3

1989 TOTAL EFFORT
(DAYS FISHED)

0 to 5000

5000 to 10000
10000 to 15000
15000 to 20000
20000 to 25000

‘t-——_ 15 Fathoms
30 Fathoms

BNESO0



and A.M. Landry, Jr. editors. Proceedings of the First International Symposium on
Kemp’s Ridley Sea Turtle Biology, Conservation and Management. Texas A&M
University, Sea Grant TAMU-SG-89-105. 260p.

Pritchard, P.C.H. 1989. Status report of the leatherback turtle pp. 145-152. In: LH.
Ogren, F.H. Berry, K. Bjorndal, H. Kumpf, R. Mast, G. Medina, H. Reichart, R.
Witham. Proceedings of the Second Western Atlantic Turtle Symposium. NOAA
Tech. Memo. NMFS-SEFC-226. 401p.

Thompson, N.B. 1984. Progress report on estimating density and abundance of marine
turtles: Results of first year pelagic surveys in the southeast U.S. NMFS MS. 30p. +
12 tables + 8 Figures.

Thompson, N.B. 1991. Preliminary information on turtle captures incidental to fishing
in Southeastern U.S. waters. NOAA Tech. Memo. NOAA-SEFSC-285. 8p.

Witzell, W.N. 1983. Synopsis of biological data on the hawsbill turtle Eretmochelys
imbricata (Linnaeus, 1766). FAO Fisheries Synopsis No. 137. FAO, Rome. 78p

WW


Table 1. Numbers of loggerhead turtles sighted in the
Northwestern and Northeastern Gulf of Mexico
during seasonal surveys from 1984-1987.

Sightings are reported by season and for each

depth stratum.

Survey

Northwestern Gulf

Fall
Winter
Spring

Summer

Northeastern Gulf

Fall

Winter

Summer

Bays

36

26

Stratum
Inshore offshore
13 2
(+) 14
8 14
3 3
284 87
193 82
371 47

Total

15
14
22

407
284

444


examination of loggerhead turtle distributions and shrimp trawling effort was com-
pleted to evaluate the overlap of seasonal and depth stratifications.

Materials and Methods

Avariety of SEFC research projects have collected information relative to the distribu-
tions of turtles in the Gulf of Mexico. The only synoptic fishery independent distribu-
tional data on turtles are available from an aerial sighting program that collected
sighting data on marine mammals and turtles at the water’s surface from Brownsville,
Texas to Key West, Florida, and from the coastline out to the 200 m isobath. Supportive
site specific data on turtle distributions were provided by the SEFC Pascagoula
Laboratory, Galveston Laboratory and include opportunistic turtle sightings and aerial
survey sightings in the Gulf of Mexico off Louisiana. Fishery dependent data have been
collected through the efforts of the Pascagoula Laboratory while obtaining information
to estimate turtle mortality incidental to commercial shrimp trawling. Turtle distribu-
tional information has been collected by the Pascagoula Laboratory during site specific
aerial surveys to census red drum. To evaluate the question on depth distributions of
turtles relative to shrimp trawling, the defined turtle distributions were compared to
the relative distribution of commercial shrimp trawling effort.

Fishery Independent Sampling for Turtles
Synoptic Aerial Surveys

From 1983 through 1986, seasonal aerial surveys were completed from the coastline of
the Gulf of Mexico out to the 200 m (100 fathom) isobath. The surveys are referred to
herein as GoMex for Gulf of Mexico. The purpose of these surveys was to provide data
for the estimation of marine mammal and marine turtle density over continental shelf
waters. Results of these surveys provide baseline information on the distributions of
loggerhead turtles in the Gulf of Mexico.

To optimize seasonal and depth coverage, the study area was divided into two sampling
areas (Fig. 1). The Northwestern Gulf, (NW) was defined as extending from the Rio
Grande River to the mouth of the Mississippi River, a total of 144,056 km’; and the
Northeastern Gulf (NE), from the Mississippi River to Key West, Florida, a total of
219,514 km’,

Sampling strata were established based on surface to bottom water depth, and were
termed "bay", "inshore", and "offshore" sampling areas (Fig. 1). "Bays" included bays,
sounds, and lagoons; inshore included waters from the coastline or oceanside from
barrier islands to 18 m and "offshore" included waters from 18 m to 200 m in depth.
This scheme optimized coverage over depth such that 15% of the bays, 12% of inshore,
and 7% of the offshore surface waters were sampled. Overall coverage averaged 10%.

