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OCR for page 42
Population Trends
The status of sea turtle species is perhaps best indicated by long-term
changes or trends in the sizes of individual populations. Because
females repeatedly return to the same beaches to nest and because
this is the time in their life cycle at which they are most available for
_ direct counting by humans, counts of nesting females or nests pro-
vide the best available long-term data on the status of their populations.
The number of nests is an index that can be correlated with population
size of mature females, rather than a direct~estimate, because sea turtles
do not necessarily nest every year and because a female usually nests sev-
eral times in a nesting season. But use of the index requires the fewest
questionable assumptions about the biology of individual species of sea
turtles. Other measures of long-term change have been made, such as
counts from oceanic aerial surveys (Appendix D), counts of carcass
strandings (Appendix E), catch per unit of effort in fishing gear, and tor-
toiseshell shipments to foreign markets. Some of these (aerial surveys,
carcass strandings, catch in fishing gear) do not differentiate individual
populations. Others depend on local changes in proximate mortality fac-
tors (carcass strandings), depend on market conditions (tortoiseshell ship-
ments), or are expensive and have poor repeatability (aerial surveys). For
those reasons, this chapter presents either trends in the number of nests
42
OCR for page 43
43
Population Trends
or trends in the number of females tagged on a nesting beach during a
season.
Short-term changes in numbers of nesting females or nests should not
be interpreted as a population trend. For example, the variation in num-
bers of nesting green turtles at Tortuguero in the late 1970s (Figure 3-1~) is
unrelated to absolute changes in the population size of these long-lived
animals. Female green turtles return to nest every 2 or 3 years, as reflected
in the year-to-year variation.
Most female Kemp's ridleys apparently nest
annually. Thus, the interyear variation in number of nesting females is
expected to be less in Kemp's ridleys than in other species (Figure 3-la).
Wide year-to-year fluctuations in numbers of nesting turtles make con-
clusions from short-term data sets misleading. A decade or more, depend-
ing on longevity, might be required to measure a real change in a popu-
lation. For example, the Little Cumberland Island, Georgia data on
loggerheads (Figure 3-1p provide the results of 26 years of intensive and
precisely replicated estimates at the same study site. A 10-year survey
from 1964 to 1973 would have indicated no change in the population
over the decade. Likewise, a 12-year survey, initiated say, in 1973 and
concluded in 1984 would have produced a similar result, even though a
substantial decrease in nesting females apparently occurred in the early
1970s. However, a survey from 1982 to 1989 would have suggested a
progressive decline of about 10% per year. Over the entire 26-year peri-
od, 1964-1989, an average decline of about 3% per year occurred in nest-
ing loggerheads. Thus, analyses of population trends can suggest different
results depending on the years surveyed.
Surveys of a decade or less may be insufficient to indicate a population
trend, as indicated above. Surveys longer than a decade become increas-
ingly valuable for management purposes, because they can transcend
short-term fluctuations that obscure long-term trends. Consequently, the
results of surveys of sea turtle species provided in Figure 3-1 must be
interpreted cautiously. Numbers of nests and nesting females are assumed
to generate comparable and useful data on all sea turtle species.
KEMP'S RlDIEY
In 1947, an estimated 40,000 female Kemp's ridleys were observed
nesting during a single day at Rancho Nuevo (Carr, 1963; Hildebrand,
1963), as judged from a motion picture taken by an amateur photogra-
pher. Data on the status of the nesting colony over the next 18 years are
lacking. By 1966, the nesting assemblages or "arribadas" (aggregations of
nesting females at a given place on a given day or series of days) were
OCR for page 44
44
Decline of the Sea Turtles
FIGURE 3-1 Trends in sea turtle populations by number of nests per year
(N) or number of nesting females per year Add. D indicates isolated
(nonconsecutive) years of data. (The committee has provided the follow-
ing values produced by linear regression analysis: r2, slope, and t~vo-
tailed p values.)
(a) Kemp's ridley
1200 ~
000
Ann
N
~\ ,,/
O ' ' ' ' ' ' ' ' ' 1
19eO 1970 1980 1990
Rancho Nuevo, Mexico
r2. .43, slope · -18.8, p · 0.02
3ooo 1
2600 t
2000 t
1600t
1000t
Ann
(c) Loggerhead
N
160r
100
In 60 ~
o ' ' ' ' ' ' ' ' ' 1 ' ' ' ' ' ' ' ' ' 1 ' ' ' ' ' ' ' ' ' 1
19eO 1970 1980 1990
Little Cumberland Island, Georgia
r 2 · .66, slope · -2.8, p · 0.0001
1
o
1970 1980 1990
Cape Island, South Carolina
rue .65, slope · -119, p · 0.0001
(b) Kemp's ridley
46000
soot
1600C
~ ~ Largest arribada
~ ,,,,,,,,, I,,,,,,,,, I,,,,, / ,\
1970 1980 1990
1940 1960 1960
Rancho Nuevo, Mexico
(d) Loggerhead
No analysis warranted
-
OCR for page 45
45
Population Trends
FIGURE 3-1
(Continued)
(e) Loggerhead
1 2000
8000
4000
O , ~l~ Be '''1''''' '''1
19eO
Melbourne Beach, Florida
(9) Green
90
so
30
N
O.
