ACCESS NUMBER: BRD 96-0004 and 96-0009

STUDY TITLE: Coastal and Marine Ecosystem Program: Distribution and Abundance 

of Marine Mammals in the Gulf of Mexico (GulfCet II) 

REPORT TITLE: Cetaceans, Sea Turtles and Seabirds in the Northern Gulf of Mexico: 

Distribution, Abundance, and Habitat Associations, Volume I: Executive Summary, 

Volume II: Technical Report, and Volume III: Data Appendix 

CONTRACT NUMBER(S): USGS BRD: 1445-CT09-96-0004 and 1445-IA09-96-0009 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Western, Central, and Eastern Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1996; 1997; 1998 

COMPLETION DATE OF REPORT: January 2000 

COST(S): FY 1996: $934,708 ; FY 1997: $712,401 ; FY 1998: $624,123 ; 

CUMULATIVE PROJECT COST: $2,271,232 

PROJECT MANAGER: R. Davis 

AFFILIATION: Texas A&M University 

ADDRESS: Department of Marine Biology, 5007 Avenue U, Galveston, Texas, 77551 

PRINCIPAL INVESTIGATOR(S)*: D. Biggs, R. Davis, W. Evans, K. Mullin, J. Norris, C. 

Ribic, J. Wormuth, B. Würsig 

KEY WORDS: Western Gulf; Central Gulf; Eastern Gulf; cetaceans; whales; dolphins; 

sea turtles; seabirds; habitats; food habits; physical oceanography; biological 

oceanography; satellite oceanography; marine ecology; acoustics; currents; distribution 

and abundance; oil and gas; surveys; zooplankton; micronekton; line transects; 

anthropogenic impacts; behavior 

BACKGROUND: Cetaceans, sea turtles, and seabirds are upper trophic level 

predators that play an important role in the pelagic marine ecosystem of the Gulf of 

Mexico. These animals are highly valued, protected by national laws and international 

agreements; knowledge of their distribution, abundance, and ecology is vital to their 

protection. The GulfCet II study was conducted to help resolve issues concerning the 

potential impacts of various deep oil and gas activities on cetaceans, sea turtles and 

seabirds on the northern Gulf of Mexico, continental slope in waters 100-2,000 m deep. 

The GulfCet II surveys (1996-1998) complemented and supplemented the spatial and 

temporal extent of the GulfCet I shipboard and aerial surveys that had been conducted 

in the north-central and northwestern Gulf from 1992-1994. The U. S. Geological 
ACCESS NUMBER: BRD 96-0004 and 96-0009

Survey, Biological Resources Division procured and administered the GulfCet II study 

based on information needs of the Minerals Management Service 

OBJECTIVES: (1) Obtain data on temporal and spatial patterns of distribution and 

minimum abundance estimates of cetaceans using line-transect and acoustic survey 

techniques directly comparable to those used in previous (GulfCet I) surveys, including 

incidental sightings of sea turtles and seabirds; (2) Identify possible associations 

between cetacean and seabird high-use habitats and the ocean environment, and 

attempt to explain any relationships of physical and biological variables to cetacean 

distributions. 

DESCRIPTION: It is generally believed that the distribution of cetaceans and seabirds 

in the Gulf of Mexico is linked, in part, by the availability of their foods. Although the 

diets of most cetaceans and seabirds in the Gulf are poorly known, it was hypothesized 

that different hydrographic regimes in the northern Gulf have different levels of potential 

prey, and furthermore, that these prey might be locally concentrated in nutrient-rich 

areas. To evaluate these hypotheses, an integrated approach was used to survey 

cetaceans and seabirds, characterize associated hydrographic and bathymetric 

features, and measure the biomass of co-occurring zooplankton and micronekton. The 

study area encompassed over 400,000 km2. The region was characterized 

hydrographically using data from expendable bathythermographs (XBTs), conductivitytemperature-depth (CTD) stations, nutrient and chlorophyll measurements, collected 

during four cruises and from tandem remote sensing of sea surface height using the 

TOPEX/POSEIDON and ERS-2 satellite altimeters. The distribution and abundance of 

potential cetacean foods were measured via nighttime net tows using a Multiple 

Opening/Closing Net and Environmental Sampling System (MOCNESS) and an IsaacsKidd Midwater Trawl. The numbers of cephalopod (squid) paralarvae and myctophids 

(an abundant family of midwater fishes) collected in the nets were used to estimate 

relative prey availability. Continuous measurements of zooplankton and micronekton 

biomass were made using an Acoustic Doppler Current Profiler (ADCP), whereby the 

acoustic backscattering strength of the ADCP signals were calibrated by regression 

analysis from the biomass obtained from the net tows. Line transect methods from 

shipboard and aerial surveys were used to make abundance estimates of cetaceans 

and seabirds. Cetacean distribution was also evaluated by recording species-specific 

vocalizations and simultaneous anthropogenic sounds (ambient noise) as detected by a 

towed hydrophone array. 

