers 9.2 

Audubon's Shearwater 4.7 

Bridled Tern 4.2 

Herring Gull 3.8 

Masked Booby 2.9 

Wilson's Storm-Petrel 2.0 

Northern Gannet 1.8 

Band-romped Storm-Petrel 1 .5 

Sooty Tern 1.5 

Total frigatebirds (1 . l ) 

* Species listed are those which compromise more than 1% of the total seabird sightings. 

Sighting percentages for species groupings listed in parentheses represent the totals of 

species categories also considered separately and should not be summed with other 

percentages . 

Appendix A for distribution map) . It is likely that the birds recorded as 

unidentified large shearwaters were Cory's Shearwaters, but sighting 

conditions did not permit separation from other possibilities, such as Greater 

Shearwater. However, it was possible to separate them from smaller 

shearwaters, such as Audubon's Shearwater. Audubon's Shearwaters were 

sighted on Pelican Cruises 3, 5, 6 (one sighting on Cruise 6 consisted of 32 

individuals), and 7 and Oregon II Cruises 199, 203, 204, and 209 (Figure 7.1). 

Sightings of small Puffin us sheanvaters were listed as unidentified small 

shearwaters if sighting conditions did not allow absolute separation of 

Audubon's Shearwater from other possibilities such as Manx Shearwater and 

Little Shearwater. Manx Shearwater is rare in the Gulf, while Little 

Shearwater has not been sighted there (Clapp et al. 1982) . 

Three species of the Family Hydrobatidae were sighted: Wilson's Storm-Petrel, 

Band-romped Storm-Petrel, and Leach's Storm-Petrel . Fifty-four Wilson's 

Storm-Petrel sightings were made on Pelican Cruise S and all three spring 

Oregon II cruises (Figure 7.2) . Forty-one sightings of the Band-romped Storm275 
Table 7.4. Mean bird sightings per day by season. 

Annual 

Species Spring Summer Fall Winter mean 

Black Tern 3.53 10.10 3.408 

Pomarine Jaeger 1 .16 2.61 2.59 1.590 

Laughing Gull 1 .75 0.50 0.28 3.76 1 .573 

Unidentified storm-petrel 5.24 0.30 0.11 0.06 1 .428 

Unidentified jaeger 0.93 2.17 2.28 1.345 

Audubon's Shearwater 0.89 3.50 0.33 0.15 1.218 

Unidentified tern 2.42 1 .30 0.78 0.04 1 .135 

Bridled Tern 0.98 2.50 0.06 0.885 

Herring Gull 0.03 0.28 1 .72 0.508 

Masked Booby 0.66 0.90 0.33 0.07 0.490 

Magnificent Frigatebird 1.50 0.375 

Sooty Tern 0.22 0.90 0.02 0.285 

Northern Gannet 0.01 0.22 0.81 0.260 

Sandwich Tern 0.10 0.50 0.06 0.165 

Wilson's Storm-Petrel 0.63 0.158 

Common Tern 0.02 0.50 0.130 

Band-romped Storm-Petrel 0.47 0.118 

Cory's Shearwater 0.05 0.20 0.11 0.090 

Royal Tern 0.09 0.10 0.15 0.085 

Ring-billed Gull 0.30 0.075 

White-tailed Tropicbird 0.01 0.20 0.06 0.068 

Least Tern 0.02 OZO 0.055 

Unidentified frigatebird 0.16 0.040 

Leach's Storm-Petrel 0.09 0.023 

Parasitic Jaeger 0.03 0.008 

Black Skimmer 0.03 0.008 

Unidentified phalarope 0.02 0.005 

Forster's Tern 0.02 0.005 

Brown Noddy 0.02 0.005 

Unidentified gadfly-petrel 0.02 0.005 

Unidentified cormorant 0.02 0.005 

Bonaparte's Gull 0.02 0.005 

Red Phalarope 0.01 0.003 

Red-billed Tropicbird 0.01 0.003 

Gull-billed Tern 0.01 0.003 

Franklin's Gull 0.01 0.003 

Brown Pelican 0.01 0.003 

276 
Table 7 .5 . Summary of bird sightings per effort-hour for 

Pelican Cruises 3, 4, S, and 6 as well as the mean for 

the four cruises combined. 

Pelican Cruise Overall 

3 4 5 6 mean 

Species (fall) (winter) (spring) (summer) 

(Birds/hour) 

Cory's Shearwater 0.030 

Audubon's Shearwater 0.074 

Unidentified gadfly-petrel 

Wilson's Storm-Petrel 

Band-romped Storm-Petrel 

Unidentified storm-petrel 0.015 

White-tailed Tropicbird 

Northern Gannet 0.029 

Masked Booby 0.044 

Magnificent Frigatebird 

Pomarine Jaeger 0.265 

Unidentified jaeger 0.40 

Laughing Gull 0.030 

Bonaparte's Gull 

Herring Gull 0.015 

Royal Tern 

Sandwich Tern 

Common Tern 

Least Tern 

Unidentified 

white backed tern 0.030 

Bridled Tern 

Sooty Tern 

Unidentified 

dark backed tern 

Black Tern 

Brown Noddy 

0.016 0.014 0.0149 

0.016 0.434 0.1309 

0.010 0.0025 

0.0000 

0.176 0.04110 

0.012 0.016 0.042 0.0212 

0.014 0.0035 

0.181 0.0526 

0.016 0.112 0.0430 

0.140 0.0350 

0.677 0.08 0.2475 

0.072 0.032 0.1360 

0.677 0.042 0.1871 

0.012 0.0030 

0.496 0.1277 

0.024 0.032 0.014 0.0175 

0.070 0.0175 

0.056 0.0140 

0.028 0.0070 

0.012 0.014 0.0139 

0.048 0.294 0.0855 

0.016 0.098 0.0285 

0.012 0.016 0.112 0.0350 

0.785 0.1963 

0.032 0.0080 

Petrel were made on Pelican Cruise S and Oregon II Cruises 199, 204, and 209 

(Figure 7.3) . Eight Leach's Storm-Petrel sightings were made on Oregon II 

Cruises 199, 204, and 209. Unidentified storm-petrels were sighted on Pelican 

Cruises 3-6 and all four Oregon II cruises (Figure 7.4) . The total of all stormpetrel (Wilson's Storm-Petrel, Band-romped Storm-Petrel, Leach's StormPetrel, plus unidentified storm-petrel) sightings represented 25 .1g'o of all of 

the GulfCet seabird sightings (Table 7.3). 

277 
Table 7.6 . Summary of the number of bird individuals seen per 

effort-hour on Pelican Cruises 3, 4, 5, and 6, 

including the mean for the four cruises. 

Pelican Cruise Overall 

3 4 5 6 mean 

Species (fall) (winter) (spring) (summer) 

(Birds/hour) 

Cory's Shearwater 0.030 0.048 0.014 0.0229 

Audubon's Shearwater 0.088 0.016 1.290 0.3485 

Unidentified gadfly-petrel 0.010 0.0025 

Wilson's Storm-Petrel 0.0000 

Band-romped Storm-Petrel 0.223 0.0558 

Unidentified storm-petrel 0.029 0.012 0.016 0.056 0.0284 

White-tailed Tropicbird 0.028 0.0070 

Northern Gannet 0.044 0.604 0.1621 

Masked Booby 0.074 0.016 0.126 0.0539 

Magnificent Frigatebird 0.196 0.0490 

Pomarine Jaeger 0.29 0.967 0.048 0.3275 

Unidentified jaeger 0.781 0.085 0.032 0.2244 

Laughing Gull 0.030 1 .170 0.042 0.3104 

Bonaparte Gull 0.012 0.0030 

Herring Gull 0.015 1.050 0.2662 

Royal Tern 0.024 0.08 0.028 0.0250 

Sandwich Tern 0.140 0.0350 

Common Tern 0.056 0.0140 

Least Tern 0.056 0.0140 

Unidentified 

white backed tern 0.044 0.012 0.014 0.0175 

Bridled Tern 0.064 0.509 0.1432 

Sooty Tern 0.032 0.504 0.1340 

Unidentified 

dark backed tern 0.012 0.032 0.140 0.0460 

Black Tern 23.200 5.8000 

Brown Noddy 0.032 0.0080 

Order Pelecaniformes 

Two species of the Family Phaethontidae were observed in the GulfCet study 

area. The Red-billed Tropicbird was spotted on Oregon II Cruise 209, and Whitetailed Tropicbirds were sighted on Pelican Cruises 3 and 6 and Oregon II Cruise 

204. All four sightings occurred in waters deeper than 1,500 meters. Clapp et al. 

(1982) have suggested that these species may be more common far offshore 

than previous records had indicated. 

278 
-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 

29 

28 

27 

26 

25 

!9 

:8 

;7 

b 

!5 

Figure 7 .1 . Audubon's shearwater sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

30 30 

29 

28 

27 

26 

29 

28 

27 

26 

25 r u 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 7 .2 . Wilson's Storm-Petrel sightings for Pelican Cruises 3-7 and 

Oregon 11 Cruises 199, 203, 204, and 209. 

279 
Figure 7 .3 . Band-rumped Storm-Petrel sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. 

Figure 7 .4. Unidentified Storm-Petrel sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199> 203, 204, and 209. 

280 
The Magnificent Frigatebird was the only member of the Family 

Fregatidae identified (Figure 7 .5) . The records for unidentified 

Frigatebirds (Figure 7.6) most likely represent Magnificent Frigatebirds, 

since no other species has been recorded in the Gulf (Clapp 

et al. 1982). Frigatebirds were noted only during the April-November time 

period. 

The Family Phalacrocoracidae was represented by one sighting of 

an unidentified Phalacrocorax spp. noted on Oregon II Cruise 203. Two 

species of the Family Sulidae were sighted, the Northern Gannet and 

the Masked Booby. Since the Gulf is primarily a winter habitat for 

the Northern Gannet (Clapp et al. 1982), it is not unexpected that all but 

one of the sightings occurred from November through February. 

Sightings of the Northern Gannet were made on Pelican Cruises 3, 4, and 7 

and Oregon II Cruises 203 and 209 (Figure 7.7) . The Masked Booby was seen 

on Pelican Cruises 3, 5, 6, and 7 and Oregon II Cruises 199, 203, 204, and 

209. However, 920 of the sightings occurred from April through November 

(Figure 7.8). 

A single Brown Pelican, Family Pelecanidae, was seen on Oregon II Cruise 199. 

This species is typically found nearshore and only rarely found farther than 

65 km offshore (Schreiber 1978). The paucity of GulfCet sightings was 

therefore as expected. 

Order Charadriiformes 

Only three sightings of Phalaropes (Family Scolopacidae, Subfamily 

Phalaropidinae) were made: one of Red Phalarope and two of 

unidentified phalarope (Table 7.2) . The Family Laridae, however, 

comprises 70.70 of all seabird sightings and contains 18 of the 32 species seen 

(Table 7.3) . 

Pomarine Jaegers were sighted 292 times on Pelican Cruises 3, 4, 5, and 7 and 

Oregon II Cruises 199, 203, 204, and 209 (Figure 7.9) . Parasitic Jaegers were 

seen only three times, all on Oregon II Cruise 204. Unidentified jaegers were 

sighted 247 times and were observed on the same cruises as Pomarine Jaeger, 

with the exception of Pelican Cruise 7 (Figure 7.10) . On 24 February 1993 

(Pelican Cruise 4), one flock of at least 100 unidentified jaegers was seen 

apparently feeding on the water surface in the vicinity of sperm whales. 

Pomarine Jaegers represented 10.80, Parasitic Jaegers 0.1°yo, unidentified 

jaegers 9.2g'o, and all jaegers together 20.0% of the seabird sightings 

(Table 7.3) . 

The Laughing Gull, with 368 sightings, was the most commonly seen 

species of gull and was observed on Pelican Cruises 3-7 and all four 

Oregon II cruises (Figure 7 .11) . Only one sighting each was made 

of the Franklin's Gull (Oregon II Cruise 199) and the Bonaparte's Gull 

(Pelican Cruise 4) . The Ring-billed Gull was seen 16 times, on Oregon 11 

Cruise 203 (Figure 7.12) . The Herring Gull was sighted 102 times, and 97% of 

sightings occurred on the cruises occurring November through February 

281 
30 

29 

28 

27 

26 

25 

.0 

9 

8 

7 

6 

5 

Figure 7 .5 . Magnificent Frigatebird sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. 

3C 

29 

28 

27 

26 

25 

10 

:9 

7 

6 

5 

Figure 7 .6 . Unidentified frigatebird sightings for Pelican Cruises 3-7 and 

Oregon II Cruises 199, 203, 204, and 209. 

282 
-96 -91) -94 -93 -92 -91 -90 -89 -98 -8 7 

30 30 

29 29 

28 

27 27 

26 - 26 

25 - 25 

7 

Figure 7.7 . Northern Gannett sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209 . 

30 

29 

28 

27 

26 

25 

30 

'9 

28 

i7 

!6 

S 

Figure 7 .8 . Masked Booby sightings for Pelican Cruises 3-7 and Oregon 11 

Cruises 199, 203, 204, and 209. 

283 
Figure 7.9. Pomarine Jaeger sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

Figure 7.10 . Unidentified jaeger sightings for Pelican Cruises 3-7 and Oregon 

II Cruises 199, 203, 204, and 209. 

284 
-96 -95 -94 -93 -92 -91 -90 -89 

30 

29 

T- t 

f 

- + + (++I + 

+ 

27 

f `~-J" \~! 

26 + 

25 

-87 

30 

29 

28 

27 

26 

25 

-96 -95 -94 -93 -92 -91 -90 -89 -88 -87 

Figure 7 .11 . Laughing Gull sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

30 

29 

28 

27 

26 

u 

.0 

!9 

;8 

:7 

s 

Figure 7 .12 . Ring-billed Gull sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

285 
(Figure 7 .13). The Laughing Gull comprised 13 .7%, Franklin's Gull and 

Bonaparte's Gull 0.03% each, Ring-billed Gull 0.69'0, and Herring Gull 3 .89'0 of 

all seabird sightings (Table 7.3) . All gulls together represent 18.49'0 of the 

GulfCet seabird sightings. 

Ten species of terns were noted during the GulfCet study, more species than 

any other group of seabirds. The Gull-billed Tern was only seen once on 

Oregon II Cruise 199. The Royal Tern was sighted 17 times on Pelican Cruises 4-

6 and Oregon II Cruises 203, 204, and 209 (Figure 7.14) . The Sandwich Tern was 

seen on Pelican Cruises 3 and G and Oregon II Cruises 204 and 209 (Figure 7.15) . 

The Forster's Tern was sighted once on Oregon II Cruise 199, and the Common 

Tern was identified on Pelican Cruise 6 and Oregon II Cruise 209 only, as was 

the Least Tern. Bridled Tern sightings were made on Pelican Cruises 3, 5, and 6, 

and Oregon II Cruises 199, 204, and 209 (Figure 7.16). The Sooty Tern was 

observed 40 times on Pelican Cruises 4 and 6 and Oregon II Cruises 199, 203, 204, 

and 209 (Figure 7.17). Sightings made on Pelican Cruises 4, S and 6 listed as 

large dark-backed terns were either Bridled or Sooty Terns. Pelican Cruise 6 

and Oregon II Cruises 199, 204, and 209 produced 412 Black Tern sightings 

(Figure 7.18). The Brown Noddy was sighted twice on Pelican Cruise 5, with 

both sightings being made in the same vicinity of the Gulf (27°15 .00`N, 

90°50.07'W and 27°15 .05'N, 91°30.03'W) . Of all GulfCet seabird records, 32.4/0 

were represented by this subfamily, but only three species, Bridled Tern, Sooty 

Tern, and Black Tern, comprise more than 1% of the total sightings; 9.39'0 of the 

seabird records were of unidentified terns (Figure 7.19, Table 7 .3). 

7.3.1 .2 Seasonality 

While birds were sighted throughout the GulfCet study area during all four 

seasons, the species composition varied during the year (Tables 7 .4-7.7, Figures 

7.20-7.24). Table 7.7 lists species diversity by season. 

Spring 

Spring produced the greatest species diversity (28 species) and the second 

highest total bird sighting rate (19 .8 bird sightings per day) (Table 7.7). If the 

Black Tern is excluded, then spring also had the greatest overall daily sighting 

rate. Many of the species sighted during this period, however, were seen very 

few times, and part of the apparent diversity could be a result of having more 

census days, enabling rare species to be observed. Figure 7.20 shows the bird 

sightings per day for the combined Pelican and Oregon II records and the 

number of sightings and individuals per hour sighted on Pelican Cruise 5 . 

Although the Oregon II cruises provided most of the records for this season, 

the conclusions were similar. Seven of the 11 most commonly sighted species 

were the same on both vessels. Storm Petrels were the most frequently sighted 

birds, with jaegers and Bridled Terns also comprising many sightings. Black 

Tern was not sighted on Pelican Cruise 5 but was noted on all of the Oregon II 

spring cruises . The most abundant species on Pelican Cruise 5 was the Bandrumped Storm-Petrel; 2.9 times as many were noted as total jaegers and 3 .9 

times the number of Bridled Tern. Finding storm-petrels to be major 

component of the avifauna was unexpected based on previous information 

286 
30 

29 

28 

27 

26 

25 

.0 

!9 

:7 

b 

5 

Figure 7 .13 . Herring Gull sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

30 

29 

28 

27 

26 

25 

.0 

;9 

x 

:7 

b 

5 

Figure 7 .14. Royal Tern sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

287 
30 

29 

28 

27 

26 

25 

30 

29 

28 

27 

!5 

Figure 7 .15 . Sandwich Tern sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209. 

30 

29 

28 

27 

26 

25 

50 

rg 

r7 

:6 

S 

Figure 7.16 . Bridled Tern sightings for Pelican Cruises 3-7 and Oregon II 

Cruises 199, 203, 204, and 209 . 

288 
-9U -39 -88- - 87 

30 

- 30 

29 29 

28 

27 + 27 

26 

- 26 

25 25 

7 

Figure 7 .17 . Sooty Tern sightings for Pelican Cruises 3-7 and Oregon II Cruises 

199, 203, 204, and 209. 