The survey platform was a twin engine Beechcraft D-18, equipped with a plexiglass
observation bubble on the nose. Four observers were rotated through the observation


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(Thompson 1984). Lohoefner et al. (1990) speculate as did Thompson (1984) that this
may be the result of turtles moving into warmer waters offshore.

Opportunistic Sightings

Sightings were reported by divers, oil company personnel, commercial and recreational
fishermen, beach goers and NMFS employees (Fig. 6 and 7). These data are biased
because the majority of the sightings were reported by NMFS observers working on oil
platforms which are concentrated in shelf waters off Texas and Louisiana. Regardless
of this sampling bias, sightings were reported beyond 60 m in the NW Gulf. These
results are remarkably similar to these from the synoptic aerial survey program off the
NW Gulf.

Fishery Dependent Sampling
Sampling Aboard Commercial Shrimp Trawling Vessesl

Henwood and Stuntz (1987) presented results of catch and mortality of turtles inciden-
tal to commercial shrimp trawling. They estimated catch and mortality of turtles relative
to commercial shrimp trawling effort. Notably, all fishing effort was in offshore waters
or those beyond the barrier islands out beyond the 30 meter (15 fathom) isobath. Their
results did not show any significant deviation in turtle CPUE between depths sampled
within the Gulf of Mexico. Loggerhead turtles were most abundant with Kemp’s ridleys
next in abundance and green turtles the least in abundance.

Aerial Surveys for Red Drum

Lohoefener et al. (1988) divided the study area 6 geographical study areas identified
as South Texas from the Texas/Mexico border to the northern end of Matagorda Bay;
North Texas to the Louisiana border; Louisiana to the mouth of the Mississippi River;
the North Central Gulf of Mexico to Perdido Bay, Al.; North Florida from Cape San
Blas, Fl. to Waccasassa Bay, F1.; Central Florida to Charlotte Harbor; and South Florida
to Key West (Fig. 8). Note that the coast between Perdido Bay, Al. to Cape San Blas,
FI. was not sampled due to military air space restrictions. Sampling was conducted in
two seasons, spring and fall, except in the Central Florida area which was only sampled
in the fall. According to Lohoefener et al.(1988), all transects were terminated within
20 minutes of flight time perpendicular to the mainland coast. Given a ground speed
of about 100 knots, survey effort was concentrated within the 60 meter isobath or are
equivalant to the inshore survey area described under the synoptic aerial surveys.
Lohoefener et al.(1988) estimated densities within study areas for each seasonal
survey. They only distinguished between hard-shelled turtles and leatherbacks. How-
ever, based on the relative ease at observing loggerhead turtles of the adult and
sub-adult size relative to green and Kemp’s ridleys we expect that the vast majority of
these sightings were loggerhead turtles. Turtles were observed in all surveys at all
depths for all seasons. Estimated turtle density in the spring ranged from a minimum
of .04 turtles/km? in the Louisiana area to a maximum of 1.13 turtles/km” in South


Over the period 1973 through 1984, direct observations of sea turtle catches were made
by observers placed on accepting shrimp trawling vessels within the Gulf of Mexico and
southeastern North Atlantic (Henwood and Stuntz 1987). Three different programs
included these observers and were: 1) the incidental catch and mortality program from
1979 to 1981; 2) the excluder trawi program from 1973 through 1984; and 3) the shrimp
fleet discards program from 1973 through 1984. These programs are discussed in detail
by Henwood and Stuntz (1987). Briefly, the incidental catch program documented the
catch and mortality of sea turtles captured accidentally by commercial shrimp trawlers.
The excluder trawl program included the experimental trawling using various
prototypical TED designs that eventually led to the acceptance of the current NMFS
TED design. The objective of the shrimp discard program was to evaluate all by-catch
associated with the commercial shrimp fleet. Henwood and Stuntz (1987) summarized
their results relative to water depth and turtle species.

Aerial Surveys to Census Red Drum

From Aprilto December 1987, aerial surveys were conducted to census red drum within
the Gulf of Mexico. Sighting information on sea turtles was also reported. Turtle
sighting information was summarized from Loheofener et al. (1988) relative to water
depth.

