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OCR for page 51
4
Distribution of Sea Turtles
in U.S. Waters
o understand the issues concerning the conservation of sea turtles in
U.S. waters, we need to view their distribution along the Atlantic and
gulf coasts on a broad spatial scale. That immediately makes appar-
ent the wide extent of the complex conservation problem even in
U.S. coastal waters. It also helps to identify, for example, which
beaches should receive priority for protection of sea turtle nesting and
where the distribution of sea turtles overlaps with human activity to cause
mortality along the coasts at various water depths in different seasons.
This chapter enlarges the general presentation on species distributions in
Chapter 2 and provides a broad analysis of the distribution of sea turtles
in U.S. waters in recent years. For our analysis, we have taken the most
quantitative published information available or have reanalyzed the most
extensive data bases available through the cooperation of individuals and
government agencies.
SOURCES OF INFORMATION
Nesting Distribution
Information on distribution of nests of loggerheads, green turtles, and
leatherbacks in the continental United States has been obtained from aeri
51
OCR for page 52
52
Decline of the Sea Turtles
al surveys and beach patrols. The committee's compilations are based on
data from the U.S. Fish and Wildlife Service, North Carolina Wildlife
Resources Commission, South Carolina Wildlife and Marine Resources
Department, Georgia Department of Natural Resources, and Florida
Department of Natural Resources. Additional data were obtained from
the U.S. Recovery Plan. Density (nests per kilometer) varies from year to
year, as does the intensity of beach surveys. Sufficient data are available,
however, to indicate the general density of nesting on beaches from
Maine to Texas.
Pelagic Aerial Surveys
Aerial surveys documenting the distribution of sea turtles in the water
have been conducted from Maine to the Mexican border. Data presented
here are from N.B. Thompson (pers. comm., NMFS, 1989) and Winn
(19823. Aerial surveys are valuable for surveying large areas in a short
time. However, interpreting data from aerial surveys is difficult for several
reasons: small turtles, particularly Kemp's ridleys, generally are not visi-
ble, and ocean conditions, such as water clarity and surface glare, can
alter visibility and therefore affect the reliability of species identification
and counts.
Sea Turtle Strandings
Volunteers in the Sea Turtle Stranding and Salvage Network (STSSN)
attempt to document every sea turtle stranding on the U.S. Atlantic and
gulf coasts. The date and location of each stranded turtle are recorded, as
well as its species, size, and condition. Distribution of strandings pro-
vides information on the distribution of turtles. However, quantification
of turtle distribution based on that data base is limited by several factors.
First, the data base is not independent of the distribution of human-
induced mortality factors, such as fishing, dredging, and boating. Second,
temporal and spatial coverages are rarely uniform. Most beaches are sur-
veyed by volunteers. Areas under contract for regular surveys since 1986
are fishing zones 17-21 (Texas), fishing zones 4 and 5 (gulf coast of south
Florida), and fishing zones 28-32 (Atlantic coast of north Florida, Georgia,
and South Carolina) (Figure 4-11. Shorelines formed by marsh or man-
grove stands, such as large sections of the Louisiana coast and the north-
western coast of the Florida peninsula, are not surveyed. Third, because
of current and wind patterns, dead turtles might float some distance
before they strand or might never strand.
OCR for page 53
53
Distribution of Sea Turtles on U.S. Waters
FIGURE 4-1 Shrimp-fishing zones along U.S. coasts of Atlantic Ocean and
Gulf of Mexico.
PA
vrg ~ 44
/NI~
1--]-~
TIC
~ 36
'sc ~ 35
<- ~
32°00'
30°00'
28°00'
coo
24°00'
1
9C r 33
go/ 32 *
GA ~ 31 *
/ LA 1 ~ ~ATLANTIC
) ~ ajar ~ ~ PA * OCEAN
AX
~;: ~
gL ~
~ 118 117 1 16 115 1 44 -
;20~
-~- GULF OF MEXICO ~/ 25
99°00' 95°00' 91°00' 87°00' 83°00' 80°00' 79°00' 76°00' 73°00'
36°00'N
34°00'N
32°00'N
30°00'N
28°00'N
26°00'N
NOTE: Asterisk indicates that zone has contractual arrangement for
observing turtle stranding.
