Eretmochelys imbricata (Linnaeus, 1766)

Meylan, Peter A., Meylan, Anne B. & Gray, Jennifer A., 2011, The Ecology And Migrations Of Sea Turtles 8. Tests Of The Developmental Habitat Hypothesis, Bulletin of the American Museum of Natural History 2011 (357), pp. 1-70 : 37-38

Eretmochelys imbricata

BERMUDA: A combination of 68 live captures and 86 strandings yielded 151 measurable hawksbills at Bermuda that varied in size from 8.7–75.7 cm SCL. The smallest Eretmochelys captured alive on the Bermuda Platform were in the 15–20 cm (n 5 1) and 20–25 cm (n 5 9) size classes (fig. 13 A), which is comparable to the minimum size seen in benthic developmental habitats for this species elsewhere (table 11). The stranding sample included some smaller posthatchlings, including five smaller than 15 cm SCL that most likely stranded from the epipelagic habitat adjacent to the Bermuda Platform. A contingency test indicates a significant difference in the size distribution of stranded Eretmochelys in Bermuda relative to that expected if they were to strand in proportion to the distribution of live captures on the Bermuda Platform (x 2 5 43.0, 12 df, P,.05). As is the case for stranded green turtles, there is a peak in the size distribution for stranded hawksbills in the smallest size classes (20– 25 cm SCL for hawksbills, fig. 13B; 25–30 cm SCL for green turtles, fig. 5C). These data for Eretmochelys , like those for C. mydas in Bermuda, suggest an increased rate of mortality during the transition between life stages, in this case between the epipelagic and benthic developmental stages. Although increased mortality during life stage transitions are well studied in vertebrates that go through metamorphosis (Searcy and Sponaugle, 2001), this may be a previously undetected phenomenon in turtles. Increased mortality with a shift in habitat has been observed in snakes. Bonnett et al. (1999) noted that snakes in their study were more likely to die when they moved away from their usual home range. In a sense, that is what is occurring when sea turtles move from one life stage to the next.

Although no laparoscopies were performed on hawksbills in Bermuda, necropsies of 30 individuals, including a 75.7 cm SCL stage 2 female, revealed no mature animals. With the exception of this largest individual (fig. 13B), all stranded Eretmochelys from Bermuda could have been considered immature on the basis of carapace length alone. Minimum size at sexual maturity for this species in figure 13 was based on laparoscopies, observations of ‘‘soft plastra’’ in reproductive males in Panama (Wibbels et al., 1991a), and emergence on the nesting beach in Panama.

Among hawksbills of known maturity status at Mona Island, Puerto Rico (n 5 17) and Panama (n 5 30), the smallest mature female and male were 71.8 and 70.5 cm SCL, respectively (fig. 21). However, among the hawkbills captured in Panama were two males (68.9 and 67.1 cm SCL) that were not laparoscoped but had long tails (36.0 cm and 31.8 cm PLTT, respectively) and showed softening of the plastron that is characteristic of reproductive male cheloniid sea turtles. Thus, males may mature at sizes as small as 67 cm SCLmin in the West Atlantic and this value is used throughout this paper as the best estimate of minimum size at sexual maturity for this species in the West Atlantic. This value is similar to estimates of minimum size at sexual maturity based on histological evidence of spermatogenesis in hawksbills from Cuba (Moncada et al., 1999) and tail length in Puerto Rico (van Dam and Diez, 1998b). See Meylan and Redlow (2006) for additional discussion of size at sexual maturity for both sexes in this species.

Although the sample size of hawksbills of known maturity status in Bermuda is small, the size distribution and maturity status observed there are consistent with the hypothesis that the Platform serves as benthic developmental habitat for this species.

PANAMA: A small sample of hawksbills (n 5 7) from Secretary varied from 40.8– 74.9 cm SCL (avg. 5 57.0 ± 14.1 cm). On the basis of size, the two largest (73.6, 74.9 cm SCL) could possibly have been mature, but this was not verified by laparoscopy. At the Zapatilla Cays, nine of 36 first captures made in nets were immatures based on laparoscopy or size. They varied at first capture from 30.0– 66.1 cm (avg. 5 56.2 ± 12.7 cm). The smallest mature individual based on laparoscopy was a 70.5 cm SCL stage 3 male. However, two smaller males (68.9 and 67.1 cm SCL) that were not laparoscoped had long tails, suggesting they may have been mature (see above).

THE LITERATURE: There are at least seven sites in the Caribbean Sea at which immature Eretmochelys are found and adults are nearly or completely absent (table 11; fig. 25). In the cases in which adults were also present, there were known hawksbill nesting beaches in the immediate vicinity of the study site. The few adults encountered at these sites could result from overlap of developmental and internesting habitat (fig. 23B).

The smallest size at which live hawksbills were captured on the Bermuda Platform (17.6 cm SCLmin) compares well to the minimum size (20.0 cm SCLn-t) seen at Mona Island, Puerto Rico, and along the southern coast of the Dominican Republic (19.5 cm SCLn-t). These are all smaller than the minimum sizes reported for two study sites in the U.S. Virgin Islands and for four sites in the Pacific (table 11). The maximum size of immature hawksbills observed in Bermuda (75.7 cm SCL) is larger than the largest immature seen in the Dominican Republic (69.7 cm SCLn-t) and Buck Island (70.5 cm CCLn-t).

Recruitment from the epipelagic to benthic stage of the life cycle appears to occur at a larger size in the Pacific than the Atlantic. Limpus (1992), Hirth et al. (1992), and Seminoff et al. (2003) reported minimum sizes of about 32 cm CCL for Pacific sites. However, at Fogg Bay, Western Australia, hawksbills as small as 26.3 cm CCL were reported at an inshore site (Whiting and Guinea, 1998).

One of the Australian study sites (table 11) illustrates the value of laparoscopy for recognizing that a specific foraging area was occupied by turtles at the benthic developmental stage. Limpus (1992) reported on a sample of 152 Eretmochelys from the Capricornia section of the southern Great Barrier Reef, Australia (fig. 25D). The maturity status of 109 of these turtles was determined via laparoscopy and only one individual (0.9 %) was an adult. This is remarkable because 20 individuals (18.3 %) were larger than the minimum size for nesting females in this population (75.0 cm CCL). Most (16) of these larger individuals were examined laparoscopically. Seven were prepubescent, eight were pubescent and one was the single adult identified in the study. Limpus (1992) pointed out that there has probably been no regular harvesting of Eretmochelys at this site and that the population is likely to represent a natural, unexploited one.