SIGNIFICANT CONCLUSIONS: The Mississippi River outflow and two major 

circulation features dominate an oceanographically complex, dynamic Gulf of Mexico 

and strongly influence the associated biotic communities. The Loop Current, 

connecting the Yucatan and Florida Currents, governs the circulation of the eastern 

Gulf. In the central and western Gulf, a warm-water anticyclonic eddy with associated 

cold-water cyclones, are the primary features. High levels of nutrients (nitrate) and 

chlorophyll were associated with the Mississippi River plume, shelf edge upwelling 

(during early summer months), and the cold-water cyclones. The trawls and bioacoustic 

surveys showed that the cyclones had locally higher stocks of zooplankton and 
ACCESS NUMBER: BRD 96-0004 and 96-0009

micronekton. Cephalopod paralarvae and myctophid abundances were relatively higher 

in the cyclone and confluence areas relative to areas with anticyclonic circulation, 

suggesting that such areas may afford enhanced foraging for cetaceans. Indeed, 

cetaceans in general were concentrated along the continental slope in areas of cyclonic 

circulation and where chlorophyll and zooplankton biomass were elevated. Cetaceans 

were less frequently observed over water deeper than 2,000 m and in anticyclones. 

Sperm whales were sighted most frequently in the area off the mouth of the Mississippi 

River along the 1000 m isobath and showed an affiliation with the edges of cyclonic and 

anticyclonic eddies. Photo-identified sperm whales showed intra- and inter-year site 

fidelity to the Mississippi Canyon, suggesting that this area is important habitat for this 

endangered species. The mouth of the Mississippi River also had relatively large 

number of certain seabirds, including the pomarine jaeger and black tern. The highest 

diversity of seabirds was found in areas of cyclonic circulation. Recordings from the 

hydrophone array included “sightings” of 11 species of cetaceans, including sperm 

whale. Based on the acoustic records, no significant difference in cetacean sighting 

frequency was found for areas of relatively low, medium, and high anthropogenic 

underwater noise levels, which included signals from seismic exploration activities.

STUDY RESULTS: The oceanic cetacean community in the Gulf of Mexico was 

composed of at least 19 species. The most numerous species were pantropical spotted 

dolphin (46,625 individuals), spinner dolphin (11,251), and Clymene dolphin (10,093) 

comprising about 50%, 12%, and 11%, respectively, of the estimated total cetacean 

abundance (94,182 individuals) in the study area. Those three species were followed in 

order of abundance by bottlenose dolphin (5,618), striped dolphin (4,858), melonheaded whale (3965), Atlantic spotted dolphin (3,213), Risso’s dolphin (3,040) and 

short-finned pilot whale 1,471), rough-toothed dolphin (852), false killer whale (817), 

dwarf/pygmy sperm whale (733), sperm whale (530), pygmy killer whale (518), killer 

whale (277), Cuiver’s beaked whale (159), Fraser’s dolphin (127), and Bryde’s whale 

(35). On a regional basis, fewer cetaceans were sighted in the western Gulf relative to 

the central and eastern Gulf. The cetaceans of the continental shelf and shelf-edge 

were almost exclusively bottlenose dolphin and Atlantic spotted dolphin. Clymene 

dolphin, pantropical spotted dolphin, spinner dolphin, and striped dolphin occurred more 

often over the lower slope and deepwater regions in areas of cyclonic or confluence 

circulation. Squid-eaters, including dwarf and pygmy sperm whale, false killer whale, 

melon-headed whale, pilot whale, pygmy killer whale, Risso’s dolphin, rough-toothed 

dolphin, and Cuvier’s beaked whale, occurred most frequently along the upper slope in 

areas outside of anticyclones. Loggerhead, Kemp’s ridley, and leatherback sea turtles 

were sighted throughout the study area but were most numerous during winter months 

and over shelf waters. Seabirds also varied seasonally. Two of the most commonly 

identified seabird species, laughing gull and royal tern, are year-round residents in the 

Gulf, while pomarine jaeger, the third most commonly identified species is a winter 

migrant. During mid-summer, black tern was the most abundant species followed by 

band-rumped storm petrel (summer migrant pelagic), frigatebird (permanent resident), 

Audubon’s shearwater (summer migrant pelagic), and sooty tern (summer resident). 
ACCESS NUMBER: BRD 96-0004 and 96-0009

STUDY PRODUCT(S): Davis, R.W., W.E. Evans, B. Würsig, eds. 2000. Cetaceans, 

Sea Turtles and Seabirds in the Northern Gulf of Mexico: Distribution, Abundance and 

Habitat Associations. Volume I: Executive Summary. U.S. Department of the Interior, 

U.S. Geological Survey, Biological Resources Division, USGS/BRD/CR-1999-0005 and 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS 

Study MMS 2000-002, 40 pp. 

Davis, R.W., W.E. Evans, B. Würsig, eds. 2000. Cetaceans, Sea Turtles and Seabirds 

in the Northern Gulf of Mexico: Distribution, Abundance and Habitat Associations. 

Volume II: Technical Report. U.S. Department of the Interior, U.S. Geological Survey, 

Biological Resources Division, USGS/BRD/CR-1999-0006 and Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2000-003, 

363 pp. 

Davis, R.W., W.E. Evans, B. Würsig, eds. 2000. Cetaceans, Sea Turtles and Seabirds 

in the Northern Gulf of Mexico: Distribution, Abundance and Habitat Associations. 

Volume III: Data Appendix. U.S. Department of the Interior, U.S. Geological Survey, 

Biological Resources Division, USGS/BRD/CR-1999-0006 and Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2000-004, 

229 pp. 

P.I.’s affiliation may be different than that listed for Project Manager(s).