OCR for page 54
54
Decline of the Sea Turtles
DISTRIBUTION
The capture of sea turtles in bottom trawls associated with commercial
and experimental or exploratory fishing provides some information on
depth and area distribution.
Nesting
The southeastern United States supports one of the two largest rook-
eries of loggerheads in the world. Some nesting occurs from North Car-
olina to Louisiana, with outliers as far north as New Jersey and west to
Texas (Figure 4-2, top); but the 330 km of beach on the Atlantic coast of
Florida between St. Augustine and Jupiter supports by far the highest den-
sity of loggerhead nesting (Figure 4-21. In recent years, from 50 to more
than 200 nests/km of beach are dug annually in this region, compared
with only a few to 50 nests/km elsewhere (Figure 4-2, top). In addition,
the same 330 km of beach is the only location where substantial (but
much lower) numbers of green turtles and leatherbacks nest on the U.S.
Atlantic and gulf coasts. Kemp's ridleys and hawksbills very rarely use
U.S. continental beaches for nesting.
Aerial Surveys
Quantitative data are available for some regions to evaluate the season-
al changes in on/offshore distribution or the depth distribution of large
individuals of the most abundant species, the loggerhead, along the
Atlantic and gulf coasts of the United States. The most general picture
comes from distributional maps compiled from aerial surveys taken in
each quarter of the year for much of the Atlantic coast and portions of the
gulf coast(Winn, 1982;Thompson, 1984; pers. comm., N.B. Thompson,
NMFS, 19891. Other aerial surveys are more spatially restricted but pro-
vide useful information for selected sites off Florida, Louisiana, and Texas
(Fritts and Reynolds, 1981; Fritts et al., 1983; pers. comm., R. Lohoefener,
NMFS, 19891.
North of Cape Hatteras to the Gulf of Maine, large loggerheads were
sighted from inshore to the offshore banks and shelf edge and continental
slope (Wine, 1982~. The distribution shifted from more inshore to more
midshelf from spring to summer. From Cape Hatteras, North Carolina to
St. Augustine, Florida, sea turtles, mostly large loggerheads with a few
adult leatherbacks, generally appeared more abundant on the inshore
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55
Distribution of Sea Turtles on U.S. Waters
FIGURE 4-2 Distribution of loggerhead nesting (per km) and seasonal
aerial surveys of loggerheads (per 10,000 km2) in shrimp-fishing zones.
Data from Appendix D.
N 200
U 150
M
B 100
R 50 _
o
21 18 15 12 9
loggerhead nests / km
~ ~ _
Fall
~ Summer
[ 1 Spring
Winter
C ~,
25 26 31 34 47 40 43
3
N
U
M 6000
B
E 3000
R
o
N 90OO
U
M 600
B
E
R
3000
o
N 90OO
U
M 600
B
E 3000
R
9000
600
N
U
M
B
E 3000
R
o
winter aerial survey (per 10,000 km2)
21 18 15 12 9 6 3 25 26 31 34 47 40 43
spring aerial survey (per 10,000 km2)
]~ ~"2~2~'"~
21 18 15 12 9 6 3 25 26 31 34 47 40 43
summer aerial survey (per 10,000 km2)
6 3 25
fall aerial survey (per 10,000 km2)
21 18 15 12 9 6 3 25 26 31
O , ,,
21 18 15 12 9
1
# -
~! ~ ~
26 31 34 47 40 43
~ _
34 47 40 43
FISHING ZONE
OCR for page 56
56
Decline of the Sea Turtles
halves of aerial transects than on offshore halves in spring and summer,
but appeared less abundant on the inshore than offshore halves in fall
and winter (Thompson, 19841. There are too many points on Thomp-
son's maps to see any obvious difference in the Cape Canaveral region.