1960 1970
\
Hutchinson Island, Florida
(I ) Loggerhead
2100
400
Ann
1970 1980 1990
r2. 00, Mope · 160, p · .35
300
|\; 1 200
100 _
O ,,,,,, .,,
1990 1980
r2. .36, slope · 2.6, p · .004
. _
0 , . . .
1960
Hutchinson Island, Florida
(h) Green
N
A
~ ~ ,
1970 1980
r2~.22, slope · 30.8, p · .07
1970 1980 1990
Melbourne Beach, Florida
r2~.052, slope · 16.3, p · .28
OCR for page 46
46
Decline of the Sea Turtles
FIGURE 3-1
(Continued)
(i) Green
9000r
N
O O O'
30
Surinam
6°r
(j) Green
6000
4000
2000
t970 1980
r2. .06, ~ope · 163, p · .21
(k) Hawksbill
N
30 r
~oL 1 ~o
2 0 ~
i
S?
/
1990 t960 19tO t980 t990
Tortuguero, Costa Rica
(1) Leatherback
Q
0~
,AI
A
r2. .26, ~ope · 1.5, p · 0.4
( m) Leatherback
60
40
20
O , 1 ~1 1 ~' ' ' ' ' 1
t960
1970 19110 1990
St. Croix, Virgin Islands
r2. .07, elope ~ 85, p ~ .~
1
A
~'~, '\1 -
o! I I I ol i I ~ I
t960 197O 19eO t990 t960 t970 1980 t990
Surinam Culebra, Puerto Rico
r2. 0.0, elope -1.2, p · .43
r2. 0.0, slope · -12, p · .43
OCR for page 47
47
Population Trends
much smaller; about 1,300 females nested on May 31, 1966 (Chavez et al.,
19671. Since 1966, the Mexican government, working with the Estacion
de Biologia Pesquera in Tampico and several other agencies, has main-
tained a presence on the beach at Rancho Nuevo throughout each nesting
season. Personnel have included government turtle biologists, fisheries
inspectors, and armed, uniformed Mexican marines. From 1967 to 1970, a
few arribadas as large as about 2,000-2,500 turtles were seen (Pritchard
and Marquez M., 19731. Archival photographs, probably from 1968, show
many hundreds of nesting females on the beach. Over the period 1947-
1970, sizes of the largest arribadas on the Rancho Nuevo beach declined
dramatically (Figure 3-1 b).
Since 1978, nests on the Rancho Nuevo beach have been counted by a
binational team of Mexican and U.S. scientists working with the U.S. Fish
and Wildlife Service. From 1978 to 1988, the number of nests (USFWS
Annual Reports, Albuquerque office, 1978-1988) declined significantly
(linear regression, p < 0.05) by about 14 nests per year (Figure 3-la).
Given that the 1947 arribada estimate was a single count of females on
the beach and that a female might be expected to lay 2.3 clutches per
season (Pritchard, 1990), the average of about 800 nests per year from
1978 to 1988 would be less than 1% of the estimated nests in 1947
(92,0001. This is the most severe population decline documented for any
sea turtle species.
LOGGERHEAD
Nesting loggerhead females on Little Cumberland Island exhibit a clear
decline in numbers over 26 years (Figure 3-1O. The average decline of
about 3% per year is not smooth, but the overall downward trend is
unmistakable. North of Little Cumberland Island, the number of nests on
Cape Island, South Carolina, also shows a decline over 17 years (Figure 3-
lc). Both populations appeared to undergo a marked decrease in the
mid-1970s, but the cause remains unknown. The number of nests per
year along the entire South Carolina coast has been estimated from aerial
surveys (pers. comm., S. Murphy, S.C. Wildlife and Marine Resources,
19891. Again, a decline is apparent, but year-to-year variability is large.
Murphy and Hopkins-Murphy (1989) summed 3-year counts (1980-1982
and 1987-1989) and compared the results; the comparison showed a 26%
statewide decrease. The same declining trend was evident for the north-
ern and southern portions of the state and for developed and undevel-
oped beaches.
About 90% of U.S. loggerhead nesting occurs in Florida from the
Canaveral area southward (Hopkins and Richardson, 19841. The impor
OCR for page 48
48
Decline of the Sea Turtles
tent nesting beaches south of Cape Canaveral do not show the declines in
nesting characteristic of Georgia and South Carolina. Combined data for
12 years from nine 1.25-km study sites on Hutchinson Island show a pos-
sible rising trend in numbers of nests from 1973 to 1989 (Figure 3-1p. The
most important loggerhead nesting beach in the United States, near Mel-
bourne Beach, Florida Jackson et al., 1988), has been surveyed for only 8
years (Figure 3-le); no clear trend is apparent.