30 

29 

28 

27 

26 

25 

30 

29 

28 

27 

26 

25 

Figure 7 .18. Black Tern sightings for Pelican Cruises 3-7 and Oregon II Cruises 

199, 203, 204, and 209. 

289 
30 

29 

28 

N 

27 

26 

25 

30 

29 

28 

27 

26 

25 

Figure 7.19. Unidentified tern sightings for Pelican Cruises 3-7 and Oregon 11 Cruises 199, 203, 204, and 209. 
Band-rumped Storm-Petrel 

Wilson's Storm-Petrel 

Masked Booby 

Audubon's Shearwater 

Unidentified jaeger 

Bridled Tern 

Pomarine Jaeger 

Laughing Gull 

Unidentified tern 

Black Tern 

Unidentified storm-petrel 

Audubon's Shearwater 

Masked Booby 

Unidentified storm-petrel 

Sooty Tern 

Cory's Shearwater 

Brown Noddy 

Unidentified jaeger 

Royal Tern 

Pomarine Jaeger 

Bridled Tern 

Band-pumped Storm-Petrel 

bl 

0 0.05 0 .1 0.15 0.2 0.25 

Per Hour 

Individuals / Sightings 

Figure 7 .20. Spring sightings. (a) Mean number of combined bird sightings 

per day for Oregon II Cruises 199, 204, 209, and Pelican Cruise S . 

(b) Number of sightings and individuals observed per hour 

during Pelican Cruise S. 

291 

0 1 2 3 4 5 6 

Si ghti ngs/Day 
Unidentified storm-petrel 

Laughing Gull 

Common Tern 

Sandwich Tern 

Sooty Tern 

Masked Booby 

Unidentified tern 

Magnificent Frigatebird 

Bridled Tern 

Audubon's Shearwater 

Black Tern 

Unidentified storm-petrel 

Laughing Gull 

Common Tern 

Sandwich Tern 

Sooty Tern 

Masked Booby 

Unidentified tern 

Magnificent Frigatebird 

Bridled Tern 

Audubon's Shearwater 

Black Tern* 

bl 

0 0.2 0.4 0.6 0.8 1 1. .2 1 .4 

Per Hour 

Individuals / Sightings 

Figure 7 .21 . Summer sightings . (a) Mean number of bird sightings per day 

for Pelican Cruise 6. (b) Number of sightings and individuals 

observed per hour during Pelican Cruise 6. *Black Terns were 

sighted at a rate of 23.2 individuals per hour, not shown. 

292 

0 2 4 6 8 10 12 

Sightings/Day 
Cory's Shearwater 

Unidentified storm-petrel 

Northern Gannet 

Herring Gull 

Laughing Gull 

Masked Booby 

Audubon's Shearwater 

Unidentified tern 

Unidentified jaeger 

Pomarine Jaeger 

Herring Gull 

Unidentified storm-petrel 

Northern Gannet 

Cory's Shearwater 

Laughing Gull 

Unidentified tern 

Masked Booby 

Audubon's Shearwater 

Pomarine Jaeger 

Unidentified jaeger 

bl 

0 0.1 O. Z 0.3 0.4 0.5 U.6 U ./ U . t3 

Per Hour 

Individuals 0 Sightings 

Figure 7 .22 . Fall sightings. (a) Mean number of combined bird sightings per 

day for Pelican Cruises 3 and 7. (b) Number of sightings and 

individuals observed per hour during Pelican Cruise 3 . 

293 

0 0.5 1 1 .5 2 2 .5 3 

Sightings/Day 
Ring-billed Gull 

Northern Gannet 

Herring Gull 

Unidentified jaeger 

Pomarine Jaeger 

Laughing Gull 

Royal Tern 

Unidentified jaeger 

Northern Gannet 

Herring Gull 

Pomarine Jaeger 

Laughing Gull 

bl 

0 0.2 0.4 0 .6 0.8 1 1 .2 

Per Hour 

Individuals / Sightings 

Figure 7.23 . Winter sightings . (a) Mean number of combined bird sightings 

per day for Oregon II Cruise 203 and Pelican Cruise 4. (b) Number 

of sightings and individuals observed per hour during Pelican 

Cruise 4. 294 

0 0.5 1 1 .5 2 2 .5 3 3 .5 4 

Sightings/Day 
Northern Gannet 

Sooty Tern 

Magnificent Frigatebird 

Masked Booby 

Herring Gull 

Bridled Tern 

Unidentified tern 

Audubon's Shearwater 

Unidentified jaeger 

Unidentified storm-petrel 

Laughing Gull 

Pomarine Jaeger 

Black Tern 

Sooty Tern 

Magnificent Frigatebird 

Band-rumped Storm-Petrel 

Masked Booby 

Unidentified tern 

Northern Gannet 

Bridled Tern 

Audubon's Shearwater 

Unidentified jaeger 

Herring Gull 

Black Tern 

Laughing Gull 

Pomarine Jaeger 

Figure 7 .24. All seasons. (a) Mean number of combined bird sightings per day 

for Oregon II Cruises 199, 203 204, 209 and Pelican Cruises 3-7 . (b) 

Number of sightings and individuals observed per hour during 

Pelican Cruise 3-7 . 

295 

0 0.5 1 1 .5 2 2 .5 3 3 .5 

Sightings/Day 

0 0.05 0 .1 0.15 0.2 0.25 0 .3 

Sightings/Hour 
Table 7.7 . Summary of seasonal species diversity and total bird 

sightings per day. 

Annual 

Season Fall Winter Spring Summer mean 

Total number species 12 12 28 14 16.50 

Total number bird 

sightings per day 8.1 12.0 19.8 23.2 15.77 

(Clapp et al. 1982). Since storm-petrels are not as easily seen as larger species, 

the relative abundance may be higher. 

This season had the fewest observation days, and all of the data were from 

Pelican Cruise 6. Summer had the second highest species diversity (14 species), 

and the highest number (23.2) of bird sightings per day (Table 7.7 and Figure 

7.21) . Black Tern was the predominant species, with 44% of the sightings. Most 

of the Black Tern sightings were restricted to an area near the mouth of the 

Mississippi River. The neat most abundant summer seabird was the Audubon's 

Shearwater. Puffin us shearwaters were 2 .6 times as numerous as both Bridled 

and Sooty Terns, 6.9 times as abundant as Magnificent Frigatebird, and 10.9 

times as common as Masked Booby. 

Eau 

The fall data come from Pelican Cruises 3 and 7, which provided the lowest total 

bird sightings per day (8.11) (Table 7.7) . With 12 species of seabirds sighted, 

fall tied with winter for the lowest seabird diversity (Table 7 .7). Since these 

cruises were both late in the season, relative abundance may be different 

earlier in the season. For example, Cory's Shearwater has been reported to be 

most abundant in the Gulf in late September through early October (Pulich 

1982), but few were seen on Pelican Cruise 3 and none on Pelican Cruise 7 . 

Figure 7.22 shows the sighting rates for the 11 most frequently sighted species 

sighted during the fall cruises. On Pelican Cruise 3, jaegers (Pomarine Jaeger 

plus unidentified jaegers) predominated; they were 5 .6 times as abundant as 

the next most common group, Puffinus shearwaters (unidentified small 

Puffinus sp. and Audubon's Shearwaters) and 15.3-times as abundant as the 

third most common species sighted, the Masked Booby. 

Pelican Cruise 4 and Oregon II Cruise 203 were made during winter which 

made this season the second most heavily sampled period. Winter ranked third 

in the number of bird sightings (12 .0 per hour) and is tied with fall for the 

lowest species diversity, with only 12 species of seabirds noted (Table 7.7) . The 

2gG 
Laughing Gull, jaegers (Pomarine Jaeger plus unidentified jaegers), the 

Herring Gull, and the Northern Gannet appear to be the most abundant birds 

during this season (Figure 7.23) . The Laughing Gull was the most abundant 

species on Pelican Cruise 4; it was 1 .1 times as abundant as both the Herring 

Gull and jaegers and 1 .9 times as abundant as the Northern Gannet. The 

remainder of the species were sighted very rarely . 

Year 

Although the Black Tern was the most commonly sighted species, it was 

present only in the spring and summer seasons (Figure 7.25) . With the 

exception of the Laughing Gull and the Herring Gull, all of the species 

observed were primarily pelagic . The dominant birds include a genus 

(SCercorarius, jaegers) and two species (the Band-romped Storm-Petrel and the 

Audubon's Shearwater) not listed by Fritts et al. (1983) as important 

components of the bird fauna of the Gulf. 

7.3 .1 .3 Habitat 

Although the complexity of the hydrography of the Gulf makes the analysis of 

habitat difficult, at least one variable (water depth) seems significantly related 

to bird distribution (Table 7.2). The difference between depth at sighting 

location for certain species pairs was significant (Table 7 .8), with the 

exception of Audubon's Shearwater and Sooty Tern, and Pomarine Jaeger, and 

unidentified jaegers. 

Although other significant habitat variables have not been identified for 

these species, depth seems unlikely to be the only important habitat variable. 

Some species, such as Northern Gannet and Masked Booby, seem to partition 

habitat further on the basis of season . The Northern Gannet was most common 

in winter when the Masked Booby was least common. The lack of a significant 

difference between water depth at sighting location for the Audubon's 

Shearwater compared to the Sooty Tern suggests a similar habitat preference. 

Table 7.8 . Comparison of selected species pairs by depth using 

the Two-Sample Wilcoxin Rank-Sum Test. 

Species Number 

sightings 

Z-Score p 

Audubon's Shearwater and Sooty Tern 165 -0.07 0.945 

Wilson's and Band-Romped Storm-Petrel 95 4.56 0.0001 

Northern Gannet and Masked Booby 123 -3.05 0.0023 

Pomarine Jaeger and unidentified jaeger 515 -1 .76 0.0785 

Pomarine Jaeger and Laughing Gull 631 10.35 0.0001 

Laughing Gull and Herring Gull 464 2.68 0.0075 

Bridled and Sooty Tern 153 2.99 0.0028 

297 
Most of the pelagic bird species were observed resting on the surface of the 

water (Cory's Shearwater, Audubon's Shearwater, storm-petrels, Masked 

Booby, jaegers, and gulls) or on flotsam (Bridled, Sooty, and Black Terns) . 

7.3.2 Non-seabirds 

Although the Gulf does not represent a significant feeding or resting area for 

most non-seabird species, it is an important migratory pathway for 

neotropical migrants (Wallace and Mahan 1975) . The Pelican and Oregon II 

cruises produced S 84 records for non-seabirds (Table 7 .9) . These sightings 

represent at least 21 species with 31 duck (Order Anseriformes) sightings, 165 

heron or egret (Order Ciconiformes) sightings, one falcon (Order 

Falconiformes) sighting, one owl (Order Strigiformes) sighting, one 

goatsucker (Order Caprimulgiformes) sighting, one hummingbird (Order 

Apodiformes) sighting, and 249 passerine (Order Passeriformes) sightings. 

Table 7 .9. Non-seabird sightings by season for Pelican Cruises 

3-7 and all four Oregon 11 cruises. 

Species Fall Winter Spring Summer Total 

Herons 

Great Blue Heron 1 2 3 

Great Egret 1 1 

Snowy Egret 1 1 

Little Blue Heron 1 S 6 

Tricolored Heron 1 1 

Cattle Egret 1 6 1 8 

Yellow-crowned Night Heron 1 1 

Unidentified Heron or Egret 130 11 141 

Passerines 

Tree Swallow 1 1 

Purple Martin 2 2 

Northern Rough-winged 0 

Swallow 1 1 2 

Barn Swallow 1 16 17 

Unidentified Swallow 1 1 2 

Unidentified Thrush 2 2 

Northern Mockingbird 1 1 2 

Yellow Warbler 2 2 

Louisiana Waterthrush 1 1 

Wilson's Warbler 1 1 

Unidentified Warbler 1 1 

Scarlet Tanager 1 1 

Orchard Oriole 1 1 

Unidentified oriole 1 1 

Unidentified Passerine 2 4 203 2 211 

Total Sightings 8 10 341 50 409 

298 
7.4 Discussion 

Since GulfCet bird observations were made on only 160 days during a 30-month 

period, conclusions about seasonal distribution will require further study. 

Observations were made during all months except March, July, and October. 

Since March and October are months when significant seabird migration 

occurs (Clapp et al. 1982), some important seabird species present in the Gulf 

during these months may be underrepresented or missing from the GulfCet 

data. 

7.4.1 Species Accounts 

The following species accounts are restricted to the seabirds which were the 

most notable in the GulfCet study area. 

Procellariiformes 

The GulfCet results supported the conclusions of Fritts et al . (1983), Pulich 

(1982) and Clapp et al. (1982) that Cory's Shearwater was a regular but 

uncommon species in the Gulf from May through November. The first sighting 

records for the state of Louisiana for Cory's Shearwater were made by Fritts 

and Reynolds (1981) in October, and subsequent sightings were obtained in 

1991 (Jackson 1992). The GulfCet sightings added three records to Louisiana 

waters. 

Audubon's Shearwater is believed to have an Atlantic breeding population of 

5,000 pairs in the Caribbean (van Halewyn and Norton 1984) . This species is 

thought to be very common seasonally off the southeastern United States 

during late spring, summer, and fall months (Clapp et al. 1982), but has been 

considered casual to regular in low numbers in the Gulf (Duncan and Harvard 

1980, Clapp et al. 1982). The GulfCet sightings for this species suggested 

greatest abundance from May through November, a period similar to that 

noted off the southeastern United States (Clapp et al. 1982) . The GulfCet data 

differed from the findings of Fritts et al. (1983), who found Audubon's 

Shearwaters most numerous between October and April . The many GulfCet 

sightings suggest a much greater abundance in the Gulf than reported by 

Fritts et al. (1983), and implied the opposite relative abundance compared to 

Cory's Shearwater (Pulich 1982) . 

The GulfCet data supported the findings of Fritts et al . (1983) that storm-petrels 

(Family Hydrobatidae) are more numerous and widespread in the Gulf than 

previously thought and are an important component of the Gulf avifauna. 

These data, however, did not support the assumption by Fritts et al. (1983) that 

Wilson's Storm-Petrel is the predominant storm-petrel species in the Gulf and 

other species are very rare. Storm-petrels were sighted in all four seasons, but 

were most common in spring . Although reported by Duncan and Harvard 

(1980) as common and sometimes abundant in eastern portions of the Gulf, the 

overall abundance of the Wilson's Storm-Petrel in the Gulf remains uncertain. 

Traditionally, the Band-rumped Storm-Petrel has been considered casual or 

rare in the Gulf (Duncan and Harvard 1980, Clapp et al. 1982) . However, the 

GulfCet data suggested that it is much more common than previously thought 

299 
and that it may be the most numerous pelagic bird species in the Gulf during 

certain times of year. The GulfCet sightings may represent the first records for 

the state of Louisiana. 

Pelicaniformes 

The three White-tailed Tropicbird records for the study supported the 

conclusion of Clapp et al. (1982) that this species occurs regularly in small 

numbers in the Gulf. Approximately 20 previous records east for the Gulf. One 

of the GulfCet records was from the area covered by the Texas Rare Bird 

Committee. Although this species has been previously reported from Texas, no 

records have been substantiated by specimens or photographs. Currently this 

species is not officially on the Texas State Bird List . Similarly, the one Redbilled Tropicbird record was from the area covered by the Louisiana State Rare 

Bird Committee, and no previous record exists for this species from Louisiana 

Although the near-shore roosting of Magnificent Frigatebirds along the Gulf 

Coast is well described, the pelagic distribution of this species has been poorly 

described (Clapp et al. 1982). The GulfCet sightings suggested that, at least in 

late summer, Magnificent Frigatebirds were widespread in low numbers in the 

pelagic waters of the Gulf. 

GulfCet records for the Northern Gannet were consistent with previous 

findings (Clapp et al . 1982) that this species is an uncommon winter resident 

in the Gulf. The GulfCet sightings were concentrated mainly near the 

continental shelf break, and the numbers of Northern Gannets wintering in 

the Gulf may be greater over the continental shelf than elsewhere in the Gulf. 

On Pelican Cruise 4, Northern Gannet sightings seemed to be concentrated in 

an area of high primary productivity. The depths where Northern Gannet and 

Masked Booby occurred were significantly different, group sizes were not. The 

GulfCet sightings of Masked Booby supported the findings of Duncan and 

Harvard (1980) and Clapp et al. (1982) that Masked Booby is an uncommon to 

common species in the Gulf. The breeding population in the Caribbean and 

southern Gulf is limited to approximately 2,500 pairs (van Halewyn and Norton 

1984). This species has demonstrated some association with oil platforms 

(Ortego 1978). The distribution and abundance of this species should be 

monitored closely for the potential effects of deep water oil and gas production 

and exploration in the Gulf. 

Charadriiformes 

Unless most of the phalaropes using the Gulf pass through during March and 

early April and again in late September and October, phalaropes are very rare 

in the Gulf. However, these are the periods of peak occurrence for the 

southeastern U.S . and the Gulf (Clapp et al . 1983). The one sighting identified to 

species was Red Phalarope rather than the species more commonly reported 

from the Gulf, the Red-necked Phalarope. The GulfCet records indicated that 

large offshore wintering populations, which have been suggested for the 

latter species (Clapp et al . 1983), are unlikely in the north-western and central 

Gulf. 

300 
While the Pomarine Jaeger has been regarded as uncommon in the Gulf 

(Duncan and Harvard 1980), the GulfCet data indicated that this species is a 

common, deep water species from November to June. It had the greatest 

relative abundance of all species in fall and was also one of the most common 

winter species. The GulfCet data suggested that the Gulf may be a major 

wintering area for this species . The Pomarine Jaeger is considered a 

kleptoparasite (Harrison 1983). However, the winter distribution in the Gulf is 

farther offshore than that of the Laughing Gull, a likely food source, and the 

difference in water depth at sightings was significantly different for these 

two species. One group of Pomarine Jaegers (seen on Pelican Cruise 4) was 

associated with a flock of Sooty Terns, but Sooty Terns are rare in the Gulf in 

winter and cannot be the primary lleptoparasitized species. 