Shrimp Trawling Effort

Commercial shrimp trawling effort was defined as "days fished", where one day equals
24 hours that nets were in the water. Thus, for this exercise, effort represents the
fishing time an individual turtle could be caught. Effort data from 1989 were used since
it is the most recent data available and provide a reasonable representation of annual
effort for all the survey efforts (E. Klima 1991 pers. comm.). Because shrimp trawling
effort were available in 1987 and coincided with the aerial surveys to census red drum,
the 1987 trawling effort data were used overlaid the turtle distributions derived from
the red drum surveys. To facilitate presentation of shrimp trawling effort, grids of total
shrimp trawling effort were defined in 5,000 days fished increments, with 0-5000 days
fished as the lowest and 25,000 the highest increment. The distributions of loggerhead
turtles from the three programs were overlaid the distributions of shrimping effort as
defined by 5,000 days fished increments. The 30 meter isobath was included in these
figures as a reference. This approach was used for both the 1989 and 1987 shrimp
trawling data.

Resuits
Fishery Independent Sampling

Synoptic Aerial Surveys

Seven seasonal surveys were flown; four in the northwestern Gulf and three in the
northeastern Gulf. The northwestern Gulf surveys were flown in Sept. to Oct. 1983
(Fall); Jan. to Feb. 1984 (Winter); April to May 1984 (Spring); and July to Aug. 1985


bubble, such that there were two observers in the bubble during surveys. All possible
transects were placed at 1.4 km intervals and randomly selected.

All surveys were conducted at an altitude of 229 m (750 ft), at about 22 km/hr (120
knots) ground speed. Four surveys were completed in the northwestern Gulf and three
in the northeastern Gulf. Data reported included species identification, location, time
and date, various environmental conditions, and animal behavior.

Site Specific Aerial Surveys

From June 1988 through June 1990 the SEFC completed site specific aerial surveys
with funding from the U.S. Dept. of Interior, Minerals Management Service (MMS).
The purpose of these surveys was to evaluate associations between turtles and oil
platforms (Lohoefener et al. 1989). Specific sampling sites off the Louisiana Coast
were divided into "inshore" and "offshore" areas. (Fig. 2.).

Sampling sites were selected based on platform density and estimated sampling time.
Water depth in the inshore sites varied from 6 to 65 m and were sampled from 1988
through 1990. Offshore sites were up to 200 m in depth and were sampled from 1989
through1990.

The survey platform was a de-Havilland (DC-8) Twin Otter airplane from NOAA’s
Aircraft Operations Center, Miami, Florida. Downward’ viewing was accomplished
through observation bubbles mounted on each side of the aircraft. Survey altitude was
229 m (750 ft) and speed was 167 m/hr (90 knots). Surveys were conducted weekly as
possible to maximize positional information on turtles. ~

Opportunistic Sightings 1985-1989

Since 1985, the Galveston laboratory has maintained a sea turtle sighting file to compile
opportunistic sightings of turtles. A sighting is defined as an event in which a sea turtle
is seen, usually swimming at the surface of the water. All sightings are reported by the
public or in conjunction with ongoing NMFS research programs. There is no sampling
plan or research program for this and the data only represent where turtles were
observed.

Fishery Dependent Sampling

As previously indicated, the interpretation of these data relative to turtle distributions
is limited. These data were collected with the primary purpose of sampling fishing
effort or a non-turtle resource, The extent of turtle distributions beyond these efforts
camnot be deduced. However, these data do provide supportive information on the
presence of turtles in sampling areas.

Sampling Aboard Commercial Shrimp Trawling Vessels.


fh

EFFORT

(]_ 0 to 5000

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fl 10000 to 15000
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The mean rate over all seasons was estimated as .0093 loggerhead turtles per kilometer.
The sighting rate in all inshore surveys was greater than all offshore surveys which was
greater than all bay surveys (Table 3). The smallest value was estimated for the winter
bay surveys (.008) and the largest value for the fall inshore survey (.0180) (Table 3).
For the bays and inshore waters, the greatest sighting rates were estimated for the fall
survey, whereas the winter surveys resulted in the greatest sighting rate in the offshore
strata (Table 3).

The stratified sighting rates in this area are all at least one magnitude higher than in
the northwestern Gulf. However, the trend is the same suggesting that turtle behavior,
environmental conditions or observer behavior was consistent throughout the Gulf
during the sampling period.