South of Canaveral, large loggerheads appear more abundant in the
inshore than offshore halves of the transects in all seasons of the year. In
the Gulf of Mexico from Key West to the Mississippi River (pers. comm.,
N. Thompson, NMFS, 1989), sightings of large loggerheads seem more
frequent in the inshore portions of aerial surveys than in the offshore por-
tions in summer and autumn, and offshore in winter. Maps of the sight-
ings of large loggerheads used in Lohoefener et al. (1988) in spring and
autumn for all gulf locations show no obvious seasonality with respect to
distance from shore, nor did Lohoefener (pers. comm., NMFS, 1989)
observe any seasonal changes in depth distribution off Louisiana from the
data used by Lohoefener et al. (19891.
Densities of large loggerheads (with a few adult leatherbacks) from
aerial sightings can also be analyzed with respect to water depth over
which the turtles were sighted within survey areas of 25,642 km2 at two
locations on the gulf coast of southern Florida in August (Fritts and
Reynolds, 1981) and seasonally both for the Atlantic coast of Florida off
the primary nesting beaches of loggerheads near Cape Canaveral and the
gulf coast of southern Florida (Fritts et al., 19831. Other sites off Louisiana
and Texas had too few turtle sightings to analyze for seasonality of
on/offshore or depth distributions.
The primary conclusion of these two aerial surveys off Florida is that
both in the Atlantic waters (Canaveral area) and the gulf waters of south-
ern Florida, the aerial sighting densities of large loggerheads are higher
throughout the year over water depths of 0-50 m than over depths from
50-1,000 m; few large loggerheads or leatherbacks were observed over
waters from 50-1,000 m in any season. Averaged over all seasons, the
sighting densities over waters 25 to 50 m deep were 78-82% of those over
0 to 25 m depths, but sighting densities over waters 50 to 100 m deep
were 9-14% of those over 0 to 25 m depths. Because the depth contours
drop off much more sharply at Canaveral than at the gulf site off south
Florida, it also appears that the distributions of large loggerheads were
related to water depth rather than to distance from shore. For both loca-
tions, the sighting density declines rapidly near the 50 m depth contour
rather than at a fixed distance from shore. An alternative explanation
might be that turtles spend more time below the surface in deeper water
and that fewer are then sighted. However, the catch in trawls, presented
below, also supports the conclusion of fewer large loggerheads and
leatherbacks being found in deeper waters.
OCR for page 57
57
Distribution of Sea Turtles on U.S. Waters
At both the Canaveral and the southern Florida gulf sites, large logger-
heads remained abundant throughout the year at depths from 0 to 50 m.
In waters less than 50 m deep, minimum sighting densities of large log-
gerheads observed in October and December averaged about 50% of
those for February, April, June, and August.
Aerial surveys of coastal waters also demonstrate the high concentra-
tion of adult loggerheads off the primary nesting beaches along the
Atlantic coast of Florida during spring and summer (Figure 4-21; sightings
range up to about 7,900 per 10,000 km2. Moderately high sighting densi-
ties, about 2,500 per 10,000 km2, also were reported in the fall off North
and South Carolina. Densities of sighted large loggerheads were low
(about 30-100 per 10,000 km2), along portions of the west coast of Flori-
da, and decreased sharply off Louisiana and Texas (to 1-30 per 10,000
km21. North of Cape Hatteras, loggerheads were absent in winter, low in
summer (about 500 per 10,000 km2, and very low (1-4 per 10,000 km2)
even in summer as far north as the Gulf of Maine.
Leatherbacks sighted in aerial surveys were uncommon throughout the
entire Gulf of Mexico, averaging about 50 per 10,000 km2 (Lohoefener et
al., 1988) and were about one-hundredth as abundant as large logger-
heads among identified sightings off the Atlantic coast south of Cape Hat-
teras (Thompson, 19841. In the Gulf of Maine, leatherbacks numbered
only 7-8 per 10,000 km2 during summer and fall; they were absent or very
sparse in winter and spring.