Although many jaegers were recorded as unidentified, most were probably 

Pomarine Jaegers because of the very small number identified as Parasitic 

Jaegers . Statistical comparison of the Pomarine Jaegers and unidentified 

jaegers by water depth revealed no significant difference. The flock of at least 

100 individuals noted on Pelican Cruise 4 apparently represented the largest 

number of jaegers sighted at one time in the Gulf. 

Laughing Gulls were most abundant during winter, less common in spring and 

fall, and rare in summer. Fritts et al. (1983) noted a similar deep-water 

abundance in winter and speculated that the birds moved further offshore to 

avoid competition with larger gulls . The GulfCet data, however, suggested a 

similar abundance and distribution for Laughing Gull and Herring Gull during 

the winter, and another explanation for the deep water distribution of 

Laughing Gulls in winter seems necessary. Several birds were seen associated 

with sargassum, and some were noted picking at the algae. 

The Herring Gull was also common during the winter months. Most sightings 

occurred near the edge of the continental shelf, but many sightings occurred 

in the deepest waters of the study area. No data were obtained to suggest this 

species' food source. 

White-backed Sterna terns (Gull-billed Tern, Royal Tern, Sandwich Tern, 

Forster's Tern, Common Tern, and Least Tern) are common in the nearshore 

areas along the Gulf coast (Clapp et al . 1983) . Gull-billed, Royal, Sandwich, 

Forster's, and Least Terns are common breeding birds in this area, but the 

Common Tern is a rare breeding species along the Gulf coast. Fritts et al . (1983) 

had very few records for any of these species in areas with water deeper than 

SO-100 m. The few GulfCet records for these species suggested that none has 

significant abundance in the deeper waters of the Gulf despite the heavy use 

of the continental shelf by these species. 

Evidence has shown the Bridled Tern to be more common in the Gulf than 

previously thought (Duncan and Harvard 1980, Clapp et al. 1983). Although 

noted to be regular in offshore Alabama waters (Duncan and Harvard 1980), 

very few sightings have been made in the waters offshore of Louisiana and 

Texas, where most of the 113 GulfCet sightings occurred. Water depth at 

sighting was significantly different for Bridled and Sooty Tern, and the group 

sizes were significantly different. The records were concentrated in spring 

and summer. 

301 
Sooty Terns breed in coastal Louisiana and Texas in small numbers, and a large 

colony exists in the Dry Tortugas, Florida (Clapp et al. 1983). Fritts et al. (1983) 

reported five sightings from the offshore waters of Louisiana and Texas. The 

40 GulfCet sightings add to the pelagic Gulf sightings . The Sooty Tern was 

observed throughout the year with the maximum number of sightings during 

summer and fall. 

The large offshore concentrations of the Black Tern noted on Pelican Cruise 6 

seemed to be located mainly over the Mississippi River plume, and most of the 

sightings on this cruise occurred in areas with low salinity surface water. One 

sighting occurred along a front of brown water and clear blue water. Black 

Terns were noted over the brown water and not beyond, whereas Bridled Terns 

were simultaneously noted over the blue water flying parallel to the front but 

not over the brown water. Many of the sightings on this cruise were of flocks 

over fish schools, but birds were also observed sitting on flotsam, such as 

water hyacinth (Eichhornia crassipes) . 

Although Fritts et al. (1983) did not describe large concentrations of Black 

Terns in offshore Louisiana waters, the study area for that survey was west of 

the Mississippi River plume. Very few Black Tern sightings were made on 

Pelican Cruise G within the study area of Fritts et al. (1983), despite the 

numerous sightings in areas east of there . Both Fritts et al . (1983) and Clapp et 

al. (1983) concluded that Black Terns have a low susceptibility to oiling in the 

Gulf. GulfCet data, however, suggest that in areas near the mouth of the 

Mississippi River during migration in August, large numbers of Black Terns 

are susceptible to oiling . 

Brown Noddv_ 

Except for an observation reported by Fritts et al. (1983) of a bird seen 220 km 

off western Louisiana, the two GulfCet records for Brown Noddy were 

apparently the only ones for this species for Louisiana not associated with a 

hurricane or tropical storm. 

7.4.2 Summary 

The bird survey conducted during the GulfCet cruises was the first extensive, 

offshore study for the north-western and central Gulf of Mexico. Duncan and 

Harvard (1980) performed a shipboard survey in the offshore waters of 

Alabama, and Fritts et al. (1983) did aerial censusing in waters offshore of 

Texas and Louisiana. Aerial census data, however, may be biased towards 

larger, more visible species (Fritts et al 1983). Most of the remaining 

information regarding pelagic seabirds in the Gulf has been based on 

occasional trips by groups of bird watchers or on records of birds occurring 

onshore after storms. Clapp et al . (1982, 1983) summarized all published seabird 

records for the Gulf except for jaegers and several species of gull. The GulfCet 

bird data not only increases and supports existing knowledge of the Gulf 

pelagic avifauna, but also adds important new information regarding the 

offshore seasonal occurrence, relative abundance, and distribution of several 

species. 

302 
The 2,692 seabird records obtained during the GulfCet Project greatly expand 

the number of published offshore bird sightings in the Gulf. Thirty-two 

species of seabirds were observed, but of the birds identified to species, 14 

species represent over 99% of the total sightings. New insight into the 

distribution and abundance has been added to Audubon's Shearwaters, stormpetrels (especially the Band-rumped Storm-Petrel), phalaropes, jaegers 

(especially the Pomarine Jaeger), Laughing Gulls, Herring Gulls, Bridled 

Terns, Sooty Terns, and Black Terns. Some conclusions regarding distribution 

relative to water depth for some species are possible. Records of species rare in 

the north-central and western Gulf, including White-tailed and Red-billed 

Tropicbird and Brown Noddy, were obtained. Although much data has been 

collected covering most of the year, information regarding species present 

during March, July, and October was low, and habitat variables for all of the 

Gulf pelagic species need further investigation. Most of the pelagic bird 

species, including species with limited Atlantic Ocean populations such as the 

Audubon's Shearwater and the Band-romped Storm-Petrel, could potentially be 

negatively affected by oil spills . 

7.5 Literature Cited 

American Ornithologists' Union . 1983. Check-list of North American birds, 6th 

ed. American Ornithologists' Union, Lawrence, KS. 877 pp. 

American Ornithologists` Union. 1985 . Thirty-fifth supplement to the 

American Ornithologists' Union check-list of North American birds. 

Auk 102: 680-686. 

American Ornithologists' Union. 1987 . Thirty-sixth supplement to the 

American Ornithologists' Union check-list of North American birds. 

Auk 104:591-596. 

American Ornithologists' Union. 1989. Thirty-seventh supplement to the 

American Ornithologists' Union check-list of North American birds. 

Auk 106: 532-538. 

Clapp, R.B., R C. Banks, D. r4organ-Jacobs, and W.A. Hoffman. 1982 . Marine 

birds of the southeastern United States and Gulf of Mexico. Part I: 

Gaviiformes through Pelecaniformes. Rept. FWS/OBS-82/01 . U.S. Fish 

and Wildlife Service, Office of Biological Services, Washington, D.C 637 

PPClapp, R.B., D. Morgan-Jacobs and R.C. Banks. 1983 . Marine birds of the 

southeastern United States and Gulf of Mexico . Part III: 

Charadriiformes. Rept . FWS/OBS-83/30. U.S . Fish and Wildlife Service, 

Office of Biological Services, Washington, D.C. 869 pp. 

Duncan, C.D., and R.W. Harvard. 1980. Pelagic birds of the northern Gulf of 

Mexico. Am. Birds 34:122-132. 

303 
Fritts, T.H., and R.P. Reynolds. 1981 . Pilot study of the marine mammals, birds 

and turtles in OCS areas of the Gulf of Mexico. Rept. FWS/OBS-81/36. U.S . 

Fish and Wildlife Service, Division of Biological Services, Washington, 

D.C. 140 pp. 

Fritts, T.H., A.B. Irvine, R.D. Jennings, LA. Collum, W. Hoffman, and M.A. 

McGehee. 1983 . Turtles, birds, and mammals in the northern Gulf of 

Mexico and nearby Atlantic waters. Rept. FWS/OBS-82/65. U.S . Fish and 

Wildlife Service, Office of Biological Services, Washington, D.C. 455 pp. 

Harrison, P. 1983. Seabirds: An identification guide . Houghton Mifflin Co., 

Boston. 448 pp. 

Heineman, D. 1981 . A range finder for pelagic bird censusing. J. Wildl. 

Manage. 45 :489-493. 

Herring, H. J. 1993 . A bathymetric and hydrographic climatological atlas for 

the Gulf of Mexico. MMS Contract No. 1412-0001-30631 . Dynalysis Rept. 

109 to the U.S . Department of the Interior, Minerals Management 

Service, Technical Analysis Group, Herndon, VA. Var. pp. 

Jackson, G.D. 1992 . The regional reports; the fall 1991 season: Central 

Southern Region. Am. Birds 4G:102-107. 

Ortego, B. 1978 . Blue-faced Boobies at an oil production platform. Auk 95 :762-

763. 

Pulich, W., Jr. 1982 . Documentation of Cory's Shearwater in the western Gulf of 

Mexico. Wilson's Bull. 94:382-384. 

Schreiber, R.W. 1978. Eastern Brown Pelican, pp. 23-24. In H.W. Kale II, ed. 

Rare and endangered biota of Florida, volume 2 : Birds. University 

Presses of Florida, Gainesville. 

Tasker, M.L., P.H. Jones, T. Dixon, and B.F . Blake. 1984. Counting seabirds at sea 

from ships: A review of methods employed and a suggestion for a 

standardized approach. Auk 101:567-577 . 

van Halewyn, R., and R.L. Norton. 1984. The status and conservation of 

seabirds in the Carribean, pp. 1G9-222. In J.P. Croaall, P.G.H. Evans, and 

R.W. Schreiber, eds . Tech. Rept. 2 - Status and conservation of the 

world's seabirds. International Council for Bird Preservation, 

Cambridge, UK. 

Wallace, G.J . and H.D. Mahan. 1975 . An introduction to ornithology, 3rd ed. 

McMillian Publishing Co., New York. 546 pp. 

304 
VIII. STUDIES OF SPERM WHALES 

B.R. Mate, M. Duncan, D.W. Weller, A. Schiro, S.K. Lynn, 

J.C. Norris, B. Wiirsig, and W.E. Evans 

This chapter describes the results of two aspects of the GulfCet Program: (1) 

satellite tagging of sperm whales, and (2) a sperm whale focal cruise 

(RN Pelican Cruise 8) . 

8.1 Satellite Tagging of Sperm Whales 

Oregon State University (OSU) attempted to place satellite-linked depth 

recorders (SLDRs) and location-only satellite telemeters on sperm whales to 

determine their movements, diving behavior, and preferred habitat. To 

accomplish this goal, three cruises were undertaken: two in the Gulf of Mexico 

(October 1992 and June 1993) and one in the Galapagos (March 1993). The 

Galapagos cruise was intended as a test for tag deployment and attachment. 

8.1 .1 Data Acquisition 

The satellite telemeters used for this project were designed and built by OSU 

using Wildlife Computers' controller boards and Telonics' ST-6 Platform 

Transmitter Terminals (PTTs) and housed in a stainless steel cylinder (5 cm 

diameter, 19 cm long, 0.8 kg in weight) . The exterior of the housing had 

attachments which consisted of two stainless steel rods (12.7 cm long, 0.6 cm 

diameter) with one pair of folding toggles mounted behind double-edged 

blades at the end of each rod (Figure 8.1). 

The transmitters were attached to whales with a compound crossbow capable of 

generating 68 kg of force. The satellite telemeter was held in a "C"-shaped cup 

at one end of an aluminum shaft. The shaft with the satellite telemeter was 

then fired from the crossbow. A line (9 kg test) attached to the aluminum shaft 

enabled the satellite telemeter to be recovered if it missed the whale. Once the 

satellite telemeter was attached to the whale, the shaft fell away. 

The TelonicsT"' PTTs transmitted a 400 mW signal every 40 seconds when in the 

programmed "on" mode. To conserve battery power, the tag was equipped with 

a saltwater switch so that it transmitted only at the surface. A small VHF radio 

transmitter was attached to the housing to enable real-time tracking at sea. 

The VHF transmitters were tuned to specific frequencies, had different 

repetition rates, and transmitted continuously. 

All satellite telemeters were identifiable by a code transmitted to the satellite as 

part of a 256 bit data stream. The SLDRs collected data over eight, three-hour 

summary periods daily. These data included three histograms: depth of dives, 

duration of dives, and time spent at various depth ranges. Other data for each 

three hour period included the longest dive, deepest dive, duration of deepest 

dive, temperature at deepest depth, longest surface duration uninterrupted by 

a submergence of greater than six seconds, and total surface duration. 

305 
0 

Figure 8 .1 . Diagram of the satellite telemeter. 

306 
Transmission was scheduled for four two-hour periods (eight hours) daily. A 

status message was relayed in lieu of the collected data every 15th 

transmission. This message provided information on battery voltage, sea 

surface temperature, number of transmissions, current zero depth offset, and 

a current assessment of saltwater resistance. All messages included a cyclic 

redundancy code for error detection purposes. 

The Wildlife Computers""' pressure transducer and software were tested 

extensively using a relay box to simulate dives to different depths and 

durations. The satellite telemeter housing was tested to 2,000 m in a pressure 

bomb. After these tests, the transmitter, batteries, and controller board were 

potted in epoxy to provide greater structural strength. 

8.1 .2 Tagging Cruise Chronology 

Cruise 1 : The first tagging cruise was conducted from 30 September to 14 

October 1992 . The R/V McGrail, a 25 m converted Coast Guard Cutter operated by 

TAMUG, was used. The McGrail arrived in Venice, LA on 31 September and left 

for Galveston 14 October 1992 . Only 4.5 of the 13 days were workable due to 

inclement weather and equipment failures on the vessel . 

The cruise covered an area where previous GulfCet cruises and aerial surveys 

had observed sperm whales, but was limited to the ship's operational range (to 

185 km offshore from Venice) . Visual contact with sperm whales was made 

only once for approximately four hours. On 9 October, 8-10 sperm whales were 

sighted. Unfortunately, the boat could not maneuver well enough at slow speed 

to get close enough to tag any animals. The whales changed their course when 

the ship approached to within 8 m. 

Cruise 2 : This cruise was conducted in the eastern Pacific Ocean, near the 

Galapagos Islands, from 20 March to 31 March 1993. The RN Odyssey, a 30 m 

sailboat owned and operated by the Whale Conservation Institute, was used. 

Three SLDRs were supplied by the GulfCet Program. The other operating costs 

for this cruise were provided by OSU's Marine Mammal Foundation. 

The purpose of this cruise was to test techniques to approach and attach SLDRs 

to sperm whales. The waters around the Galapagos were an ideal testing 

ground because, unlike the Gulf of Mexico, the seasonality and distribution of 

large numbers of sperm whales has been well documented for this area (e.g ., 

Whitehead et al . 1989, Whitehead and Waters 1990). 

Several hundred sperm whales were located and followed over a five-day 

period using visual and acoustic contacts. Whales occasionally changed 

direction during close vessel approaches, but did not show a "flight" response 

to the boat. 

On 26 March, a SLDR was successfully attached to a sperm whale. The telemeter 

was placed about 0.5 m from the whale's dorsal ridge and appeared to be flush 

against the animal's skin. The animal did not appear to startle or take flight 

after attachment of the telemeter, but continued its initial submergence 

pattern and surfaced a few minutes later, 100 m from the boat. 

307 
Two other tagging attempts were unsuccessful: in the first instance, the 

telemeter hit the dorsal ridge of the animal and glanced off. In the second 

case, the animal arched suddenly so the tag missed its target completely. The 

animal then fluked and broke the retrieval line, preventing tag recovery. 

Cruise 3 : The second GulfCet tagging cruise used the R/V Acadiana, a twin 

diesel, 18 m vessel chartered from LUMCON. The OSU team arrived in Cocodrie, 

LA on 1 June 1993 . Construction of a tagging platform and some remaining 

LUMCON charter activities were completed by 5 June. The ship left Cocodrie on 

6 June and returned on 29 June. Fourteen of the scheduled 24 days were 

workable; four days were used for transit between Cocodrie and Port Eads, LA 

(6, 14, 16, and 29 June); one day the ship fulfilled a previous charter obligation 

(15 June); five days were spent in port during Tropical Storm Arlene (17 to 21 

June). 

The tagging platform was constructed from a two-piece, 9-m long, fiberglass 

extension ladder with a pulpit at the end made of wood. The platform was 

stabilized with tension wires and extended 3.5 m off the starboard side of the 

ship. The platform was extremely stable, and it was possible to pull it in while 

underway and during docking . 

Visual observations and sonobuoys were used to locate whales. The areas 

surveyed were based on previous GulfCet aerial and shipboard sightings. 

During 24-hour operation, watches were held from 0600-2000 daily with two 

OSU persons on watch at all times. All cetacean sightings were recorded. At 

night, the scientific crew stood 2-hour watches that included acoustic stations 

(monitoring a suspended hydrophone) and maintaining vessel safety. 

When whales were spotted, one observer remained in visual contact with the 

animals while the other scientists prepared the tagging equipment, 35-mm 

cameras, video recording equipment, and data sheets . VHF radio headsets were 

worn by the captain and scientific crew to communicate the whale's location 

and to coordinate the ship's movements for tagging. 

The vessel covered 2,331 km searching for sperm whales (Figure 8 .2) . Sperm 

whales were seen on seven days and heard on 11 days. The number of sperm 

whales ranged from 4-22 per day with up to eight animals seen at one time 

(Table 8.1) . A maximum of 87 individuals was seen during the cruise . Animals 

were sighted most often in the afternoon. 