When relating these turtle distributions to shrimping effort, it is obvious that the
distribution of turtles overlaps and extends beyond the extent of commercial shrimp
trawling. Waters in which there is no shrimp trawling and turtles are present, do not
require regulation of the shrimp trawling industry. In the northwestern Gulf, the
relative intensity of shrimp effort decreases when moving to offshore waters. However,
the majority of sightings in offshore waters was greater than inshore waters suggesting
the turtle CPUE might be higher in offshore relative to inshore waters. The results of
Henwood and Stuntz (1987) are representative of the catch of turtles in inshore waters
rather than offshore and bay waters, because this is where they were able to sample.
Thus, their CPUE estimates are probably not applicable to offshore and bay waters of
the northwestern Gulf.

In the northeatern Gulf, turtles tended to concentrate in inshore waters but were
present in relatively large numbers in offshore as compared to bay waters. However,
shrimp trawling effort appears to be more uniform over these waters with the exception
of waters near the Dry Tortugas and in inshore waters from the Mississippi River to
Mobile Bay. Over both areas, turtles were present over all depths during all seasons.

Site Specific Surveys

A total of 229 loggerhead turtles were sighted during this study. Turtles were sighted
in all sampling areas except area 6 and 63% (145) were sighted within area 1. (Table
4). Within the sampling period, we define spring as April through June; Summer from
July through September; Fall from October through December; and winter from
January through March. Given this seasonal stratification, of the total 229 turtles, 40%
(92) were sighted an the spring; 23% (53) in the summer; 28% (63) in the fall; and 9%
(21) in the winter (Lohoefner et al. 1990). Results from this seasonal sampling effort
are consistent with those from the GoMex sampling.

Turtles were observed in all depth strata within a given sampling area (Fig. 5).
According to Lohoefener et al. (1990), the distribution of turtles by depth did not differ
seasonally, although sighting rates increased with increasing depth in the fall and
winter. This result is similar to that observed along the southeast Atlantic seaboard


Table 2. Total transect kilometers flown for each season in the
Northwestern and Northeastern Gulf of Mexico. Linear

distance is reported for each depth stratum within each

season.
Survey Stratum
Bays Inshore Offshore Total
Northwestern Gulf
Fall 2,011.1 2,701.4 5,101.5 9,814.6
Winter 1,810.6 3,198.6 5,543.8 10,553.0
Spring 2,688.9 3,269.5 4,847.1 10,805.5
Summer 3,293.0 3,863.1 5,743.3 12,889.4
Northeastern Gulf
Fall 3,354.9 6,479.8 6,150.9 15,985.6
Winter 5,371.6 6,800.4 3,338.7 15,510.7

Summer 5,932.8 8,396.7 4,592.8 18,922.3


NOAA TECHNICAL MEMORANDUM

Se
NMFS-SEFSC-286 Gg

Turtles in the Gulf of Mexico: Pelagic Distributions

and Commercial Shrimp Trawling

by
Nancy B. Thompson
Elizabeth Scott Denton
Dennis B. Koi
Anthony Martinez

Keith Mullin
May 1991

The Technical Memorandum series is used for documentation and timely com-
munication of preliminary results, interim reports, or special-purpose information.
Although the memoranda are not subject to complete formal review, editorial con-
tril, or detailed editing, they are expected to reflect sound professional work.
U.S. Department of Commerce
Robert A. Mosbacher, Secretary
National Oceanic and Atmospheric Administration
John A. Knauss, Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

William W. Fox, Jr. Assistant Administrator for Fisheries


Based on the examination of turtle and shrimp trawling effort distributions, it appears
that the distribution of turtles extends beyond the extent of trawling and throughout
the shrimp trawling season. Thus, wherever shrimping occurs, loggerhead turtles are
observed in significant numbers, The absence of sightings in bay waters from these
aerial surveys does not mean juvenile turtles or green Kemp’s ridely turtles were not
present. These aerial.surveys only target adult and sub-adult loggerhead and leather-
back turtles. Notably, Hildebrand (1982) suggested that the Kemp’s ridley historically
was the "Louisiana" turtle, and the green turtle, the "Texas" turtle. Both species .are
rately sighted using current survey methods but, both species utilize inshore and
nearshore waters (Hildebrand, 1982). Thus, within bays of the Gulf of Mexico green
and Kemp’s ridley turtles are present and cannot be been detected via our aerial survey
efforts (Ogren 1989). :


Florida. In the fall, turtle density ranged from 05 turtles/km? in Louisiana to .64
turtles/km” in South Florida. Overall leatherback turtles were rarely reported.