Kemp's ridleys are not usually visible and identifiable from aerial sur-
veys, so this survey method provides no information on their distribution.
Seasonality of sighting densities varies with the geographic location
along the coast. Off the primary nesting beaches of Florida's Atlantic
coast, sighting densities were about 15 times higher during spring and
summer than during autumn and winter (Figure 4-29; the lowest sighting
densities occurred in the winter, when they were about 2.5% of highest
summer densities. That pattern reflects the aggregation of the mature log-
gerheads for breeding and access to the nesting beaches. Farther north,
off North Carolina, sighting densities were not maximal during the sum-
mer nesting season, but rather were 2-4 times higher during spring and
autumn than during winter or summer. Seasonal coverages of aerial sur-
veys are insufficient to permit speculation about other regions.
Slrandings
According to 1987 and 1988 data from the STSSN, the most common
turtle carcasses found on the outer beaches from Maine to Texas were
OCR for page 58
58
Decline of the Sea Turtles
those of loggerheads (1,522 and 1,150 in these years), followed by
Kemp's ridleys (141 and 176), green turtles (105 and 150), leatherbacks
(119 and 63), and hawksbills (22 and 20) (Appendix E). Those numbers
understate the number of dead turtles in the area, in that many dead tur-
tles do not drift ashore or are not found. The highest stranding rates of
loggerheads occurred along 500 km of Atlantic beaches of Georgia and
northern Florida (Figure 4-31. Other areas with many strandings of log-
gerheads were the beaches of Mississippi, Alabama, and Texas. Carcasses
of Kemp's ridleys were found most frequently on beaches of Texas, the
Atlantic coast of northern Florida, and North Carolina (Figure 4-31. Green
turtles were stranded most frequently along the Atlantic coast of Florida;
leatherbacks along the coasts of Delaware, New Jersey, and New York;
and hawksbills along the coasts of Texas and Florida (Figure 4-31.
Seasonality of strandings differs with species of turtle and geographic
region. Loggerheads strand most frequently in May-December on Atlantic
beaches, in April and May on the Texas coast, and in May and June in
Mississippi and Alabama. In some locations and seasons, few turtle car-
casses are reported. In some areas, that is accounted for by the absence
of beach surveys or by ocean current patterns; in others, it might be relat-
ed to an overall lack of turtles in the region. For example, in northern
Florida and Georgia, only 1.5% and 3.6% of the annual totals of logger-
head strandings in 1987 and 1988, respectively, occurred in winter (Jan-
uary-March) along the 500 km characterized by maximal strandings during
May through September. That is consistent with the aerial survey data on
turtles off this coastal region, where winter sighting densities were 2.5%
of maximal summer sighting densities. In addition, few sightings in the
region were on the inshore portions of the aerial surveys in winter, but
many during the spring and summer (Thompson, 19843.
Seasonality of strandings of Kemp's ridleys also appeared to differ with
region (Figure 4-31. On Texas beaches, stranding occurred in February-
December, with maximums in April and May and again in August and
September, but few strandings occurred on the Atlantic coasts of Florida
to Maine in lanuary-May.
ONSHORE, OFFSHORE, AND DEPTH DISTRIBUTION
Turtles caught in bottom trawls also provide information on depth dis-
tribution that is consistent with the marked decrease of large loggerheads
and leatherbacks at increasing depths observed in the aerial surveys.
Twenty-nine loggerheads were captured off Georgia and Florida (to Key
West) in 1306 hours of trawling (Bulks and Drummond, 19781. The high-
est catches, about 0.0015-0.0045 turtles/hour of trawling, were taken in
OCR for page 59
s9
Distnb?~tion of Sea Turtles on U.S. Waters
FIGURE 4-3 Sea turtle stranding, by species, 1987-1988.