Animals were approached to within 75 m, at which time the vessel was slowed 

and one engine shut down to reduce noise for the final approach. The sperm 

whales were generally small; most were judged by eye to be less than 8 m in 

length and were considered too small to tag; a few were up to 8 m. Even these 

presented a small target and needed to be within S m of the ship and 

perpendicular to the tagging platform (approximately parallel to the vessel's 

starboard side) before a shot could be attempted. Positioning was critical for 

successful tagging. Because there were subdermal anchors at each end of the 

cylindrical tag, the tag's trajectory had to be perpendicular to the whale or the 

tag would not attach properly. Tagging attempts were made only when the 

animal's back was well out of the water and not arched. 

308 
w 

Figure 8.2. Sperm whale sightings, 6 to 29 June 1993, aboard K/V Acadiana. 

-91 .0 -90.5 -90.0 -89.5 -89.0 -88.5 -88.0 -87.5 -87.0 -86.5 -86.0 
Table 8.1. Sperm whale sightings, 7-29 June 1993, RN Acadiana. 

Date Time Latitude Longitude Number 

7 June 1993 1608 29°02.35 88°01 .54 7 

7 June 1993 1710 29°02.23 88°03 .50 3 

7 June 1993 1900 29°02.26 88°05 .15 4 

12 June 1993 1520 28°49.32 88°35 .48 3 

12 June 1993 1750 28°50.40 88°35.71 3 

12 June 1993 1810 28°40.68 88°39.67 1 

12 June 1993 1945 28°50.36 88°40.09 4 

13 June 1993 0815 28°42.47 88°52.71 2 

13 June 1993 0828 28°42.01 88°52.24 2 

13 June 1993 0845 28°41.68 88°52.14 1 

13 June 1993 1637 28°43.85 88°22.01 3 

13 June 1993 1740 28°46.40 88°26.66 1 

13 June 1993 1900 28°50.47 88°34.07 5 

13 June 1993 1915 28°50.29 88°34.40 4 

23 June 1993 1240 28°56.25 88°11 .17 2 

23 June 1993 1345 28°57.70 88°12.43 2 

23 June 1993 1426 29°00.12 88°12.91 3 

23 June 1993 1430 29°00.39 88°12.81 3 

23 June 1993 1508 29°00.13 88°12.33 1 

23 June 1993 1725 28°56.56 88°11.26 3 

23 June 1993 1740 28°56.70 88°11.57 1 

23 June 1993 1835 28°59.58 88°16.94 4 

23 June 1993 1908 28°58.59 88°17.53 3 

24 June 1993 1145 29°00.37 88°12.41 2 

24 June 1993 1308 29°02.34 88°12.09 2 

24 June 1993 1347 29°04.42 88°11.47 3 

24 June 1993 1450 29°03.63 88°11.62 4 

29 June 1993 1805 28°39.70 88°41 .00 2 

29 June 1993 1830 28°38.80 88°41 .55 2 

mean 2.8 

SD 1.35 

n 29 

Two animals were tagged. The first whale (about 8 m in length) was tagged on 

7 June with an SLDR. Only one message was heard from this tag. Photos 

revealed that the tag was located on the dorsal ridge with the forward tine of 

the housing implanted 5-8 cm in the blubber and the rear tine only implanted 

2 .5 cm. It is believed that this tag fell off the animal shortly after attachment 

due to incomplete penetration of the tines into the blubber. The second animal 

(about 7 m in length) was tagged on 11 June with a location-only telemeter. 

The telemeter placement was good. Although penetration was not complete, it 

was judged to be adequate. Further shock tests have been conducted without 

failures, so at present it is not known why this telemeter failed . All other 

opportunities (12-13 June and 23-24 June) for tagging were with animals 

judged to be too small. No whales were seen on four of the last five days despite 

excellent weather and sighting conditions (25-29 June) . 

310 
A seismic vessel, the Acadian Commander, began seismic surveys on 23 June in 

an area where whales had been seen routinely (Figure 8.3). The seismic 

surveys were expected to continue for 30 days. Whales were seen on the 

periphery of the seismic survey area on the 23 and 24 June (Figure 8.4), but 

not in the middle of the area where we had seen many whales regularly before 

the seismic work began. No whales were seen in or near this area for the 

survey days after 24 June (Figure 8.5) (Mate et al. 1990. While the change 

observed in whale distribution may have been due to normal movements or a 

change in prey concentration, it did coincide with the onset of seismic 

activity. Therefore, there may be a cause-and-effect relationship, and the 

possibility can only be resolved with further investigation. Six species of 

other cetaceans were seen during this cruise (Table 8 .2) . A log of sea bird 

sightings was not maintained. 

8.1 .3 Summary 

Previous information about sperm whales in the Gulf has indicated that they 

are sparsely distributed and have very small pod sizes (see Chapter 2) . The 

sperm whales sighted during the tagging cruises were in a patchy distribution 

over a large geographic region and were usually in loose groups of 2-8 

animals. 

Of particular interest was the small size of the sperm whales sighted. None of 

the animals were thought to be over 8 m. Four whales appeared small enough 

to be calves, which may have been recently weaned. At one point, the ship was 

in an area with about eight small animals at the surface. The ship stayed in 

this area for two hours and there was no evidence of any larger animals. 

Large animals would be expected if these small ones were part of a mixed 

group of females, calves, and juveniles. This juvenile group social structure 

may be unique to this area. It has never been reported in the scientific 

literature and certainly deserves more attention. The Smithsonian's stranding 

records were examined (Mead, personal communication), and the conclusions 

drawn from those data were that sperm whales of normal size do exist in the 

Gulf, and that the animals seen on the tagging cruises were not merely from a 

population of small individuals. 

While searching for sperm whales in the Gulf of Mexico in 1993, some 

circumstantial evidence was obtained that seismic vessel activity may affect 

the distribution of sperm whales (Mate et al. 1994). During five of the first 

nine survey days, sperm whales were consistently sighted, generally in a 

localized geographic area. During this time, the Acadian Commander was 

preparing to begin seismic testing. During the first two days of seismic 

activity (34 guns shooting every 10 seconds at 1800 psi, 24 hours a day), only a 

few sperm whales were located on the margins of the seismic survey area. No 

whales were found for the next five days in that region. Although 

observations represent circumstantial evidence, the change in whale 

sightings after the onset of seismic activity is sufficient to warrant concern 

and additional studies. Richardson et al. (1995) summarize data on marine 

mammal reactions to seismic activity . 

311 
w 

N 

Figure 8 .3 . R/V Acadiana cruise track and sperm whale sightings 6 June to 22 June 1993, prior to seismic activity. 
W 

W 

Figure 8 .4 . R/V Acadlana cruise track and sperm whale sightings 23 June to 24 June 1993, with seismic activity 

beginning. 
W 

N 

Figure 8.5 . R/V Acadiana cruise track and sperm whale sightings 25 June to 29 June 1993, with seismic vessel 

activity . 
Table 8.2 . Cetaceans other than sperm whales seen on a sperm whale tagging cruise 7-29 June 

1993 (10096 confidence) . 

Date Time Latitude Longitude Species Number 

7 June 1993 1420 28°40.64 89°05 .69 Bottlenose dolphin 2 

7 June 1993 1940 28°3G.70 88°43 .03 Pantropical spotted dolphin 15-20 

12 June 1993 1350 28°54.29 88°23 .85 Clymene dolphin 25-30 

23 June 1993 1930 28"59.40 88°17 .64 Fraser's dolphin 3 

24 June 1993 0745 29°07.13 87°58 .52 Pantropical spotted dolphin 25 

24 June 1993 1010 28°58.11 88°02 .64 Yantropical spotted dolphin 35-40 

24 June 1993 1345 29°04.H0 88°10.94 Rough-toothed dolphin 8 

27 June 1993 1920 28°46.40 88°57 .81 Risso's dolphin 5 

w 

VI 
Satellite telemeters were attached to two small animals on this cruise : an SLDR 

and a location-only telemeter. The lack of penetration of the tines appeared to 

be due to the tough slain and blubber on the animal's dorsal ridge. The small 

size of the animals that were tagged may have exacerbated this problem. The 

attachment methods have worked very well on right whales and bowhead 

whales (Mate et al. 1992, Mate and Krutzikowsky in prep.), but may have to be 

modified for sperm whales. 

8.2 Sperm Whale Focal Cruise 

Sperm whales have been consistently found in a small area off the mouth of 

the Mississippi River, both historically (Chapter 2) and in the present study 

(Chapters 3 and 4) . This situation provided the opportunity to locate whales for 

the tagging program, and also raised a question as to why these animals should 

be so consistently found at this location. In order to begin addressing this 

question, a sperm whale focal cruise was undertaken (Pelican Cruise 8) . The 

purpose of the cruise was to monitor sperm whale movements and behavioral 

patterns and to begin describing individuals in order to determine whether 

they utilize this area over multiple seasons and years. Both acoustic and visual 

means were used to locate and follow whales in the vicinity of the mouth of the 

Mississippi River. 

The R/V Pelican was used from 20-28 August 1994 to follow sperm whales for 

visual and acoustic observation in an area approximately 40 km southeast of 

the mouth of the Mississippi River. There were two aspects of the acoustic 

effort : using the horizontal towed array to locate and follow animals, 

particularly at night when no visual effort was possible, and using a 

combination of horizontal and vertical arrays to localize vocalizing sperm 

whales. The visual effort concentrated on locating and following animals, 

describing behavior, and obtaining fluke identification photos. Sightings and 

effort are summarized in Appendix A. 

8.2.1 Locating and Tracking Sperm Whales 

Survey effort started on 20 August at the head of the Mississippi Canyon. An 

absence of sperm whale sightings and acoustic contacts during 10.52 hr of 

effort prompted a move east to an area which had produced a large number of 

sperm whale sightings and acoustic contacts during earlier GulfCet cruises. 

The first contact with sperm whales was made at 28°38.49'N, 89°00.69'W at 

1920 hr on 20 August, very near the location on Pelican/Longhorn transect 

leg 12 where whales had previously been located. For the next four days, until 

the evening of 24 August, whales were followed using visual and acoustic 

methods. Contact was maintained approximately 800 of the time, within an 

area of 12 x 21 NM (Figure 8.6) . During the tracking period, vertical 

hydrophone arrays were deployed for acoustic localization, and photos were 

taken for individual identification. By the evening of the fourth day it was 

clear that many animals were concentrated around the location where they 

had initially been contacted. After completing day time efforts to obtain fluke 

identification photographs, the nights of 24, 26, and 27 August were spent 

transiting east, southeast, and south, respectively, to acoustically determine 

the dispersion of sperm whales in the area. In each case, when we departed 

316 
28.80 

-89.20 -89.10 -89.(l() -88. 

/ f 28.80 

28.70 -

28.60 -

28.50 

w 

1 

28.40 

28.30 - 

28.20 

28.70 

28.60 

0 

0 

x 

0 

0 

0 

0 

0 

0 

-89.00 s c 

o 

o,*4? 

0 

N 

I 

28.50 

28.40 

28.30 

28.20 

Figure 8 .6 . Cruise track of R/V Pelican, 20-28 August 1994 (TIO Cruise 8), and sperm whale sightings . The 

presence (filled circles) or absence (open circles) of sperm whales is shown in half-hour 

intervals (whether detected visually or acoustically) . Triangles indicate no effort . 
the vicinity of 28°70'N, 88°85`W we lost contact with whales, but upon 

returning to the area they were again located and followed, suggesting that 

they were the only sperm whales in the area. The southwestern leg followed a 

preferred depth contour (1,200 m), yet no new animals could be located. Sperm 

whales were consistently located and followed within the area of 

concentration through 28 August, after which the ship returned to port. The 

ability to locate, follow, and maintain visual and acoustic contact with sperm 

whales day and night over an eight day period is an indication of the efficacy 

of the dual use of acoustic and visual methods. 

8.2 .2 Acoustic Localization of Vocalizing Sperm Whales 

The towed linear hydrophone array and vertically deployed sonobuoy triads 

were used in an attempt to localize vocalizing sperm whales at depth. A total of 

forty-nine sonobuoys were deployed, thirty-three of which were used in 

three-buoy vertical arrays. The linear array was towed near the surface while 

the sonobuoy arrays were deployed at either 20, 100, and 300 m or 100, 120, and 

300 m depths (Figure 8.7) . The localization analysis was based on a matched 

field processing approach and performed at sea. Given an estimate of the 

acoustic environment, known receiver positions, and assuming linear ray 

paths, the most probable location of the source was determined by finding the 

best match between observed arrival time differences and time differences 

between several hundred pre-defined locations in the acoustic field. The 

location associated with the set of travel time differences most closely 

matching the set produced by the source was presumed to be an approximate 

source location. Travel time differences for these points were computed taking 

into account the temperature effect on the vertical sound speed profile. Thirty 

XBTs were deployed and provided temperature vs. depth data. Salinity was 

assumed to be 35 ppt. 

Results from one trial analysis (at midnight, 24 August) show three of eve 

vocalizing animals near 600 m deep (Figure 8.8), in water 1,247 m deep. The 

shallowest localization was at approximately 100 m. There were no indications 

of animals vocalizing at or near the surface. 

8.2 .3 Photo-identification of Sperm Whales 

Arnbom (1987) showed that individual sperm whales can be photographically 

identified from distinctive marks and notches on their flukes. Photoidentification was conducted from a 4.2 meter inflatable Avon and individual 

sperm whale flukes were photographed with 35-mm cameras equipped with 

200- and 300-mm zoom lenses, as well as HI8 video. Opportunistic photographs 

were also taken from the main research vessel . Photo-identification from the 

small inflatable was conducted at irregular intervals during the study, with 

major efforts on 23, 25, and 28 August. Three researchers independently 

agreed on individual identifications. 

Twenty individual sperm whales were identified from photographic and video 

images, two of them calves (Schiro et al. 1995). Identified individuals slowly 

meandered within the study area each day. However, over the study period, 

identified individuals moved several kilometers to the east then back to the 

318 
Y 

Fore and aft hydrophones on the 

towed array 

/ 3sonobuoys 

simulating a 

/ vertical 

hydrophone 

/ array 

/ / i ~ / 

/ i 

Sperm whale 

(acoustic source) 

Z 

Figure 8.7. Illustration of the use of the horizontal and vertical hydrophone 

arrays to locate sperm whales in 3-dimensional space. The 

horizontal towed array was composed of two receivers separated 

by 60 m. The vertical array was composed of 3 sonobuoys deployed 

in close proximity to each other at depths of 100, 120, and 300 m. 

west near the original contact position. By the last day of observations 

(28 August), some sub-groups with identified individuals had moved several 

kilometers to the southwest. 

Nine of the 20 identified individuals were resighted (photographed again in a 

new subgroup) during the cruise (Figure 8.9) . Six of these individuals were 

resighted only on the same day during which they were initially sighted. 

Sighting intervals for 4 individuals resighted over multiple days ranged from 

1-4 days. One individual was sighted on 3 different days. Whales sighted on 

multiple days had a mean distance between resightings of 19.3 ± 6.73 (SD) lm 

(n = 5 resightings, 4 whales). Distances ranged from 10.3-26 .2 km. 

It was the impression of the researchers that only a maximum of 50 animals 

(possibly fewer) was seen and that many of the same animals were sighted 

throughout the survey. All sightings on this survey were made within 20 NM 

of the location where sperm whales were first encountered (near 28°38'N, 

89°00'W). 

319 
Range (m) 

1000 2000 3000 4000 5000 6000 

-100 

-200 

-300 

Q 

D 

-400 

-500 

-600 

Figure 8.8. The results of localizing vocalizing sperm whales using vertical 

and horizontal arrays. Asterixes = sonobuoy locations and 

stars = estimated whale locations. 

8.2.4 An Interaction Between Sperm Whales and Pilot Whales 

On 24 August 1994 an unusual interaction between pilot whales and sperm 

whales was observed at 28°43 .20'N, 88°44.13'W (Weller et al. in press) . This 

location is within the general region where we had been observing sperm 

whales throughout the cruise . The defense reaction of sperm whales to the 

presence of pilot whales is described here. While the true nature of the 

interaction is difficult to interpret, this account provides suggestive evidence 

that short-finned pilot whales may aggress toward or at least threaten sperm 

whales. Pilot whales are not generally known to prey on other marine 

mammals; however, records from the eastern tropical Pacific document that 

this species does chase, attack, and may occasionally eat young dolphins 

during fishery operations (Perryman and Foster 1980) . In captivity, pilot 

whales have been noted to aggress toward humans (Norris 1967), and to eat 

still-born or young dolphins (Brown et al. 1966, cited in Perryman and Foster 

1980). A male pilot whale off Hawaii bit into the thigh of a woman, and took 

her at least 12 m below the surface in possibly aggressive or play related 

behavior (Shane et al. 1993) . 

The following report describes the general behavior state and salient 

behavioral events recorded during the 150 minute interspecific interaction. 

Real time field notes, 35-mm photographs, and video tape with running spoken 

commentary were used as the basis for this description. The term "adult" sperm 

320 
ID# 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

Date sighted 

21 Aug 22 Aug 23 Aug 24 Aug 25 Aug 26 Aug 27 Aug 28 Aug 

7 

Figure 8 .9. Sighting dates of identified sperm whales. The absence of 

identifications on 26 and 27 August was due to the lack of 

photographic effort . 

whale refers to presumed adult females and immatures (as opposed to small 

calves) . Based on size estimates, the presence of a mature male was not 

observed. 

At 1410 hr (28°39 .20'N, 88°41 .91'W) a large school of approximately 30 shortfinned pilot whales was sighted via 25 x 150 binoculars. Individuals were 

dispersed in numerous subgroups and spread over a 2-S km area, with some 

animals approaching the research vessel within 200 m and paralleling its 

course directly abeam. Low directional leaping and rapid surfacing was noted 

at this time, and two of the observers (B. Wursig and D.W. Weller) commented 

that the apparent size of these pilot whales exceeded what they had observed 

in other geographic locations. 