Discussion

Results from these surveys efforts serve as a baseline for the seasonal and spatial
distribution of loggerhead turtles in the Gulf of Mexico. Large loggerhead turtles are
clearly a successful target for pelagic aerial surveys. Results of these surveys show that
the distribution of sub-adult and adult turtles is skewed toward the eastern Gulf, from
the Mississippi River Delta to Key West, Florida. The relative lack of sightings in the
western Gulf is not to be interpreted to mean that there are no marine turtles in this
area. Historically, the green turtle was known as the Texas turtle and was the focus of
an active fishery in the Corpus Christi area at the turn of the century (Hildebrand 1982).
The Kemp’s ridley turtle was known historically as the Louisiana turtle and while not
fished with the same intensity as the green, was taken opportunistically (Hildebrand
1982). Results of our own research have produced data on both Kemp’s ridley and
green turtles in Gulf waters ( Henwood and Stuntz 1987). Thus, both species are
present throughout the Gulf by were not detected by pelagic aerial surveys. The
carapace coloration of the green turtle does no provide the same contrast as that of the
loggerhead and is probably easily overlooked. The Kemp’s ridley coloration also does
not provide a visual contrast against the water and being the smallest species is probably
not large enough for detection at 229 m altitude and at 222 km/hr speed.

This east-west dichotomy in the distribution of loggerhead turtles is supported by
results of previous aerial surveys in the Gulf conducted by the US Fish and Wildlife
Service (USFWS) in 1980-1981 (Fritts et al. 1983). In these surveys loggerhead turtles
were sighted 50 times more frequently off the southwest coast of Florida compared to
off the Louisiana and Texas coasts (Fritts et al. 1983). Results of these bimonthly
surveys indicated that this distribution showed seasonal shifts as well, with the highest
frequently of sightings occurring in the spring and the lowest in the winter (Fritts et al.
1983). Our results suggest that there may be seasonal movements also from the central
Florida inshore and offshore waters in the fall to the southwest Florida inshore and
primarily offshore waters in the winter. Fritts et al. (1983) estimated turtle density off
the southwest coast of Florida and the highest was estimated for February 1981 (.22
turtles per km?) and lowest in October 1980 (.061 turtles per km’). Our results also
demonstrated an aggregation of turtles in southwest Florida waters during the winter.

The USFWS surveys extended from the coastline out to the approximate 1000 m
bathymetric curve with some sightings beyond the 100 m curve (Fritts et al. 1983). Both
inshore and offshore waters of central and southwest Florida support numerous turtles
out to the western boundary of the offshore sampling areas (Fig. 4). Thus, it appears
from these most recent surveys, that loggerhead turtles are abundant and conspicuous
in the eastern Gulf off the coast of Florida and significantly less abundant or less
conspicuous in the northern and western Gulf waters where they are probably replaced
by green and/or Kemp’s ridley turtles. Loggerhead turtles probably move from north
central Florida to the southwest offshore waters in response to reduce temperatures in
the winter or as they follow an available resource.


(Summer). The northeastern Gulf surveys were flown June to Aug. 1985 (Summer);
Sept. to Oct. 1985 (Fall); and Jan. to Feb. 1986 (Winter). The total linear kilometers
flown was estimated to be 94,490.5 km in both surveys areas, with 44,071.9 km flown in
the northwestern Gulf and 50,418.6 km flown in the northeastern Gulf.

The total number of loggerhead turtles sighted at the surface of the water was tabulated
by survey and for each depth stratum (Table 1). The total number of turtles recorded
was 1,192 with 6% of these recorded in the bays; 73% in inshore waters; and 21% in
the offshore waters. It is likely that at 229 m altitude the majority of turtles sighted are
in the sub-adult and adult size classes. During a previous aerial survey program
targeting marine turtles, it was determined empirically that the average minimum size
of turtles that are sighted at a survey altitude of 153 m is about 61 cm (2 ft.) straight line
carapace length (Thompson 1984). Therefore, using a simple linear relationship at a
survey altitude of 229 m, we expect that the minimum observation size was at least 92
cm carapace length. This length, 92 cm, is certainly well within the average length of
nesting females throughout the southeast U.S. (Dodd 1988).