Fishing zones
are shown on horizontal axes (see Figure 4-19. Source: STSSN (see
Appendix E).
| · fall ~II~ summer
~ spring ~ winter |
300
D 200
Z 1 00
o
60
-
~ 40
Z 20
o
90
D 60
E
Z 30
o
loggerhead strandings 1987
2 1 18 15 12 9 6 3 25 28 31 34 37 40 43
Kemp's ridley strandings 1987
green strandings 1987
300
b2oo
Z 100
o
60
0) 40
Z on
90
b6o
Z 30
loggerhead strandings 1988
. . .-~ ~ .
21 18 15 12 9 6 3 25 28 31 34 37 40 43
Kemp's ridley strandings 1988
O O O O O O O 0 1 0 0 0 0 0
green strandings 1988
~ ~,.--. - , - ~- , . . .
21 18 15 129 6 3 25 28 31 34 37 40 43
30
b2o
Z 10
Q) 8
z 4
leatherback strandings 1987
n
2 ~18 15 12 9 6 3 25 28 31 3i 37 40 4a
12
o
300 T
turtle strandings 1987
n
_
. . . .
21 18 15 12 9 6 3 25 28 31 34 37 40 43
30
Z 10
n
leatherback strandings 1988
~. . ~. . . _~ 1L
21 18 15 12 9 6 3 25 28 31 34 37 40 43
12 T .
hawkshill strandings 1987
1- -,-, ,, , ~_. . .
21 18 15 12 9 6 3 25 28 31 34 37 40 43
D~ 8
z 4
o
hawksbill strandings 1988
. . . . . . . .
21 18 15 12 9 6 3 25 28 31 34 37 40 43
21 18 15 12 9 6 3 25 28 31 34 37 40 43
3oo T
O_
turtle strandings 1988
37 40 43
21 18
15 12 9 6 3 25 28 31 34
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60
Decline of the Sea Turtles
0-40 m of water compared with catch rates of 0-0.0025 turtles/hour in 40-
100 m. Thirteen sea turtles, mostly loggerheads and Kemp's ridleys, were
caught in the Gulf of Mexico off Louisiana or the west coast of southern
Florida in the NMFS observer program in 1988-1989. Catch rates were
0.006 turtles/hour (in 2,007 hours) at 0-27 m and 0.0008 turtles/hour
(1,285 hours) at more than 27 m. Henwood and Stunts (1987) also
showed that the catch of sea turtles per net per hour was lower at depths
of 27-99 m than 2-27 m. They had 976 trawling hours in the deeper
water and 5,177 trawling hours in the shallowest water. They did not pre-
sent data on catch rates of turtles by depth, but the catch per effort at
depths greater than 27 m was less than the catch per effort for all but two
of the other depth intervals. As with the aerial surveys, turtle abundance
in deeper water appeared to be about one-tenth that in shallower water.
SUMMARY
Data on distribution of sea turtles come from observations of nesting
turtles, aerial surveys, the STSSN, and incidental captures in fishing gear.
Nesting is most common on the Atlantic coast of Florida, and the log-
gerhead is the greatly predominant species. Loggerheads aggregate off
the nesting beaches in spring and summer, and move up and down the
coasts and a little more offshore in fall and winter. Some leatherback and
green turtle nesting occurs in eastern Florida. According to stranding
data, sea turtles in order of decreasing abundance in U.S. coastal waters
are loggerheads, Kemp's ridleys, green turtles, leatherbacks, and hawks-
bills. Some strandings occur from the Gulf of Maine to all the states along
the Gulf of Mexico. Adult turtles are apparently less abundant in deeper
waters of the Gulf of Mexico than in waters less than 27-50 m deep, and
they are usually uncommon near the shore off northern Florida in fall and
winter. In the eastern gulf, turtles are less abundant inshore in winter
than in summer, but even in winter they are common in inshore waters.
Representative terms from entire chapter:
sea turtles