At 1446 hr (28°43 .20'N, 88°44.13'W) a subgroup of sperm whales was sighted 

near the horizon just prior to their fluke-up dives. Eighteen minutes later, at 

1504 hr, a mixed aggregation of sperm whales (referred to as "group 1") and 

pilot whales was sighted. The observation of several large pilot whales tail 

lunging and tail slapping at the head of an adult sperm whale suggested that 

this might be an agonistic encounter. As the research vessel approached, the 

composition of the aggregation was noted to consist of a sperm whale 

321 
mother/calf pair surrounded by 8-10 pilot whales. The pilot whales appeared 

excited, as evidenced by rapid swimming and surfacings, fluke-up dives, and 

variable movements along the flanks, heads, and flukes of the sperm whales. 

The behavior of the mother/calf pair appeared distressed, as suggested by 

high head rises, frequent respirations, fluke swiping, and erratic changes in 

body orientation and posturing. 

Also in view at this time, and at an approximate distance of 500 m, was a mixed 

group of six adult sperm whales and one calf (referred to as "group 2") and 

approximately 20 pilot whales. At 1532 hr, after 28 min of observation on 

group l, the research vessel motored to within 300 m of group 2, attempting to 

stay near the whales but not approaching them directly. The remaining 

122 min of behavioral observation was done at a distance no greater then 

300 m and at zero or minimal vessel power. 

Considerable fusion in the composition of group 2 was noted across the 

observation period . At 1600 hr, group 1 and attendant pilot whales joined 

group 2, increasing the overall sperm whale group size to seven adults and two 

calves . At least three additional lone adult sperm whales, also escorted by pilot 

whales, approached group 2 at rapid swim speeds, and eventually joined the 

interaction (one as late as 1705 hr), resulting in a total of 10 adults, two calves, 

and 30-45 pilot whales . The increase in the number of sperm whales appeared 

to be correlated with a decrease in pilot whale activity . The presence of calves 

may explain why these sperm whales did not attempt to flee by vertical 

descent. 

The spatial proximity of individual sperm whales remained close, often 

huddled together and touching, throughout the observation . This huddling 

behavior included the creation of numerous marguerite formations 

(Nishiwaki 1962) over the duration of the interaction. The marguerite 

formation was assembled horizontally at the water surface with heads in and 

flukes out, and at times vertically, providing a three dimensional marguerite 

in which whales were pitch-poling with heads at the surface and flukes 

suspended below. In most cases, one or both of the young calves were directly 

in the center of these formations. The marguerite was not always a complete 

circle and seemed to wax and wane with increases and decreases in overall 

pilot whale activity. 

During marguerite formations and periods of tight huddling, the following 

behavioral events were observed for sperm whales: (1) open mouth behavior: 

lower jaw agape exposing the teeth and associated white mouth and lips; 

(2) inverted surface posturing: inverted, ventrum-up body position at the 

surface of the water exposing underside of lower jaw; (3) lateral fluke swishes: 

a portion of a fluke blade above the water surface and rapidly thrust in a 

lateral or sideways orientation; (4) peduncle arching: caudal peduncle arched 

above the surface of the water (particularly frequent in calves) ; 

(5) underwater bubble clouds: underwater exhalation of air; (6) tail slapping: 

repeated horizontal fluke slaps on surface of water; (7) spy hopping: lifting of 

head above surface of water; and (8) inverted underwater posturing: inverted 

body position directly below a calf at the surface situated between two adults . 

372 
Harassment strategies of the pilot whales remained mostly veiled . The pilot 

whales remained near and among the sperm whales for the majority of the 

observation period. Many fluke-up dives and caudal arches were noted and 

some rapid "surge" swimming (in which a burst of white water was created at 

the surface by the forward movement of a pilot whale) was observed. The pilot 

whales appeared to take particular interest in attempts to penetrate the 

marguerite formation of the sperm whales. In one instance two pilot whales 

swam toward an adult sperm whale, inverted themselves just prior to 

interspecific body contact and slid over the sperm whale's dorsum and back 

and directly into the center of the marguerite. At no time were either species 

seen to bite the other, nor was any blood or any other sign of injury observed, 

including fresh rake marks. 

The pilot whales did not seem to act in any particularly coordinated fashion 

except for a "stand off" in which approximately 25 pilot whales simultaneously 

clustered behind the flukes of the sperm whales who were in a distinct 

staggered-line-abreast formation. At other times, groups of both species 

formed lines facing each other, at a distance of less than one sperm whale 

body length. 

At 1715 hr (28°43.45'N, 88°45 .21'W) five rough-toothed dolphins approached 

the interaction, at first swimming among the pilot whales and sperm whales, 

and eventually remaining directly below the bow of the vessel . At 1734 hr all 

of the pilot whales had departed and most of the sperm whales had sounded. 

Several sperm whales remained rafting or pitch-poling vertically in the 

water. At 1820 hr all sperm whales had departed. Remains of two partially 

digested and apparently regurgitated squid were seen floating in the water 

column at this time and one was collected. 

Vocalizations of both species were gathered using sonobuoys deployed within 

1 km of the interaction and recorded on a Racal V-Store tape recorder. Sound 

recordings were made at 3-3/4 ips with an associated bandwidth of DC-12.5 kHz. 

The sperm whales maintained a stereotypical steady vocal pulsing throughout 

the interaction while pilot whale vocalizations were mostly infrequent 

whistles, with occasional burst pulse 'tonal' signals. While there were no direct 

correlations between vocalizations and behavior, the sperm whales produced a 

number of four, six, and seven pulse codas at the start of our observations and 

at the end of the interaction. No rough-toothed dolphin vocalizations were 

heard until at least 10 minutes after they had joined the interaction. Once the 

pilot whales departed the area, and after a series of codas, the sperm whales 

gradually became silent . 

Our interpretation of this interspecific interaction as agonistic is based upon 

the defense behavior displayed by the sperm whales . Accounts of how this 

species reacts to deleterious stimuli such as whalers, killer whales, false killer 

whales, and sharks closely parallel the behaviors observed here. 

Nishiwaki (1962) described the marguerite formation after observing all 

members of a sperm whale group circle a harpooned affiliate in a heads-in 

and flukes-out arrangement resembling the petals of a marguerite flower. 

Berzin (1971) reports a similar account from whalers in the northern Pacific 

in which a group of hunted sperm whales maintained a large circle of adults 

323 
surrounding young animals . Best et al. (1984) described an apparent calving 

episode in which a tightly bunched group of sperm whales, with all calves in 

the center, were threshing their flukes. These authors then state that outside 

of the "circle" (it is difficult to assess if this was indeed an actual marguerite) 

were numerous killer whales and dozens of sharks. Palacios and Mate (1994) 

observed the marguerite formation during an attack by false killer whales on 

sperm whales near the Galapagos Islands. 

Indications of the generality and rate of occurrence of instances such as that 

observed in the Gulf of Mexico can be obtained from observations during 

tracking of sperm whales in the South Pacific between 1985-1993 (see Smith 

and Whitehead 1993, Dufault and Whitehead, in press, for some details of the 

research). During 165 24-hour days of tracking sperm whales from 10-12 m 

auxiliary sailing vessels (principally off the Galapagos Islands and mainland 

Ecuador) there were 18 instances in which sperm whales and short-finned 

pilot whales were visible at the same time. In five of these cases, apparent 

harassment of the sperm whales by 12-50 pilot whales was observed, and in 

two of them (both off mainland Ecuador, and, as determined by individual 

identification studies [Arnbom 1987], containing different groups of sperm 

whales) the sperm whales were observed to adopt the marguerite formation. In 

neither observation of the marguerite formation were first-year sperm whale 

calves present. In four of the five harassment cases, including both times the 

marguerite was observed, the pilot whales were accompanied by 12-SO 

bottlenose dolphins; however, it seemed to be the pilot whales that were most 

affecting the behavior of the sperm whales. Harassment incidents lasted 

between 10-60 min, and behavior of both species was generally similar to that 

observed in the Gulf of Mexico, although observations were less complete and 

detailed. The two observations of the marguerite formation during apparent 

pilot whale harassment were the only two clear observations of this behavior 

during the South Pacific studies. 

L. Ballance (personal communication, National Marine Fisheries Service, La 

Jolla, CA) observed a group of eight sperm whales, including one calf, form a 

marguerite as a possible response to killer whales in the eastern tropical 

Pacific . Interpretation of this account is complicated, however, by the 

presence of a mixed aggregation of pilot whales and bottlenose dolphins 

swimming around the sperm whales in an excited manner. Ballance suggests 

that these smaller delphinids may have been seeking refuge from the killer 

whales by associating with the sperm whales . However, it may also be true that 

the sperm whales formed the marguerite in response to the pilot whales (and 

possibly the bottlenose dolphins). 

In contrast to all of the above accounts, Arnbom et al. (1987) report the 

reaction of sperm whales to an attack by killer whales. These sperm whales did 

not create a defense marguerite but rather faced their aggressors in more of 

an offensive manner. However, similar to the acoustic behavior of sperm 

whales reported here, Arnbom et al. (1987) also found that the sperm whales 

eventually became silent after the encounter. 

Reports of sperm whales forming the marguerite are relatively uncommon in 

the literature . Most existing accounts suggest that the marguerite is a defense 

response to some perceived threat to injured or particularly vulnerable 

324 
individuals (calves), similar to what is commonly reported for terrestrial 

animals. While the defense reaction of sperm whales reported here and by 

others (Nishiwali 1962, Berzin 1971, Best et al. 1984, Palacios and Mate 1994, 

Ballance personal communication) varies from that of Arnbom et al. (1987), 

differences may simply reflect divergent strategies activated by perceived 

risk and potential vulnerability at both the individual and group level. 

Attack responses of sperm whales toward whaleboats are well documented and 

include inverted body posturing, lateral fluke swipes, head rises, and inverted 

open mouth behaviors (Caldwell et al. 1978, Norris 1967) . Many of these 

discrete behavioral events, reported for whales in obvious distress, were also 

prevalent throughout the interaction reported here. 

The sperm whale defense responses described here suggest that these animals 

were reacting to a perceived threat. No actual combat or overt fighting was 

observed, and no evidence of injury to either species was noted. Therefore, we 

hypothesize that the pilot whales were testing the vulnerability of these 

sperm whales to assess the potential for separating particularly weak or young 

individuals from the group. The pilot whales were cautious in their threats (as 

are most terrestrial mammals) due to the potential for injury. Thus, no blatant 

or brazenly obvious attacks were attempted by the pilot whales, most likely as a 

result of a perceived lack of general sperm whale vulnerability. It is also 

possible that the pilot whales were engaged in play or practice of predation, 

with no real intent to harm or kill the sperm whales. The presence of apparent 

blackfish tooth rakes on the dorsal fins and flukes of sperm whales from both 

the Gulf of Mexico and South Pacific suggests that this type of non-lethal 

predation may be occurring. Killer whales are known to teach cooperative 

hunting strategies to their young (Lopez and Lopez 1985), but we have no 

evidence for this point in the present case. A final alternative explanation 

may be that of competitive exclusion occurring between two squid-eating 

species. 

In combination with the few accounts of pilot whales aggressing towards 

other marine mammals and evidence that several blaclfish species including 

false killer whales and pygmy killer whales may attack and eat other 

cetaceans (Perryman and Foster 1980, Hoyt 1981, Palacios and Mate 1994) it is 

not unreasonable to speculate that this interaction was aggressive in nature. 

8.2.5 Summary 

Sperm whales were located and followed within a small area off the mouth of 

the Mississippi River from 20-28 August 1994. Estimated at no more than SO 

animals, they were followed for approximately 80% of the time using a 

combination of visual and acoustic means. Three-dimensional localizations 

showed that whales vocalized at depths as great as 600 m (in 1,250 m of water). 

The results of the two aspects of acoustic effort, whale tracking and 

localization, demonstrate that sperm whales can be both tracked as well as 

localized in three dimensions using acoustic means. 

Twenty individuals were photographically identified. Sighting intervals for 

four individuals resighted over multiple days ranged from 1-3 days. The mean 

distance between resights over multiple days was 19.3 km, ranging from 10.3- 

325 
26.2 km. A rarely observed aggressive interaction between pilot whales and 

sperm whales was also observed. 

Historical records show no seasonal change in the distribution or abundance 

of sperm whales in the north-central Gulf of Mexico (Chapter 2), and it is 

possible that individual females, calves, and immature whales, at least, reside 

permanently in the Gulf of Mexico. Questions about site fidelity and subgroup 

isolation must remain unresolved until a significant proportion of the 

population is identified and more is known about long-term movements of 

sperm whales in the Gulf. This will require continued photo-identification 

effort to allow the description of the population's age, sex, and social 

structures. 

8.3 Literature Cited 

Arnbom, T. 1987. Individual identification of sperm whales. Rep. Int. Whal. 

Comm. 37:201-204. 

Arnbom, T., V. Papastavrou, I..S . Weilgart, and H. Whitehead. 1987 . Sperm 

whales react to an attack by killer whales. J. Mammal. 68:450-453 . 

Berzin, A.A. 1971 . The sperm whale. Edited by A.V. Yablokov. Pishchevaya 

Promyshlennost Moscow. English Translation, 1972, Israel Program for 

Scientific Translations, Jerusalem. 394 pp. 

Best, P.B., P.A.S . Canham, and N. MacLeod. 1984. Patterns of reproduction in 

sperm whales, Physeter macrocephalus, pp. 51-80. In W.F. Perrin, R.L 

Brownell, and D.P. DeMaster, eds . Reproduction in whales, dolphins and 

porpoises. Rep. Int. Whal. Comm. Special Issue 6, Cambridge, UK. 

Brown, D., D.K. Caldwell, and M.C. Caldwell. 1966. Observations on the behavior 

of wild and captive false killer whales, with notes on associated 

behavior of other genera of captive delphinids. Los Angel. Co. Mus. 

Contrib. Sci. 95 :1-32. 

Caldwell, D.K., M.C. Caldwell, and D.W. Rice. 1978. Behavior of the sperm whale, 

Physerer catodon L, pp. 667-717. In K.S . Norris, ed. Whales, dolphins, 

and porpoises. University of California Press, Berkeley. 

Dufault, S., and H. Whitehead. In press. The geographic stock structure of 

female and immature sperm whales in the South Pacific. Rep. Int. Whal. 

Comm. 

Hoyt, E 1981 . The whale called killer. E.P. Dutton, New York. 

Lopez, J.C., and D. Lopez 1985. Killer Whales (Orcinus orca) of Patagonia, and 

their behavior of intentional stranding while hunting nearshore. 

J. Mammal 66:181-183. 

326 
Mate, B.R., and G.K. Krutzikwosky. In prep. Application of remote methods of 

large cetacean tracking: Bowhead whales. OCS Study MMS 95-0053. Dept. 

of the Interior, Minerals Management Service. 169 pp. 

Mate, B.R., S. Nieukirk, R. Mescar, and T. Martin. 1992 . Application of remote 

sensing methods for tracking large cetaceans: North Atlantic right 

whales (Eubalaena glacialis) . OCS Study MMS 91-0069. Dept. of the 

Interior, Minerals Management Service. 167 pp. 

Mate, B.R., K.M. Stafford, and D.K. Ljungblad. 1994. A change in sperm whale 

(Physeter macrocephalus) distribution correlated to seismic surveys in 

the Gulf of Mexico. One Hundred Twenty-eighth Meeting of the 

Acoustical Society of America, 28 November-2 December, 1994, Austin, 

TX. J. Acoust. Soc. Am. 96:3268-3269. 

Nishiwaki, M. 1962. Aerial photographs show sperm whales' interesting 

habits. Norsk Hvalfangst-Tidende 51 :395-398. 

Norris, K.S . 1967 . Aggressive behavior in cetacea, pp. 225-241 . In C.D. 

Clemente and D.B. Lindsley, eds . Aggression and defense. University of 

California Press, Berkeley. 

Palacios, D.M., and B.R. Mate. 1994. Ataque de orcas falsas a cachalotes en las 

Islas Galapagos, Ecuador, pp. 82-83. In A. Ximenez and P.C. Simoes-Lopes, 

eds. Anais da 6a Reuniao de Trabalho de Especialistas em Mamiferos 

Aquaticos do Sul. 24 a 28 de Outubro de 1994, Florianopolis, Brasil. 

Perryman, W.L, and T.C. Foster. 1980. Preliminary report on predation by 

small whales, mainly the false killer whale, Pseudorca crassidens, on 

dolphins (Stenella spp. and Delphinus delphis) in the eastern tropical 

Pacific . NMFS Admin. Rept. LJ-80-05 . National Marine Fisheries 

Service, La Jolla, CA. 

Richardson, W.J ., C.R. Greene, Jr., C.I . Malme, and D.H. Thomson, with 

contributions by S.E. Moore and B. Wursig. 1995. Marine mammals and 

noise. Academic Press, New York. 576 pp. 

Schiro, A., B. Wursig, D. Weller, J . Norris, and M. Armstrong. 1995 . 

Preliminary findings on sightings and movements of individually 

identified sperm whales (Physeter macrocephalus) in the north central 

Gulf of Mexico. Presented at the Ninety-eighth Annual Meeting of the 

Texas Academy of Science, 2-4 March, Waco, TX. 

Shane, S. H., L Tepley and L Costello. 1993. Life-threatening contact between 

a woman and a pilot whale captured on film. Mar. Mamm. Sci. 9:331-335 . 

Smith, S.C., and H. Whitehead. 1993 . Variations in the feeding success and 

behaviour of Galapagos sperm whales (Physeter macrocephalus) as they 

relate to oceanographic conditions. Can. J. Zool. 71:1991-1996. 

327 
Weller, D.W., B. Wiirsig, H. Whitehead, J.C. Norris, S.K. Lynn, R.W. Davis, N. 

Clauss, and P. Brown. In press. Observations of an interaction between 

sperm whales and short-finned pilot whales in the Gulf of Mexico. Mar. 

Mamm. Sci. 

Whitehead, H., and S. Waters . 1990. Social organisation and population 

structure of sperm whales off the Galapagos Islands, Ecuador (1985-

1987), pp. 53-55. In P. S. Hammond, S. a Mizroch, and G. P. Donovan, eds. 

Individual recognition of cetaceans: Use of photo-identification and 

other techniques to estimate population parameters . Rep. Int. Whal. 

