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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
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
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 -
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
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
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
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
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.
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.