Chelonia mydas

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 : 50-53

Chelonia mydas

BERMUDA: After hatching on the nesting beach, hatchling green turtles are thought to spend an extended period, in the range of 3– 6 years, in pelagic habitats ( Carr et al., 1978; Carr, 1987; Reich et al., 2007). Formerly called the Lost Year stage of the life cycle, this is better termed the epipelagic stage. It remains the least well-known stage for this and all sea turtle species. Green turtle hatchlings are known from Sargassum drift lines that exist up current from Bermuda ( Carr, 1987; Witherington, personal commun.), and a single green turtle recovered by the stranding network in Bermuda was smaller (18.8 cm SCL) than the smallest size at first capture on Bermuda’s benthic foraging grounds (22.3 cm SCL). Thus, there are small green turtles in the pelagic environment around Bermuda that approach the size at which they switch over from a pelagic to a benthic life style.

When considered in light of residency data, the shape of the histogram in figure 5 A suggests that C. mydas arrive at Bermuda in the 22–30 cm SCL size range, remain there until they reach about 60–65 cm SCL, and then depart. Because it is only the size classes from 30–60 cm that do not differ significantly in representation (G 5 5.996, P, 0.05), it can be hypothesized that departure begins with the 60–65 cm size class. If the shape of the right side of the histogram in figure 5 A is due only to departure, then the structure of the histogram suggests that about 14 % leave at an average size of 62.5 cm SCL, 32 % leave at an average size of about 67.5 cm SCL, 34 % leave at an average size of 72.5 cm SCL, 15 % leave at an average size of 77.5 cm SCL, and essentially all remaining individuals have left before they reach 80 cm SCL. A weighted average for these figures gives an estimated average size at departure of 70.6 cm SCL. This is about 6 cm smaller than the minimum size at sexual maturity for male C. mydas , and 10 cm smaller than the minimum size of sexual maturity for females (based on 178 laparoscopies performed in Panama, this study; Meylan and Meylan unpubl. data). The shortest time intervals (, 1 yr) between the last capture in Bermuda and recapture on a foreign foraging ground (fig. 5D) are also for turtles in the 65–80 cm size range. These could be assumed to be the turtles for which size at departure is most certain.

Puberty is a possible cue for departure. No size class of C. mydas from Bermuda (fig. 5 A) is made up entirely of pubescent individuals. About 60 % of turtles in the largest size class (75–80 cm SCL) were pubescent; turtles in this size class were observed less often than those in smaller size classes. This suggests that pubescent turtles are leaving Bermuda. The onset of puberty and the earliest apparent departures both occur at about 60 cm SCL. Schmid et al. (2003) cited the work of Gregory and Schmid (2001) as providing evidence that the onset of puberty and associated endocrinological changes in L. kempii correspond to a habitat shift in maturing individuals of that species.

Tag-return data also provide useful information on departure of C. mydas from Bermuda. There is a strong association between the distribution of Bermuda inter- national tag returns and known adult foraging grounds for this species in the Atlantic. The majority of returns (56) have come from Nicaragua, the principal foraging grounds of adult green turtles in the western Caribbean ( Carr et al., 1978; Bass et al., 1998). Two tag returns came from the eastern Caribbean ( St. Lucia and Grenada) where green turtles that hatch on Aves Island ( Venezuela) are known to forage as adults ( Carr et al., 1978). Six additional tag returns were made off the Venezuelan portion of the Guajira Peninsula, which is a known foraging area of adult green turtles that nest in Tortuguero, Costa Rica ( Carr et al., 1978). One Bermuda green turtle was recaptured in St. Lucie County, Florida, fitted with a satellite transmitter and subsequently tracked to foraging grounds west of the Marquesas Islands (50 km W of Key West) where it remained for at least eight months (D. Bagley, personal commun.). This area west of the Marquesas has recently been shown to serve as adult foraging habitat for C. mydas ( Bresette et al., 2010) .