Comm. (Special Issue 12). Cambridge, UK. 

Whitehead, H., L Weilgart, and S. Waters. 1989. Seasonality of sperm whales 

off the Galapagos Islands, Ecuador. Rep. Int. Whal. Comm. 39 :207-210. 

328 
IX. CETACEAN HABITAT 

R.W. Davis, G.S . Fargioil, W.E. Evans, 

N. May, and T.D. Leming 

9.1 Introduction 

There are many factors that influence the spatial and temporal distribution 

and abundance of marine mammals. These factors may be broadly divided into 

those that are environmental, biotic, and anthropogenic (Borcard et al. 1992). 

Environmental variables include those that are physicochemical, 

climatological, acoustic, and geomorphological . Environmental factors operate 

on time scales ranging from less than a day to many millennia. Diel, seasonal, 

interannual, and decadal patterns of variability or periodicity may occur for 

each factor. Biotic variables include competition among animals, reproduction, 

and predation. Anthropogenic factors include, among others, historical 

hunting, pollution, ship activity, commercial and recreational fishing, oil and 

gas development and production, and seismic exploration. One of the major 

impacts of many of these factors is the alteration of the acoustic environment, 

vital to cetaceans for navigation, foraging, and social communication (Evans 

1971) . The spatial structuring of marine mammal communities may be 

influenced by many of these factors, the relative contributions of which are 

difficult to quantify. 

The correlation of environmental features with sighting data can improve our 

understanding of cetacean ecology and indicate which, if any, oceanographic 

variables may be driving cetacean distribution (Smith and Gaskin 1983, Hui 

1985, Smith et al. 1986, Brown and Winn 1989, Reilly 1990, and Waring et al. 

1993). Shipboard and aerial sightings were analyzed to determine an average 

species habitat profile. Seven environmental variables were chosen to 

characterize habitat based on their ability to represent oceanographic, 

topographic, and acoustic variables: 1) sea surface temperature (SST) from 

ship data and AVHRR infrared satellite images, 2) sea surface temperature 

gradient, 3) temperature at 100 m, 4) depth of the 15° C isotherm, S) surface 

salinity, 6) bottom depth, and 7) bottom depth gradient. Environmental profiles 

were assembled for 13 species or species categories that had at least 10 

sightings with most of these variables. Environmental profiles were assembled 

for the other species or species categories that had fewer sightings (see 

Appendix A), but these were not included in the statistical comparisons. 

Several of the seven variables are standard hydrographic indices used in 

water column analyses . SST, SST gradient, temperature at 100 m, and depth of 

the 15* C isotherm, when combined with dynamic height data, can provide 

information on the position, depth and size of distinctive mesoscale 

oceanographic features, such as the Loop Current, warm-core eddies and coldcore eddies (Biggs et al. 1995 ; see also Chapter 6 of this report) . These variables 

are also useful in locating thermal fronts or upwelling areas that may be 

associated with increased primary productivity and prey concentrations, 

potentially attracting marine mammals. These variables may also cause 

acoustic propagation anomalies (Smith et al. 1986, Biggs 1992, Biggs and 

329 
Miiller-Krager 1994), that could affect foraging behavior (Evans 1971, Au 

1993). 

In the northern Gulf, salinity is an important hydrographic variable because 

of the high fresh water discharge from the Mississippi River. This discharge 

can extend over the continental slope and, in conjunction with wind, can 

affect the circulation, salinity, and acoustic propagation patterns of the northcentral Gulf (Walker et al. 1994, see also Chapter 6 of this report). 

Water depth and subsurface topography appear to influence cetacean 

distribution (Hui 1979, 198 ; Kenney and Winn 1987) . Since several of the 

species in the GulfCet study area were known to be deep divers based on their 

diet (Clarke 1986, Jefferson et al. 1993), bottom depth and bottom depth 

gradient were considered to be important variables for the analysis . The 

continental shelf bread: and continental slope of the GulfCet study area contain 

some of the most variable topography in the Gulf. Although smaller cetaceans 

may be confined to the upper 200 m of the water column (Williams et al . 1993), 

many prey on deep scattering layer (DSL) organisms that are accessible at 

night when the DSL migrates toward the surface (Ridgway and Harrison 1994). 

Sperm whales, pygmy/dwarf sperm whales and beaked whales are capable of 

diving much deeper and probably exploit much of the water column down to 

and below 1,000 m (Clarke 1986, Jefferson et al . 1993) . 

We would have liked to include sea-surface chlorophyll concentration as one 

of the environmental variables, as it has been shown to correlate with the 

distribution of zooplankton and larger nekton that may influence cetacean 

distribution (Smith et al. 198G) . However, without the satellite-based Coastal 

Zone Color Scanner (CZCS) to determine surface pigment concentrations, the 

resolution of our data from water samples was insufficient for quantitative 

inclusion in the habitat analysis. 

9 .2 Methods 

9.2 .1 Geographic Information System (GIS) 

The data used in the characterization of habitat were based on the results from 

Chapter 3 (Visual Surveys Aboard Ships and Aircraft) and Chapter 6 

(Oceanography). A GIS was used to integrate the marine mammal sightings and 

oceanographic data. The locations of cetacean sightings were recorded in 

latitude and longitude coordinates with a global positioning system (GPS) or 

LORAN-C aboard the survey ships and aircraft (see Chapter 3 for details) . SST 

(°C) data were collected by the Advanced Very High Resolution Radiometer 

(AVHRR) carried aboard the NOAH polar orbiting satellites (see Appendix B for 

images) . Subsurface water temperature and salinity (psu) were measured with 

CTD casts (temperature only with XBT casts) at stations along the survey lines 

(see Chapter 6 for details) . The point measurements acquired during an entire 

cruise were contoured for each depth interval using Surfer' software. Bottom 

depth and bottom depth gradient were determined from a bathymetric map of 

the Gulf of Mexico with a resolution of 5 minutes of arc (Herring 1993). The 

maps were imported into an AGIST"' Geographic Information System (Delta Data 

Systems, Inc., Picayune, N1S) . Mean values for the seven variables for the study 

area were extracted from the GIS. 

330 
9.2 .2 Statistical Methods 

The seven environmental variables were analyzed using the Kruskall-Wallis 

test with a posteriori comparisons (significance considered at a = 0.05) for the 

13 species or species categories that had 10 or more sightings for most of the 

variables (Tables 9.1 and 9.2). 

9 .3 Environmental Profiles 

Only eight of the 13 species or species categories had 10 or more sightings for 

SST (Tables 9.1 and 9.2), and there was a significant difference among these six 

(KW = 20.7, df = 7, p = 0.004) . There was a gradient of species found in cooler 

water to those found in warmer water (Figure 9.1) . Atlantic spotted dolphins, 

striped dolphins and sperm whales occurred in the coolest water and, as a 

group, were significantly different from pantropical spotted dolphins, but 

overlapped with the group comprising bottlenose dolphins, Risso's dolphins, 

pygmy/dwarf sperm whales, and beaked whales. The mean annual SST for the 

study area was 24.6° C ± 2 .5 SD. 

As with SST, there were only eight species or species categories that had 10 or 

more sightings for SST gradient (Tables 9.1 and 9.2), and there was a 

significant difference among these eight (KW = 15.7, df = 7, p = 0.03). There was 

a gradient of species found in shallower SST gradients to those found in 

steeper SST gradients (Figure 9 .2) . Atlantic spotted dolphins and striped 

dolphins occurred in the shallowest SST gradients and, as a group, were 

significantly different from beaked whales, but overlapped with bottlenose 

dolphins, Risso's dolphins, pantropical spotted dolphins, sperm whales and 

pygmy/dwarf sperm whales. The mean annual SST gradient for the study area 

was 0.03°/1 .1 km ± 0.07 SD 

Depth of the 15* C Isotherm 

There was no significant difference among nine species or species categories 

(KW = 11 .8, df = 8, p = 0.16) with regard to the depth of the 15° C isotherm 

(Tables 9.1 and 9.2) . The overall mean for the nine species or species categories 

was 205.3 m ± 30.2 SD. The mean annual depth of the 15°C isotherm for the study 

area was 199.0 in ± 33 .3 SD. 

Water Temperature at 100 m 

There was no significant difference among nine species or species categories 

(KW = 14.6, df = 8, p = 0.07) with regard to water temperature at 100 m (Tables 9.1 

and 9.2) . The overall mean for the nine species or species categories was 

19.5° C±0.44 SD. The mean annual water temperature at 100 m for the study 

area was 19 .8° C ± 1 .5 SD. 

331 
Table 9.1 . Cetacean species or species categories that had 10 or more sightings for most of the 

seven environmental variables used in the statistical comparison (indicated with an 

X) . 

Species or species category SST SST Depth Temperature Surface Bottom Bottom 

(°C) gradient of the at 100 m salinity depth depth 

(°C/1 .1 km) 15° C (°C) (psu) (m) gradient 

isotherm (m) (m/1 .1 km) 

Atlantic spotted dolphin X X X X - X X 

Bottlenose dolphin X X X X X X X 

Risso's dolphin X X X X X X X 

Pygmy/dwarf sperm whale X X X X - X X 

Sperm whale X X X X X X X 

w Alesoplodon spp. - - X X - X X 

N Clymene dolphin - - X X - X X 

Pantropical spotted dolphin X X X X X X X 

Striped dolphin X X X X - X X 

Spinner dolphin - - - - - X X 

Rough-toothed dolphin - - - - - X X 

Short-finned pilot whale - - - - - X X 

Beaked whale X X - - - X X 
Table 9 .2 . Environmental profiles for the cetacean species or species categories that had 10 or 

more sightings for most of the variables. 

Depth of Bottom 

SST 15°C Temperature Surface Bottom depth 

Species or species Sample SST gradient isotherm at 100 m salinity depth gradient 

category statistic (°C) (°C/1 .1 km) (m) (°C) (psu) (m) (m/1 .1 km) 

Atlantic spotted mean 22.6 0.09 184.20 19.30 34.94 197.1 11 .2 

dolphin median 22.5 0.02 185.20 19.19 36.00 173.4 7.1 

n 19 19 18 18 8 30 30 

minimum 17 .9 0.0 141 .2 18.2 27.4 102 1 

maximum 28.7 0.6 216.6 22.6 36.2 589 37 

Bottlenose dolphin mean 24.2 0.08 188.70 19.06 33.60 293.5 16.4 

median 23.8 0.05 190.75 18.83 35.93 216.6 12.0 

n 84 81 68 68 19 149 149 

minimum 14.6 0.0 150.9 16.9 15.8 101 0 

maximum 30.7 0.5 271 .6 22.6 36.5 1226 120 

Risso's dolphin mean 24.4 0.09 194.54 19.32 34.88 713.7 23.3 

median 24.3 0.05 195 .40 18.92 35.72 571 .0 20.9 

n 38 38 31 31 11 67 67 

minimum 19.0 0.0 145.7 18.1 24.8 150 3 

maximum 29.5 0.3 256.4 23.0 36.6 1997 58 

Pygmy/dwarf sperm mean 24.6 0.09 205 .41 19.72 35.57 928.5 20.6 

whale median 24.5 0.07 199.7 19.75 35.75 860.8 17.1 

n 34 34 29 28 4 72 72 

minimum 18.9 0.0 145 .3 17.7 34.7 176 2 

maximum 29.7 0.3 277.1 22.0 36.1 1989 91 
Table 9.2 . Environmental profiles for the cetacean species or species categories that had 10 or 

more sightings for most of the variables. (continued) 

Depth of Bottom 

SST 15°C Temperature Surface Bottom depth 

Species or species Sample SST gradient isotherm at 100 m salinity depth gradient 

category statistic (°C) (°C/1 .1 km) (m) (°C) (psu) (m) (m/1 .1 km) 

Sperm whale mean 23 .7 0.07 194.80 19.91 35.82 1104.9 24.2 

median 23.3 0.05 194.60 19.59 36.21 1009.3 18.8 

n 37 36 41 38 15 65 65 

minimum 18.1 0.0 160.9 17.3 33.4 480 3 

maximum 29.5 0.4 254.8 24.4 36.3 1957 90 

w Mesoplocion spp. mean 25.2 0.09 195.35 20.25 36.01 1196.9 14.8 

median 26.6 0.09 188.40 19.64 35.86 1126.5 13.5 

n 6 6 10 10 3 13 13 

minimum 18.2 0.0 160.5 18.9 35.8 680 4 

maximum 28.6 0.2 256.4 22.4 36.4 1933 26 

Clymenc dolphin mean 24.3 0.04 190.35 19.22 36.15 1261 .0 17.5 

median 23.5 0.02 175.90 19.19 36.39 1304.0 16.1 

n 9 9 17 17 5 22 22 

minimum 21 .3 0.0 150.4 17.6 35.5 612 2 

maximum 27.4 0.1 284.1 22.5 36.4 1979 40 

Pantropical spotted mean 25.3 0.07 197.82 19.24 35.64 1241 .9 19.0 

dolphin median 25.4 0.05 196.30 19.24 35.99 1287.2 16.5 

n 60 58 69 57 31 103 103 

minimum 19.1 0.0 145.3 17.0 31.8 364 2 

maximum 29.7 0.5 266.9 22.4 36.6 1999 120 
Table 9.2 . Environmental profiles for the cetacean species or species categories that had 10 or 

more sighting s for most of the variables. (continued ) 

Depth of Bottom 

SST 15°C Temperature Surface Bottom depth 

Species or species Sample SST gradient isotherm at 100 m salinity depth gradient 

category statistic (°C) (°C/1 .1 km) (m) (°C) (psu) (m) (m/1 .1 km) 

Striped dolphin mean 23.6 0.02 200.47 19.24 34.72 1235.2 24.5 

median 22.2 0.01 199.05 18.93 34.82 1214.9 23.0 

n 13 12 12 12 7 18 18 

minimum 19.6 0.0 160.6 17.9 32.6 570 6 

maximum 30.0 0.1 248.9 22.4 36.3 1997 71 

Spinner dolphin mean 24.1 0.53 187.01 18.96 36.09 1111 .0 23.3 

median 23.4 0.03 184.80 18.89 36.09 927.0 22.4 

n 7 7 8 8 2 13 13 

w minimum 18.1 0.0 154.8 17.3 36.0 526 7 

v maximum 29.7 0.1 230.0 21 .5 36.2 1776 38 

Rough-toothed mean 25 .1 0.11 208.07 19.92 35.23 950.5 18.4 

dolphin median 25.4 0.02 197.10 19.24 34.83 1066.6 13.3 

n 9 9 7 7 3 16 16 

minimum 21.9 0.0 180.5 18.5 34.7 194 7 

maximum 27.9 0.6 267.3 22.9 36.1 1524 73 

Shirt-finned mean 24.0 0.03 174.69 19.09 35.72 863.4 17.0 

pilot whale median 22.8 0.01 165.20 18.87 35.98 716.8 11 .5 

n 9 8 8 8 3 21 21 

minimum 21 .7 0.0 157.1 17.9 35 .0 246 3 

maximum 29.0 0.1 203.6 21 .0 36.2 1906 69 

Beaked whale mean 24.8 0.12 173.47 18.66 35.84 1273.7 17.9 

median 24.7 0.10 165.35 18.79 35.80 1292.7 19.3 

n 12 12 6 7 4 16 16 

minimum 21 .5 0.0 159.3 17.9 35.5 253 3 

maximum 28.8 0.3 203.1 19.3 36.2 1852 33 
30 

U 

d 25 

m 

m 

E 

m 

FU 

C 

m 20 

15 

rn 

r-n 

m m 

- rn 

m 

i :u 

U 

:U 

UJ 

W 

STFR STCO PHMA TUTR GRGR KOGR ZIGR STAT 

Figure 9 .1 . Box plots of the sea surface temperature associated with sightings 

of eight cetacean species or species categories. The mid-line is 

the median, the box encompasses the interquartile range, and the 

dotted lines with brackets are 1 .5 x the interquartile range. 

Outlying points are shown individually by horizontal bars. Codes: 

STFR = Atlantic spotted dolphin, STCO = striped dolphin, 

PHM4 = sperm whale, TUTR = botUenose dolphin, GRGR = Risso's 

dolphin, KOGR = pygmy/dwarf sperm whale, ZIGR = beaked whale, 

STAT = pantropical spotted dolphin. 

336 
o.s 

0.4 

E 

Y 

r 

U 

0.3 

c 

m 

H 

0.2 

0.1 

0.0 

0.5 

m m 

m 

r:-I 

u u u u , : u 

STCO STFR STAT TUTR GRGR PHMA KOGR ZIGR 

Figure 9 .2 . Box plots of the sea surface temperature gradient associated with 

sightings of eight cetacean species or species categories . The 

mid-line is the median, the box encompasses the interquartile 

range, and the dotted lines with brackets are 1 .5 x the 

interquartile range. Outlying points are shown individually by 

horizontal bars . Codes: STFR = Atlantic spotted dolphin, 

STCO = striped dolphin, PHMA = sperm whale, TUTR = bottlenose 

dolphin, GRGR = Risso's dolphin, KOGR = pygmy/dwarf sperm 

whale, ZIGR = beaked whale, STAT = pantropical spotted dolphin. 

337 
Surface Salinity 

Only four of the 13 species or species categories had 10 or more sightings for 

surface salinity, and there was no significant difference among these four 

(KW = 2.4, df = 3, p = 0.49) (Tables 9.1 and 9 .2) . The overall mean was 

34.98 psu ± 0.99 SD. The mean annual surface salinity for the study area was 

35 .31 psu ± 2.16 SD. 