No important nesting has been observed over the roughly 200 km
from New Smyrna Beach to Jacksonville Beach; this gap constitutes some
evidence of discrete northern and southern U.S. populations, an idea sup-
ported by morphometric differences (Stoneburner et al., 1980) and recent-
ly reported genetic differences (pers. comm., B. Bowen, University of
Georgia, April 19901. If the separation is genuine, trend data indicate a
decline in loggerhead populations from the northern nesting assemblage,
but no decline or a possible increase in the southern assemblage. More
years of nesting and data and population biology studies are needed to
assess trends in the southern assemblage.
GREEN TURTLE
The status and history of green turtle nesting in Florida have been
reviewed by Dodd (1982), who found little evidence of past large-scale
nesting in the area. Thus, current nesting rates cannot be compared with
historical records. The numbers of nests have increased on Hutchinson
Island over the period 1971-1989 (Figure 3-1,¢). Considerable nesting also
occurs on Melbourne Beach (Figure 3-lh), but nests have been counted
for only 8 years, a period that is not long enough to confirm a trend.
Green turtles exhibit wide year-to-year fluctuations in numbers of nest-
ing females, and that makes statistical analysis of trends particularly diffi-
cult. The year-to-year variation is also apparent in green turtle nesting
data from Surinam and Tortuguero, Costa Rica (Figure 3-li,p. The only
other substantial regional nesting population, on Aves Island, Venezuela,
has not been surveyed long enough for determination of trends, although
qualitative observations during visits over many years suggest a heavy
decline (Pritchard and Trebbau, 19841.
HAWKSBILL
The hawksbill is an exceedingly difficult species to monitor for long-
term trends, for a number of reasons. Small numbers of animals nest on
a wide variety of beaches across a broad geographic area. Hawksbill
OCR for page 49
49
Population Trends
beaches tend to be remote, inaccessible, and sometimes so narrow that
the turtle leaves no crawl trace. Hawksbills also exhibit the large year-to-
year fluctuations in nesting counts characteristic of green turtles and log-
gerheads. Thus, few trend data are available. Nests on Buck Island in
the Virgin Islands (Hillis and Mackay, 1989b) and Long Island in Antigua
(Corliss et al., 1989) have been counted accurately only for the past few
years. Mona Island, Puerto Rico, is a concentrated nesting area that has
proved to be too remote for consistent assessment (pers. comm.,
Richardson, University of Georgia, 19891. A survey of nests in Surinam
(Figure 3-lk) has provided a series of 13 annual estimates over 15 years.
The trend is positive, but the small number of turtles and the absence of
recent data make the trend questionable.
LEATHERBACK
Leatherbacks do not nest with enough frequency on the U.S. mainland
(Florida) to permit a trend analysis, although they occur commonly off
shore. The nesting beaches nearest the U.S. mainland are those at St.
Croix in the Virgin Islands and Culebra, Puerto Rico (Figure 3-ll,m). The
short records (9 and 6 years) do not indicate trends. Most leatherbacks in
U.S. coastal waters are thought to come from Surinam and French Guiana
nesting beaches (pers. comm., P. Pritchard, Florida Audubon Society,
19891. Nests on those beaches have been counted since 1967, but the
results (as an indicator of population trends) are questionable, because
the nesting population has apparently been shifting between the two
countries (pers. comm., P. Pritchard, Florida Audubon Society, 19891.
Similarly, the small nesting populations in Trinidad and Guyana in the
1960s showed a significant increase by the 1980s, although again a shift
from the major beaches in French Guiana cannot be ruled out.
SUMMARY
The committee concluded that population trends are often challenging
to interpret, and adequate surveys spanning 10 years or more are usually
required to demonstrate with some certainty a change in absolute popula-
tion numbers. However, much can be deduced about sea turtle trends
from the studies of nesting densities to date.
· The Kemp's ridley population has experienced a major decline since
1947, and in the last decade its numbers have continued to
decrease.
OCR for page 50
50
Decline of the Sea Turtles
Loggerhead nesting populations have declined over the last 20-30
years on northern U.S. nesting beaches (Georgia and South Caroli-
na). On southern Florida Atlantic beaches, however, loggerheads
have not shown a decline, and might even be increasing.
· Green turtle nestings on Florida beaches are low but are increasing
at Hutchinson Island, Florida.
· Hawksbill nesting is too sparse in U.S. waters for trend analysis.
Nesting in Surinam appears to have increased somewhat over the
last 15 years, but absolute numbers have been very low throughout.
· Leatherbacks nest in small numbers in the United States, principally
in the Virgin Islands and Puerto Rico. Although records are too few
to detect trends, the numbers do not appear to be declining. Inter-
pretation of trends on the important Surinam and French Guiana
beaches is complicated by population shifts as beaches erode and
accrete.
Representative terms from entire chapter:
sea turtle