Surveys in the northwestern Gulf netted a total of 57 reported sightings of loggerhead
turtles, with sightings per survey and depth stratus presented in (Table 1; Fig.3). No
sightings were recorded in the bay waters of the northwestern Gulf sampling area. This
does not mean that there were no turtles in these waters, but that no adult, sub-adult
or large juvenile turtle were visible at the surface under survey conditions. Water color
or turbidity could not be ruled out as a factor because it is highly variable within bay
waters. However, it is more likely that turtles were either too small for detection within
these water (L. Ogren 1990; personal observation).

The total linear kilometers flown is presented in Table 2. Sightings per kilometer flown,
or sighting rate, was estimated as an index to compare between seasons, depth strata,
and areas. The sighting rate for the northwestern Gulf was estimated as .0013. Within
the inshore and offshore waters, the sighting rate ranged from 0 to .0048 turtles per km
(Table 3). A chi-square computation to test the independence of sighting rate from
stratum and survey resulted in acceptance of the null hypothesis; that sighting rate was
independent of survey or stratum (X 2= 23, p. 950; Fleiss 1973). While there appears
to be a greater likelihood of sighting turtles in the inshore and offshore, waters during
the fall, this is not supported statistically because so few turtles were recorded during
any one survey. The distribution throughout this sampling area is presented in Figure

A total of three surveys were flown in the northeastern Gulf with a total of 1,135
loggerhead turtles recorded during these surveys. The number of turtles recorded by
surveys and stratum are presented in Table 1 and illustrated in Figure 4. As before,
sightings per unit of linear transect flown was estimated by season and depth stratum
and included in Table 3. A statistical comparison of rates demonstrated that. there
were significant difference in sighting rates between season and strata (x? = 11.88, p.
005; Fleiss 1973).


1987 TOTAL EFFORT
(DAYS FISHED)

0 to 5000

5000 to 10000
10000 to 20000
20000 to 30000
30000. to 40000

[—— 15 Fathoms
30 Fathoms

BSE



introduction

The loggerhead turtle is commonly found within waters of the Gulf of Mexico (Hil-
debrand 1982; Hildebrand 1983). Nesting of this species is limited along the Gulf of
Mexico coast and is considered minor except along the Florida west ‘coast (Marquez
1990; Hildebrand 1982). Information on the pelagic distribution of this species tends
to be anecdotal and limited to hatchling and small juveniles associated with ocean fronts
(Marquez 1990). The distribution of adults and sub-adults is usually referred to as
"coastal" (Marquez 1990; Ehrhart 1989). Hildebrand (1982) reported that turtles have
been observed in association with oil platforms, rock reefs and obstructions, and "at (a)
considerable distance from the coast" while feeding on Portuguese man-of war. The
association between turtles and offshore oil platforms has recently been re-confirmed
(Klima et al. 1988).

Hildebrand (1982) also discusses the relative distributions of the leatherback, Kemps’
ridley, green, and hawksbill turtles within the western Gulf of Mexico. Historically, the
green turtle supported a turtle fishery along a portion of the Texas coast. Hildebrand
(1982) remarks that at one time, the Kemp’s ridley turtle was referred to in the Gulf of
Mexico as the Louisiana turtle. The hawksbill has never been numerous within the
Gulf of Mexico (Witzell 1983). Very little information on the relative importance of
leatherback turtles exists and Pritchard (1989) indicated that contrary to historical
reports, this species probably does not nest in the Florida Keys and Dry Tortugas. No
sampling program targetting the presumably deep water leatherback turtles has been
conducted, thus, the distribution of this species in the Gulf of Mexico is not well defined.
In fact, the only information on this species appears to be from capture incidental to
long line fishing (Thompson 1991).