Bottom Depth 

There was a significant difference among the 13 species or species categories 

(KW = 362.6, df = 12, p < 0.001) with regard to bottom depth (Tables 9.1 and 9.2). A 

posteriors comparisons indicated that there were three distinct groups: 

Group 1 Atlantic spotted dolphins (mean = 197.1 m, SE = 19.3, n = 30) 

Group 2 bottlenose dolphins (mean = 293 .5 m, SE = 17.0, n = 149) 

Group 3 Risso's dolphins (mean = 713 .7 m, SE = 52.4, n = 67) 

short-finned pilot whales (mean = 863.4 m, SE = 86.8, n = 21) 

pygmy/dwarf sperm whales (mean = 928.5 m, SE = 57 .9, n = 72) 

rough-toothed dolphins (mean = 950.5 m, SE = 109.7, n = 16) 

spinner dolphins (mean = 1111 .0 m, SE = 112.3, n = 13) 

sperm whales (mean = 1104.9 m, SE = 41 .9, n = 65) 

striped dolphins (mean = 1235.2 m, SE = 111 .1, n = 18) 

rlesoplodon spp. (mean = 1196.9 m, SE = 100.2, n = 13) 

pantropical spotted dolphins (mean = 1241 .9 m, SE = 42 .0, n = 103) 

Clymene dolphins (mean = 1261 .0 m, SE = 83 .6, n = 22) 

beaked whales (mean = 1273.7 m, SE = 97.8, n = 16) 

The mean water depth for Atlantic spotted dolphins was significantly 

shallower than that of bottlenose dolphins . On average, Atlantic spotted 

dolphins occurred on the continental shelf while the bottlenose dolphins were 

seen along the shelf break. 

The species in Group 3 occurred in the deepest water along the continental 

slope. Within this group, there was a gradient of species found in shallower 

water to those found in deeper water (Figure 9.3) . Risso's dolphins and shortfinned pilot whales occurred in shallower water along the upper slope and, as 

a subgroup, were significantly different from striped dolphins, Mesoplodon 

spp., pantropical spotted dolphins, clymene dolphins and beaked whales which 

occurred in the deepest water. Pygmy/dwarf sperm whales, rough-toothed 

dolphins, spinner dolphins and sperm whales occurred at depths in the middle 

of these two subgroups and linked them together. The mean depth in the study 

area was 986 m ± 570 SD. 

Bottom Depth Gradient 

There was a significant difference among the 13 species or species categories 

(KW=53 .1, df=12, p < 0.001) with regard to bottom depth gradient (Tables 9.1 

338 
- m 

n 

rn 

1 

m 

n 

E 

r 

a 

m 0 

E 1000- 

m° u 

n 

u 

u 

u 

500 - Lij 

u 

w -

W lJ 

lJ 

LJ L:J 

5'z Q'C`)~C~P~ C~~~ .~~ c~~~ 

c~"N0Q~oP ~G 

oPG 

g 

9.3 . Box plots of the bottom depth associated with sightings ~Figure of eight 

cetacean species or species categories. The mid-line is the 

median, the box encompasses the interquartile range, and the 

dotted lines with brackets are 1 .5 x the interquartile range . 

Outlying points are shown individually by horizontal bars . Codes: 

STFR = Atlantic spotted dolphin, STCO = striped dolphin, 

PHMA = sperm whale, TUTR = bottlenose dolphin, GRGR = Risso's 

dolphin, KOGR = pygmy/dwarf sperm whale, ZIGR = beaked whale, 

STAT = pantropical spotted dolphin . 

339 
and 9 .2) . There was a gradation from species found over shallower bottom 

depth gradients to those found over steeper bottom depth gradients (Figure 

9.4) . Atlantic spotted dolphin occurred over the shallowest bottom depth 

gradient and were significantly different from the group with the steepest 

gradient which included sperm whales, Risso's dolphins, striped dolphins, and 

spinner dolphins. The other species occurred over bottom depth gradients in 

the middle . The mean bottom depth gradient in the study area was 

16.5 m/1.1 lm ± 14.6 SD. 

9.4 Discussion 

The mean, annual marine environment in the upper layer of the study area 

may be described as subtropical to tropical in temperature with normal 

seasonal variation in the depth of the mixed layer (i .e ., deepest [35-110 m] in 

the winter and shallowest [< SO m] in the summer). Salinity ranges from 34.9-

36 .5, excluding areas affected by the Mississippi River effluent (see Chapter 6 

and Appendix B for details) . The surface circulation is dominated by persistent 

warm-core (anticyclonic) eddies 100-300 km in diameter that separate from 

the Loop Current in the eastern Gulf and drift westward until they dissipate 

along the continental slope (Hofmann and Worley 1986, Biggs 1992) . Generally, 

one or more of these warm-core eddies is present in the Gulf at any one time. 

These dynamic circulation features transport large quantities of high-salinity, 

nutrient-poor water across the near-surface environment of the northern 

Gulf. Cold-core (cyclonic) eddies can be generated as the anticyclonic eddy 

interacts with the continental margin . As a result, the temperature and 

salinity in the upper 200-300 m can vary depending on whether 

measurements are taken inside or outside of these distinctive but ephemeral 

circulation features. Below a depth of several hundred meters, the Gulf has 

stable temperature-salinity characteristics that are independent of warm-core 

or cold-core eddies. 

Primary productivity was low (< 0.4 mg chlorophyll a/m3) throughout most of 

the study area (see Chapter 6) . Although the data from this study were 

insufficient to detect a seasonal phytoplankton bloom, pigment concentrations 

in Gulf waters beyond the shelf break do exhibit a seasonal cycle with highest 

concentrations between December and February and lowest values between 

May and July (Mullet-Karger et al. 1991) . The general oligotrophic conditions 

observed in this study are in agreement with previous reports of low 

concentrations of phytoplankton, nutrients, and zooplankton in the Gulf (IISayed 1972, Muller-Karger et al. 1991, Biggs 1992). However, higher nutrient 

concentrations associated with cold-core eddies and effluent from the 

Mississippi River increase plankton biomass and result in local "hot spots" of 

primary (> 5 mg chlorophyll a/m3) and secondary productivity, including fish 

species of commercial significance (Parsons et al . 1985, Park et al . 1989, Govoni 

et al. 1989, Ortner et al . 1989, Chapter 6 and Appendix C) . In addition, offshore 

jets of shelf-borne phytoplankton created by the confluence of warm-core 

and cold-core eddy pairs can also increase production beyond the continental 

slope (Biggs and Miiller-Karger 1990 . 

340 
120 

100 

Y 80 

rr 

C 

m 

T 60 

L 

d 

d 

O 

E 0 

M° 40 

20 

0 

Figure 9.4 . Box plots of the bottom depth gradient associated with sightings 

of eight cetacean species or species categories. The mid-line is 

the median, the box encompasses the interquartile range, and the 

dotted lines with brackets are 1 .5 x the interquartile range. 

Outlying points are shown individually by horizontal bars. Codes: 

STFR = Atlantic spotted dolphin, STCO = striped dolphin, 

PHN'1A = sperm whale, TliTR = bottlenose dolphin, GRGR = Risso's 

dolphin, KOGR = pygmy/dwarf sperm whale, ZIGR = beaked whale, 

STAT = pantropical spotted dolphin. 

341 
The cetaceans in this study occurred in water with a relatively narrow range 

of annual SSTs, similar to those reported previously for cetaceans in the 

northern Gulf (Fritts et al . 1983) and for pantropical spotted and spinner 

dolphins in the eastern tropical Pacific (Au and Perr-yman 1985, Perrin and 

Hohn 1994, Perrin and Gilpatrick 1994, Perrin et al . 1994b) . The seasonal 

variation in SST in the Gulf typically ranges from S-7° C, with little 

interannual variation (Tluller-Krager et al . 1991). It is noteworthy that the 

deep diving species (e.g ., Risso's dolphins, sperm whales, pygmy/dwarf sperm 

whales, and beaked whales) in this study occurred in water with the steepest 

SST gradient. These species are known to feed on squid and may be foraging 

along thermal fronts associated with eddy systems. These areas may be 

associated with upwelling events that are more productive than the warmer, 

oligotrophic surface water (Biggs 1992, Biggs and Mullet-Karger 1994). SST 

gradient also seems to be a reasonable correlate to the distribution and 

migration of yellowfin tuna which feed on many of the same organisms as the 

dolphin species encountered (Perrin et al . 1973, Sharp and Dizon 1978) 

The mean values for the depth of the 15'C isotherm (< 250 m), the temperature 

at 100 m (< 22° C) and surface salinity (< 36.6 psu) indicate that most of the 

cetacean sightings were outside of the Subtropical Underwater that flows into 

the Gulf through the Yucatan Straits and is found in the region of the Loop 

Current and warm-core rings derived from that current (Hofmann and Worley 

1986). As the warm-core rings move westward across the northern Gulf, they 

mix with Gulf Common Water. Discharge from the Mississippi and Atchafalaya 

rivers gives rise to a band of low-salinity water that usually flows westward 

over the continental shelf but can have a freshening influence as far south as 

26°N (Nowlin 1972). The relatively stable, mean surface salinity beyond the 

shelf edge is probably responsible for the absence of any significant 

difference for this environmental variable among cetacean species in the 

study area. However, the large range of salinities recorded (some as low as 15 .8 

psu) for those species frequently observed along the shelf break in the northcentral Gulf reflects the miring of the Mississippi River discharge and nearshore water. 

Bottom depth showed the clearest indication of habitat partitioning in the 

study area. Atlantic spotted dolphins were consistently found in the shallowest 

water on the continental shelf and along the shelf break. In addition, the 

bottom depth gradient was less for Atlantic spotted dolphins than for any other 

species . This agrees with observations of this species along the west Florida 

shelf (Mullin unpubl. obs.) . rlullin et al . (1994) sighted Atlantic spotted 

dolphins in offshore waters along the Louisiana coast that were in deeper 

water (mean depth of 367 m) than those seen in this study. However, the 

continental shelf is very narrow with a steep bottom gradient along the 

Louisiana coast, so that small movements offshore result in a rapid change in 

depth. Overall, it appears that Atlantic spotted dolphins prefer shallow water 

with a gently sloping bottom typical of the continental shelf, although they 

may also occur along the shelf break and upper continental slope. Their 

occurrence in shallow, shelf waters may be related to prey preference and 

foraging strategies . Atlantic spotted dolphins are known to feed on small 

cephalopods, fish, and benthic invertebrates (Perrin et al. 1994a) . Fertl and 

Wiirsig (1995) described the behavior of Atlantic spotted dolphins feeding in a 

coordinated manner on a school of clupeid fish. A rehabilitated Atlantic 

342 
spotted dolphin that was monitored with a satellite-linked time-depth recorder 

along the Texas coast for 24 days spent most of its time at depths less than 10 m. 

However, the dolphin consistently made dives that were on or near the 

seafloor in 30 m of water (Davis et al. in press) . 

The bottlenose dolphins in this study were found most commonly along the 

upper slope in water significantly deeper than that for Atlantic spotted 

dolphins. This species also occurs in nearshore waters where it is more 

abundant than the Atlantic spotted dolphin (Perrin et al. 1994a) . However, 

morphometric, hematological and hemoglobin differences indicate that the 

larger, offshore bottlenose dolphins may not mix with the nearshore 

population (Hersh and Duffield 1990) . The mean depths from this study for 

offshore bottlenose dolphins and Atlantic spotted dolphins are biased towards 

deeper depths because eve did not survey the entire range of their habitat that 

includes shallower waters of the continental shelf. Pelagic bottlenose dolphins 

feed on a variety of epipelagic and mesopelagic fish, squid, and crustaceans 

(Walker 1981, Barros and Odell 1990) 

The deep water cetaceans of Group 3 are diverse in size, diving ability, and 

prey preference. Large species such as the sperm whale, Afesoplondon spp. 

and beaked whales are known or believed to be deep divers that feed on squid 

and mesopelagic or deep water benthic fish (Raun et al. 1970, Berzin 1971, Rice 

1989, Heyning 1989, Clarke 1986, Findlay et al . 1992) . Sperm whales are capable 

of exploiting most of the water column in the study area and regularly occur 

in water over 1,000 m deep. Previous sightings of sperm whales in the Gulf 

were in waters of similar depth to that of this study (Fritts et al. 1983, Collum 

and Fritts 1985) . Their frequent occurrence near the Mississippi Canyon 

suggests that this may be an important part of their habitat in the northcentral Gulf. However, sperm whales were not commonly sighted near DeSoto 

Canyon in the Gulf, and submarine canyons were apparently not important 

habitat for sperm whales off the North Atlantic coast (Kenney and Winn 1987) . 

The combination of deep water within 20 km of the Mississippi River delta and 

the enhanced primary productivity associated with the river discharge (see 

Chapter 6) may increase the abundance of squid and be responsible for the 

year-round occurrence of sperm whales in this part of the north-central Gulf. 

Of the 42 species of cephalopods known to occur in the Gulf, most are 

contiguous with species from the North Atlantic (Voss 196). Less than 109'6 of 

these species are endemic to the Gulf and apparently these are confined to 

benthic species . 

Risso's dolphins and short-finned pilot whales occurred most commonly along 

the mid-to-upper slope, often in areas with a steep bottom gradient. A similar 

range of bottom depths was observed for these species by Fritts et al . (1983) 

and Mullin et al. (1994) for the northern Gulf, by Dohl et al. (1981) along the 

California coast, and by Findlay et al . (1992) along the South African coast. The 

deeper water and steep bottom gradient characteristic of Risso's dolphin and 

pilot whale habitat may be linked to their diet of squid (Evans 1987, Jefferson 

et al . 1993). However, no information on the diving and feeding behavior of 

these species is available. 

Pygmy/dwarf sperm whales feed on squid, benthic and mesopelagic fish, and 

crustaceans (Fitch and Brownell 19G8) . Their bottom feeding habits and 

343 
considerable body oxygen stores for their size suggest that they make deep 

dives along the continental slope (Raun et al . 1970, Caldwell and Caldwell 1989) . 

The apparent preference of dwarf sperm whales for areas over or near the 

shelf break is consistent with their greater frequency of sightings in this 

study. Nevertheless, pygmy sperm whales have historically stranded more 

frequently in the Gulf than dwarf sperm whales (Schmidly and Scarbrough 

1990, see Chapter 2 of this report) . Without surveying the entire Gulf, the 

habitat differences between these two species remain speculative. 

Rough-toothed, spinner, clymene, pantropical, and striped dolphins are all 

small cetaceans that occur over deep water beyond the continental shelf 

(Jefferson et al. 1993), although their small size probably limits them to the 

upper 200 m of the water column (Williams et al. 1993). Their occurrence in 

deep water may be linked to the offshore location of their prey (Ridgway and 

Harrison 1994). For example, striped dolphins off the coast of Japan feed on 

myctophid fish and squid, many with luminous organs, associated with the 

deep scattering layer (Tliyazaki et al. 1973). In the eastern tropical Pacific, 

spinner dolphins also feed on vertically migrating fish, especially myctophids 

(Fitch and Brownell 1968) . It is difficult to say anything definitive about the 

habitat of the species not discussed above (see Appendix A) because of the 

relatively small number of sightings. The average SST, SST gradient, depth of 

the 15* C isotherm, water temperature at 100 m, and surface salinity were 

similar to those of the species in Table 9.2 . The false killer whales were 

generally sighted over the mid-slope, although they could occur from the 

shelf break to the lower slope. Fraser's dolphins, melon-headed whales, and 

killer whales generally occurred from the mid-to-lover slope over areas with 

a steep bottom gradient. A Bryde's whale was sighted in shallow water on the 

shelf break . Similar depth ranges for many of these species have been 

recorded by Fritts et al . (1983) and Mullin et al. (1990 for the northern Gulf. 

9 .5 Conclusions 

Although the study area covered 154,621 km2 (about 109'0 of the Gulf, it may 

represent only a part of the home range of the species observed. Even during 

normal years, the oceanographic features of the north-central and western 

Gulf are very complex and dynamic due to the formation of warm-core rings 

from the Loop Current and seasonal fresh water discharge from the 

Mississippi and other rivers . The GulfCet aerial and shipboard surveys 

occurred during a period when several warm-core rings moved through the 

northern Gulf. At the same time, there was an unusually large influx of fresh 

water from the Mississippi River due to record rainfall in the midwestern 

states in 1993 . As a result, some of the oceanographic variables were atypical 

during the two years of this study, and the data were insufficient to address 

seasonal and interannual variability in habitat. 

The mean, annual marine habitat for cetaceans in the study area is subtropical 

to tropical in the upper 200-300 m, with relatively low primary productivity. 

Cooler, nutrient-rich water below this layer is brought to the surface by the 

upwelling influences of cold-core eddies. Nutrients are also introduced into 

the Gulf by the Mississippi River. This results in a patchy distribution of 

primary productivity with "hot spots" of chlorophyll a that may increase 

secondary productivity and attract cetaceans. Additional studies will be needed 

344 
to examine the distribution of cetaceans around these areas of high 

productivity. 

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Perrin, W.F ., D.K. Caldwell, and M.C. Caldwell . 1994a. Atlantic spotted dolphin 

Stenella frontalis (G . Cuvier, 1829), pp. 173-190. In S.H. Ridgway and 

R. Harrison, eds. Handbook of marine mammals, volume 5 : The first 

book of dolphins. Academic Press, San Diego. 

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coeruleoalba (Meyen, 1833), pp. 129-159. In S.H. Ridgway and 

R. Harrison, eds. Handbook of marine mammals, volume 5 : The first 

book of dolphins. Academic Press, San Diego. 

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Kogia, on the Texas coast. Tex. J. Sci. 21:269-274. 

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66:1-11 . 

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177-233 . In S.H . Ridgway and R. Harrison, eds. Handbook of marine 

mammals, volume 4: River dolphins and the larger toothed whales . 

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volume 5: The first book of dolphins. Academic Press, San Diego. 416 pp. 

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Mexico: Past, present, and future, pp. 59-76. In Sea turtles and marine 

mammals of the Gulf of Mexico: Proceedings of a workshop held in New 

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LA. 

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Academic Press, New York. 485 pp. 

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348 
Smith, R.C., P. Dustan, D. Au, K.S . Baker, and E.A. Dunlap. 1986. Distribution of 

cetaceans and sea-surface chlorophyll concentrations in the California 

Current. Mar. Biol. 91 :38-402. 

Voss, G.L 1956. A review of the cephalopods of the Gulf of Mexico. Bull. Mar. 

Sci. Gulf Caribb. 6 :85-140. 

Walker, N., L Rouse, G.S . Fargion, and D.C. Biggs . 1994. Circulation of 

Mississippi River water discharged into the northern Gulf of Mexico by 

the Great Flood of Summer 1993. EOS 7 :51 . 

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bottlenose dolphins (Tursiops) in the eastern North Pacific. Admin. 

Rept. W-81-03C. National Marine Fisheries Service, La Jolla, CA. 54 pp. 

Waring, G.T., C.P. Farifield, C.M. Ruhsam, and r1. Sano. 1993 . Sperm whales 

associated with Gulf Stream features off the north-eastern USA shelf. 

Fish. Oceanogr. 2 : 101-105. 

Williams, T.M., W.A Friedl, J.E. Haun, and N.K. Chun. 1993 . Balancing power 

and speed in bottlenose dolphins (Tursiops rruncarus), pp. 383-394. In 

I .L Boyd, ed. Marine mammals - advances in behavioral and population 

biology. Oxford University Press, New York. 

349 
X. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE STUDIES 

R.W. Davis, W.E . Evans, B. Wiirsig, and G.S . Fargion 

The GulfCet Program was the first attempt to determine the distribution and 

abundance of cetaceans along the entire continental slope in the northcentral and western Gulf of Mexico. Although the scope of this program was 

greater than previous studies in this part of the Gulf (Fritts et al. 1983, hiullin 

et al. 1994), the total area surveyed represented only 109'0 of the entire Gulf and 

was small (0.0089'0) in comparison to cetacean surveys and habitat 

characterization in the eastern tropical Pacific Ocean (ETP) (Wade and 

Gerrodette 1993). 

Cetaceans were commonly observed throughout the GulfCet study area during 

all four seasons. However, the determination could not be made whether 

animals were in transit or resident in the study area for extended periods, and 

it is likely that the small study area encompasses only a portion of the home 

range for many of the species observed. For example, a radio-tagged 

pantropical spotted dolphin in the ETP moved over 500 km (3-times the 

maximum latitudinal dimension of the GulfCet study area) in 27 days (Pemn et 

al. 1979). Without additional information on daily movement patterns and 

feeding behavior, we can only speculate whether the association of animals 

with mesoscale oceanographic features such as cold-core rings is biologically 

meaningful. 

The diversity of cetaceans in the study area was comparable to that along the 

continental slope of northeastern United States and in the EI'P (Rain et al . 

1985, Wade and Gerrodette 1993). However, the overall density of cetaceans in 

the GulfCet study area was significantly lower (<2590) than in these latter two 

regions. In addition, baleen whales, especially fin whales (Balenoprera 

physal us), made up a significant number (ca. 1096) of identified sightings 

along the northeastern United States (Hain et al . 1985), but were practically 

absent from the GulfCet study area. Group density in the study area was 

approximately the same as that found in the E'I'P. 

The lower densities of whales in the Gulf compared to the northeastern United 

States and ETP could be related to the more oligotrophic conditions and a 

smaller food base that cannot support a high density of cetaceans. The 

estimated biomass of cetaceans in the study area was 9,131 metric tons (Table 

10.1) . Of this biomass, sperm whales represented 68.6%. Together, sperm 

whales, pantropical spotted dolphins, short-finned pilot whales, bottlenose 

dolphins, melon-headed whales, killer whales, and striped dolphins constituted 

92.7% of the cetacean biomass. The minimum estimated food requirement for 

all cetaceans (assuming adult body masses for each species) vas estimated to be 

80,694 metric tons/year, of which sperm whales consumed 4290. Dividing the 

minimum food requirement by the size of the study area gives a cetacean food 

consumption rate of 0.52 metric tons/km2/year . This value is about 2096 of the 

estimated annual food consumption per km2 for cetaceans luring along the 

continental slope in the northeastern United States, and it may reflect the 

lower primary and secondary productivity of the Gulf (Rain et al . 1985) . 

351 
Table 10.1 . Cetaceans ranked by abundance with mean body mass, 

species-total biomass (percent of total biomass shown 

in parentheses), and minimum food requirements . 

Species-total Min. food 

Mean biomass requirement 

Species or Abun- adult body (kg) 3 (metric 

species category dancer mass (kg)-' (with % total) tons/yr)4 

Pantropical spotted dolphin 7,10 90 639,450 (7.00) 13,261 

Bottlenose dolphin 2,538 150 380,700 (4.17) 6,98 

Striped dolphin 2,091 90 188,190 (2 .06) 3,903 

Melon-headed whale 2,076 160 330,720 (3 .62) 5,940 

Clymene dolphin 1,69 50 8=x,750 (0.93) 2,036 

Atlantic spotted dolphin 1,145 100 114,500 (1 .25) 2,312 

Spinner dolphin 840 SO 42,000 (0.46) 1,008 

Risso's dolphin 529 300 158,700 (1 .74) 2,436 

Sperm whale 313 20,000 6,260,000 (68.56) 33,624 

Short-finned pilot whale 215 2,000 430,000 (4.71) 4,107 

Rough-toothed dolphin 177 100 17,700 (0.19) 358 

Unidentified Ziphiidae 124 1,30.5 167,400 (1 .83) 1,764 

Dwarf sperm whale 88 135 11,880 (0.13) 223 

Killer whale 71 3,000 213,000 (2 .33) 1,838 

Fraser's dolphin 65 160 10,400 (0.11) 187 

Pygmy killer whale 36 110 3,960 (0.04) 78 

Pygmy sperm whale 19 315 5,985 (0.07) 91 

Cuvier's beaked whale 14 1,800 25,200 (0.28) 247 

False killer whale 10 1,000 10,000 (0.11) 114 

Bryde's whale 3 12,000 36,000 (0.39) 220 

Total 9,130,535 80,695 

1 Annual abundance estimated from GulfCet shipboard visual survey data. 

Z Data published by Carwardine (1995). 

3 Total biomass was derived by multiplying the species abundance by the mean body 

mass. 

° Estimated minimum food requirement (N1FR, metric tons/year) for each species or 

species category was calculated as: T4FR = [(A ~ B~C) +(D ~ E)], where A = annual species 

abundance in the study area; B = species resting metabolic rate (kcal/day), equal to 

70M°'75 (M = mass in kg); C = 2, the multiple of resting metabolic rate used to estimate 

the actual field metabolic rate (Hinga 1979) ; D = 0.8, the assimilation efficiency 

(Lockyer 1981) ; and E = 1 kcal/g wet weight, the assumed caloric density of prey (Hinga 

1979, Hain et al . 1985). 

5 Average of body masses for Cuvier's beaked whale (1,800 kg) and Blainville's beaked 

whale (900 kg) . 

Without synoptic data on the abundance and distribution of cetaceans in the 

entire Gulf of Mexico, we cannot determine the importance of the GulfCet study 

352 
area for the 20 species sighted. However, approximately 19,000 cetaceans 

utilize the study area annually, which indicates that the continental slope in 

the north-central and western Gulf is of some importance to these animals . 

The dynamic hydrography of the north-central and western Gulf resulting 

from the formation and movement of warm-core and cold-core rings and the 

outflow of fresh water from the Mississippi River makes a description of 

average habitat difficult and may obscure important associations of cetaceans 

with distinctive hydrographic features associated with feeding. 

To obtain a more complete understanding of the seasonal and annual 

distribution, abundance, and habitat utilization of cetaceans, a survey of the 

entire Gulf and the satellite and conventional radio-tracking of the 

predominant species, such as pantropical dolphins, could be conducted . In 

addition to location at sea, the satellite telemeters can record information on 

diving behavior that could provide clues concerning potential prey species 

and resource partitioning among cetaceans (Evans 1971, Tanaka 1987, Mate 

1989, Merrick et al . 1994). This information, in addition to trophic level studies 

of primary and secondary productivity and prey distribution, could enable 

researchers to gain a better understanding of the factors that influence the 

distribution of cetaceans. The diet of a significant number of the cetaceans in 

the Gulf is dominated by cephalopods and mesopelagic fishes associated with 

the vertically migrating acoustic deep scattering layer (Perrin et al . 1973). A 

long-term monitoring program is needed to obtain baseline information on 

cetaceans before oil and gas development moves further onto the continental 

slope . Concurrent with synoptic surveys, focal studies could examine the 

presence of cetaceans around distinctive oceanographic features, such as coldcore eddies and the Mississippi River freshwater plume, in order to better 

understand the influence of these features on cetacean distribution. 

During the implementation of the focal studies, behavioral data could be 

gathered to determine whether animals are using certain areas for specific 

activities, such as social/sexual behavior, foraging, resting, or transiting . The 

behavioral studies could be conducted concurrently with aerial surveys and 

shipboard visual and acoustic surveys . However, dedicated cruises are also 

needed to study: 1) behavioral patterns, 2) at-sea movements and diving 

behavior, and 3) the reaction of cetaceans to oil and gas exploration and 

development activities. During these cruises, sperm whales chould be further 

photo-identified and skin and blubber biopsies taken to improve our 

understanding of population biology and town loads of sperm whales. 

Seventy-eight percent of the oil and 97g'o of the gas production in United States 

federal waters occurs in the Gulf of Mexico, primarily along the TexasLouisiana continental shelf (Minerals Management Service, New Orleans, LA, 

November 1995) . During the period 1984 to 1994, the MTiS western and central 

Gulf regions produced about 3 .4 x 109 barrels of crude oil and 50.2 x 109 million 

cubic feet of natural gas (Technical Information Management System 

Database, Offshore Systems Center, Minerals Management Service, New 

Orleans, LA, July 1995). In addition to oil and gas exploration along the 

continental shelf, this area has considerable commercial shipping traffic that 

enters the northern Gulf ports . The long-term forecast for petroleum 

transportation is for the total volume to increase into the next century. This, 

coupled with the move of the petroleum industry into deeper waters in their 

353 
continuing quest for new oil and gas reserves, may result in significant 

impact on cetaceans along the continental slope of the Gulf (Tucker and 

Associates, Inc. 1990) . The long-term consequences of human activity cannot 

be predicted with certainty. However, it can be anticipated that cetaceans will 

encounter oil and gas exploration and production activity as these move 

further onto the continental slope. These activities include construction, oil 

spills, ship traffic, seismic exploration, and underwater noise. 

Major construction activities will include the installation of drilling rigs, 

platforms, and pipelines. There are three primary concerns associated with 

construction activities on the continental shelf and slope . These involve sea 

floor disturbance, the attraction of fish and invertebrates to submerged 

structures, and the potential interaction of these structures with resident or 

migrating cetaceans (Phillips and James 1988). Stationary rigs may alter 

habitat by acting as fish attractants, enhancing prey availability for certain 

species of cetaceans. Negative impacts to cetaceans may result from seismic 

exploration activities (Richardson et al. 1995), the sounds produced by rig 

construction and oil and gas exploration and production, and the potential for 

collisions with increased ship activity . The only way to determine the long 

term effects of these activities is through a monitoring program that 

commences ahead of the widespread implementation of deep water exploration 

and production. Such a monitoring program would involve traditional aerial 

and shipboard visual surveys, shipboard acoustic surveys, behavioral 

observations of the cetaceans encountered, and satellite and conventional 

radio telemetry studies of the predominant cetacean species. The shipboard 

acoustic surveys are particularly useful because they monitor the presence of 

vocalizing cetaceans as well as ambient noise levels . In addition, this kind of 

data can be archived for later analysis as exploration and production activities 

develop and change over long periods of time. 

The Gulf of Mexico is rich in species occurring throughout the food chain that 

are acoustically very active . It is unfortunate that so little information is 

available on ambient noise levels, source levels from fish and dolphins, and 

especially accurate data on exposure levels as a function of frequency during 

the explosive removal of platforms and other noise associated with oil and gas 

development. This fact makes the northern Gulf of Mexico ideal for using 

acoustic monitoring to study the seasonal movements, distribution, and 

abundance of cetaceans. 

The sound reception and production capabilities of many suborders and 

families of marine animals are well known (Richardson et al . 1995). Every 

group of cetacean studied has been found to vocalize and to have broad-band 

hearing sensitivity. This includes all of the 28 species thought to occur in the 

Gulf of Mexico, even though quantitative data on hearing exists for only four 

or five of these (i .e ., bottlenose dolphins, false killer whales, killer whales, 

rough-toothed dolphins, and short-finned pilot whales). 

Lack of quantitative data on the responses of marine animals, especially 

marine mammals, to man-made noise emphasizes the concerns expressed by 

the National Research Council's Ocean Studies Board Committee and the need 

for better and precise measurements of the acoustic environment of marine 

animals (National Reseach Council 199 ; Richardson et al . 1995) . In their 1994 

354 
Report to Congress, the Marine Mammal Commission stated these same 

concerns and noted that acoustic monitoring would be useful. In addition to 

the comprehensive libran~ of marine animal sounds that the GulfCet study has 

already archived, a great deal of data on ambient noise associated with 

shipping and oil and gas exploration have also been recorded. These data await 

analysis and interpretation. 

In conclusion, the continental slope in the north-central and western Gulf of 

Mexico is an area that supports a diverse cetacean community, but one whose 

density does not equal areas such as the northeastern United States and the 

ETP. Very complex and dynamic oceanographic and mesoscale features typify 

this area of the Gulf and show large annual and inter-annual variability. This 

males it difficult to predict the distribution of cetaceans based on existing data. 

However, the GulfCet program has demonstrated that any future monitoring 

programs would need to be long-term, with relatively intensive sampling 

effort in order to detect significant changes in the density and distribution of 

cetaceans. Of special interest is the demonstration of acoustic monitoring 

techniques, which hold great promise for long-term monitoring programs of 

cetacean distribution and abundance. 

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Carwardine, M. 1995 . Whales dolphins and porpoises - the visual guide to all 

the world's cetaceans. Dorling Kindersley, New York. 256 pp. 

Evans, W.E 1971 . Orientation behavior of delphinids: Radio telemetric studies. 

Ann. NY Acad . Sci . 188:142-1G0. 

Fritts, T.H., A.B. Irvine, R.D. Jennings, LA. Collum, W. Hoffman, and M.A. 

McGehee. 1983 . Turtles, birds, and mammals in the northern Gulf of 

Mexico and nearby Atlantic waters. Rept. FWS/OBS-82/65. U.S . Fish and 

Wildlife Service, Office of Biological Services, Washington, D.C. 455 pp. 

Hain, J.H.W., M.A.T1. Hyman, R.D. Kenney, and H.E Winn. 1985 . The role of 

cetaceans in the shelf-edge region of the northeastern United States . 

Mar. Fish. Rev. 47:13-17. 

Hinga, K.R. 1979 . The food requirements of whales in the southern 

hemisphere. Deep Sea Res. 26A:569-577 . 

Lockyer, C. 1981 . Estimation of the energy costs of growth, maintenance, and 

reproduction in the female minke whale (Balaenoptera acurostrata) 

from the southern hemisphere. Rep. Int. Whal. Comm. 31 :337-343 . 

Mate, B.R. 1989. Satellite-monitored radio tracking as a method for studying 

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Merrick, R.L, T.R. Loughlin, G.A. Antonelis, and R. Hill . 1994. Use of satellitelinked telemetry to study Steller sea lion and northern fur seal 

foraging . Polar Sci . 13:10-144. 

355 
Mullin, K.D., W. Hoggard, C.L. Roden, R.R. Lohoefener, C.T1 . Rogers, and 

B. Taggart. 1994. Cetaceans on the upper continental slope in the 

north-central Gulf of Mexico. U.S. Fish. Bull. 92 :773-78G. 

National Reseach Council. 1994. Low-frequency sound and marine mammals -

current knowledge and research needs . National Academy Press, 

Washington, D.C. 75 pp. 

Perrin, W.F., R.R. Warner, C.H. Fiscus, and D.B. Holts. 1973 . Stomach contents of 

porpoise, Stenella spp., and yellowfin tuna, Thunnus albacores, in 

mimed-species aggregations. U.S . Fish. Bull. 74:1077-1092. 

Perrin, W.F., W.E. Evans, and D.B. Holts. 1979 . Movements of pelagic dolphins 

(Stenella spp.) in the eastern tropical Pacific as indicated by results of 

tagging, with summary of tagging operations 1969-76 . NOAH Tech. Rept. 

Nr1FS-SSRF-737 . National Oceanic and Atmospheric Administration, 

Rockville, MD. 14 pp. 

Phillips, N.W., and B.M. James, eds . 1988. Offshore Texas and Louisiana marine 

ecosystem data synthesis, volume II : Synthesis report. OCS study N1MS 

88-0067. U.S . Dept. of the Interior, Minerals Management Service, Gulf 

of Mexico OCS Regional Office, New Orleans, LA. 477 pp. 

Richardson, W.J ., C .R. Green, Jr., C.I . Tialme, and D.H . Thomson, with 

contributions by S.E. Moore and B. Wursig. 1995 . Marine mammals and 

noise. Academic Press, San Diego. 576 pp. 

Tanaka, S. 1987. Satellite radio tracking of bottlenose dolphins, Tursiops 

truncatus. Nippon Suisan Galkaishi 53 :1327-1338. 

Tucker and Associates, Inc. 1990. Sea turtles and marine mammals of the Gulf 

of Mexico, proceedings of a workshop held in New Orleans, August 1-3, 

1989. OCS Study MMS 90-0009. U.S . Dept. of the Interior, Minerals 

Management Service, Gulf of Mexico OCS Regional Office, New Orleans, 

IA. 211 pp. 

Wade, P.R., and T. Gerrodette. 1993 . Estimates of cetacean abundance and 

distribution in the eastern tropical Pacific. Rep. Int. Whal. Comm. 

43:477-493 . 

356 
ENT FTy~ 'y~ The Department of the Interior Mission 

a - p As the Nation's principal conservation agency, the Department of the Interior has responsibility for 

g most of our nationally owned public lands and natural resources. This includes fostering sound use 

of our land and water resources; protecting our fish, wildlife, and biological diversity ; preserving the 

_ aA 

q ~ 

environmental and cultural values of our national parks and historical places; and providing for the 

RCH 3 ~ enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration . 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

. i (OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

,r ~ew~ Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury . 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.