Since 1983, the Southeast Fisheries Science Center (SEFSC) of the National Marine
Fisheries Service (NMFS) has collected sighting information on the sub-adult and adult
life history stages of marine turtles within Gulf of Mexico waters. Because of the
comparative ability to see loggerhead turtles from the most commonly utilized fishery
independent sampling platform, airplanes, the vast majority of these data are specific
to loggerhead turtles. Additional information on green and Kemp’s ridley turtles were
collected during fishery dependent sampling conducted to evaluate mortalties of turtles
incidental to commercial shrimp trawling. While the majority of the fishery inde-
pendent turtle data can be statistically evaluated relative to the stratification of logger-
head distributions regionally, seasonally, and by depth, most of the fishery dependent
data are only representative of where we were able to sample within the constraints of
commercial trawling effort in the late 1970’s and early 1980’s. Examination of the
distributions of the loggerhead from the fishery independent data provides insights into
the ecology and movements of this species within and beyond the Gulf of Mexico.

Recently, the National Research Council Committee on Sea Turtle Conservation
concluded that sea turtle abundance was 10 times greater in shallow than deeper water
during shrimp trawling season. This conclusion prompted speculation about the
seasonal and depth distribution of turtles relative to shrimp trawling effort. An


List of Figures

Figure 1. Survey area for synoptic aerial sampling of the Gulf of Mexico. Line at the
mouth of Mississippi River separates northwestern Gulf and northeastern Gulf. Bays
includes bays and sounds; inshore waters includes waters from the coast or coeanside
of barrier islands to18 m in surface to bottom depth; and offshore waters are 18 m to
200 m in depth. ‘

Figure 2. Aerial survey sampling area from site specific surveys to evaluate turtle-oil
platform distances. Shrimp trawling effort is overlaid in 5,000 days fished grids.

Figure 3. Turtle sightings for the northwestern Gulf of the synoptic surveys. Each dot
represents a single sighting of a loggerhead turtle. Shrimp trawling effort is overlaid in
5,000 days fished grids.

Figure 4. Turtle sightings from the northeatern Gulf of the synoptic survey. Each dot
represents a single loggerhead turtle sighting. Sightings in offshore blocks line up along
sampled transects. Shrimp trawling effort is overlaid in 5,000 days fished grids.

Figure 5. Aerial surveys sightings from site specific sampling. Each dot represents a
single sighting. Shrimp trawling effort is overlaid in 5,000 days fished grids.

Figure 6. Opportunistic sightings of turtles in the northwestern Gulf. Shrimp trawling
effort in 5,000 days fished grids are overlaid

Figure 7. Opportunistic sightings of turtles in the northeastern Gulf. Shrimp trawling
effort in 5,000 days fished grids are overlaid.

Figure 8. Turtle sightings from red drum aerial overflights over the western Gulf of
Mexico. Shrimp effort grids in 5,000 days fished increments from 1987 effort data are
overlaid.

Figure 9. Turtle sightings from red drum aerial overflights over the eastern Gulf of

Mexico. Shrimp effort grids in 5,000 days fished increments from 1987 effort data are
overlaid.

12




Executive Summary

e Available data on the distribution of turtles was evaluated relative to com-
mercial offshore shrimp trawling effort.

The majority of data on turtle distributions was collected through pelagic
aerial surveys targetting loggerhead turtles. Remaining information was
provided from fishery-dependent sampling provides information on the
presence of turtles within limited sampling areas.

The distribution of loggerhead turtles overlaps and extends beyond the
distribution of offshore commercial shrimp trawling.

e Aerial surveys sampled inshore and bay waters and where there were rela-
tively few turtles as compared to nearshore and offshore waters. It is likely

. that within these waters, smaller turtles, including green and Kemp’s ridley
turtles are more common and not detected through aerial observation.


Table 3. Mean number of turtles sighted per linear kilometer of
selected transects flown. These sighting rates are
presented for the Northwestern and Northeastern Gulf
study areas by depth stratum and season. The mean

sighting rate over all states for each season is

included.
Survey Stratum
Bays Inshore Offshore Total
Northwestern Gulf
Fall oO -0048 -0004 -0015
Winter oO 0 .0025 -0013
Spring oO -0025 .0029 -0020
Summer 10) . 0008 -0005 .0005
Northeastern Gulf
Fall -0051 -0180 -0070 201121

Winter +0008 +0131 -0109 -0083

Summer .0019 .0158 .0051 0085


Notice

The National Marine Fisheries Service (NMFS) does not approve, recommend, or
endorse any propretary product or material mentioned in this publication. No ref-
erence shall be made to NMFS, or to this publication furnished by NMFS, in any
advertising or sales promotion which would indicate or imply that NMFS approves,
recommends, or endorses any proprietary product or proprietary material men-
tioned herein or which has as its purpose any intent to cause directly or indirectly
the advertised product to be used or purchased because of NMFS publication.

This report should be cited as follows:

Thompson, N.B» E,Scott Denton”, D-B. Koi, A. Martinez’, and K. Mullin?, 1991,
Turtles in the Gulf of Mexico: Pelagic Distributions and Commercial Shrimp Trawl-
ing. NOAA Tech. Memo. NMFS-SEFSC-286, 12 p.

Copies may be obtained by writing:

Dr. N.B. Thompson
Southeast Fisheries Science Center
75 Virginia Beach Dr.

Miami, FL 33149
or
NTIS
5258 Port Royal Rd.
Springfield, VA 22161
1 Miami Laboratory, Miami, FL 33149
Galveston Laboratory, Galveston, TX 77550

3pascagoula Laboratory, Pascagoula, MS 39567


Literature Cited
Dodd, C.K. Jr. 1988. Synopsis of the biological data on the loggerhead sea turtle Caretta
caretta (Linnaeus 1958). U.S. Fish and Wildlife Serv., Biol. Rep. 88 (14). 110p.

Fleiss, J.L. 1978. Statistical methods for rates and proportions. Wiley, New York 223p.

Fritts, T.H., A.B. Irvine, R.D. Jennings, L.A. Collum, W. Hoffman, and M.A. McGehee.
1983. Turtles, birds, and mammals in the northern Gulf of Mexico and nearby Atlantic
waters. U.S. Fish and Wildlife Service, Division of Biological Services, Washington,
D.C. FWS/OBS-82/65. 455p.

Henwood, T.A. and W.E. Stuntz. 1987. Analysis of sea turtle captures and mortalities
during commercial shrimp trawling. Fish. Bull. U.S. 85(4):813-817.

Hildebrand, H.H. 1982. A historical review of the status of sea turtle populations in
the Western Gulf of Mexico. In: K.A. Bjorndal, editor, Biology and Conservation of
Sea Turtles. Smithsonian Press, Washington, D.C. 447-453p.

Hildebrand, H.H. 1983. Random notes on sea turtles in the Western Gulf of Mexico.
In: D.W. Owens, D. Crowell, G. Dienberg, M. Grassman, S. McCain, Y. Morris, N.
Schwantes, and T. Wibbels, editors, Western Gulf of Mexico sea turtle workshop
proceedings. Office of Sea Grant, Texas A&M Univ., Publ. TAMU-SG-84-105.

Klima, EF, G.R. Gitschlag, and M.L. Renaud. 1988. Impacts of the explosive removal
of offshore petroleum platforms on sea turtles and dolphins. Mar. Fish. Rev. 50(3):
33-42,

Lohoefener, R.R., W. Hoggard, C.L. Rhodin, K.D. Mullin, and C.M. Rogers. 1988.
Distribution and relative abundance of surfaced sea turtles in the north-central Gulf
of Mexico: Spring and fall 1987. Published abstract in: B. A. Schroeder, compiler.
Proceedings of the Eighth Annual Workshop on Sea Turtle Conservation and Biology.
NOAA Tech. Memo. NMFS-SEFC-214. pp. 47-50. -

Lohoefener, R., W. Hoggard, K. Mullin, C. Roden and C. Rogers. 1990. Association
of sea turtles with petroleum platforms in the north-central Gulf of Mexico. OCS
Study/MMS 90-0025. U.S. Dept. of the Interior, Minerals Mgmt. Service, Gulf of
Mexico OCS Regional Office, New Orleans, LA. 90p.

Marquez M., R. FAO special catalogue. Vol.11: Sea Turtles of the world. An an-
notated and illustrated catalogue of sea turtle species known to date. FAO Fisheries
Synopsis. No. 125, Vol. 11. Rome, FAO. 1990. 81ip.

Ogren, LH. 1989. Distribution of juvenile and subadult Kemp’s ridley sea turtles:
preliminary results from the 1984-1987 surveys. pp. 116-123. In: C. W. Caillouet, Jr.

10


98°w

96°W 94°W 92°W

90°W 8B°w 86°W

84w

82°W

30°N

28°N

Northwestern Region

HB Bay Zone
BR INSHORE ZONE
OFFSHORE ZONE

Northeastern Region

GULF OF MEXICO

24°N'