There is a significant limitation to the use of tag returns to monitor individual turtles through departure from benthic developmental habitat. For any international tag recovery, data exist for a first capture, any recaptures at the tagging study site, and the foreign recapture. The length of time between the last observation at the tagging site and the foreign recapture may be known, but the respective portions of time spent in residence at the tagging site, in migration, and in residence at the foreign recapture site are not. Additional error affecting tag-return data results from the poor accuracy and precision sometimes associated with the reporting by turtle fishermen who return the tags. With these caveats, it is clear that an inverse relationship exists between time to recapture and size at last observation. Individuals that were smaller the last time they were observed in Bermuda took the longest time between that observation and foreign recapture (y 5 20.201, x + 17.955, r 2 5 0.42788, P, 0.001). Individuals with the shortest time between their last observation in developmental habitat and their foreign recapture provide the best estimates of the size and timing of departure from developmental habitat.

One of seven C. mydas equipped with a satellite transmitter, a 78.6 cm SCL female (PTT 11674, table 7), was successfully tracked during departure from Bermuda ; the resulting developmental migration was characterized by highly directed travel (fig. 12). The turtle took a SSW heading and maintained a nearly straight course for about three weeks to reach Hispaniola. At least one valid Argos location was obtained on 35 days of the 38-day migration. Minimum distance traveled and average travel speed were calculated using the single best location per day of the ‘‘hybrid’’ filtered dataset. The total migratory distance (greatcircle distance between subsequent locations) was 2048 km and the mean speed of travel during migration was 2.3 km /hr (SD 5 0.97, n 5 35). Along its route, the turtle crossed the Silver Bank , and while it was off the Dominican Republic , it was exposed to Hurricane Georges , which passed close by. The turtle’s intended destination remains unknown, but if it had continued at the rate it was traveling, it could have reached Nicaragua —the site with the greatest number of tag returns of Bermuda turtles—after about 2 months. When the turtle was captured, it was near the Windward Passage between Haiti and Cuba, through which it could have entered the Caribbean Sea .

The absence of sexually mature C. mydas in Bermuda waters, the distribution of tag returns from the Caribbean, and the satellite telemetry data collected during this study all indicate that the foraging grounds in Bermuda are not the final habitat occupied by the green turtles that are captured there. That is, Bermuda waters do not serve as adult resident habitat. Given the geographic isolation of the island, C. mydas that grow up in Bermuda must make a significant developmental migration to move into the next stage of their life cycle.

SECRETARY AND ZAPATILLA CAYS, PAN- AMA: The smallest green turtles observed at Secretary and the Zapatilla Cays (46.7 cm and 46.2 cm SCL, respectively) were much larger than the smallest individuals (22.3– 34.6 cm) observed at 19 other sites in the greater North Atlantic (table 10) that apparently represent benthic developmental foraging grounds for this species. Data from these studies suggest that the smallest turtles at these two sites in Panama may be recruiting from other benthic developmental foraging grounds rather than from epipelagic environments. Smaller C. mydas are known from Bocas del Toro Province and benthic foraging grounds for these smaller size classes are reported by local fishermen to be present at sites that have not been sampled. Lahanas et al. (1998: 350) reported that Inagua, Bahamas, is a site from which immature C. mydas depart long before they approach maturation. They also suggested the possibility that a series of foraging areas may be used at this stage. Thus, observations at our two Panama study sites support the Carr et al. (1978) model that portrays benthic developmental foraging grounds as a series of separate sites.

To better estimate the size at which green turtles departed the Secretary and Zapatilla Cays study sites, their size at last sighting in Panama was compared to the time elapsed between that last sighting and the reported foreign recapture date (fig. 14C). As is the case for foreign recaptures of Bermuda green turtles, smaller animals were reported recaptured on foreign foraging grounds after longer intervals than larger turtles, suggesting that smaller turtles may have resided in Panama for longer periods after being tagged. Because there is more certainty about the timing of departure of turtles that were recaptured after shorter intervals, they are more likely representative of the actual size at departure.

Except for one turtle (77.1 cm SCL), the last size recorded for turtles that departed was less than the minimum size at sexual maturity based on laparoscopies (76.7 cm SCL). Many C. mydas in the 60–80 cm size range, the size range of departing animals, were observed to be in puberty (fig. 15). Although the average size at which immature C. mydas were last seen at the Panama study sites before foreign recapture was 61.4 ± 8.42 cm SCL (n 5 11) at Secretary, and 68.9 ± 7.0 cm (n 5 22) at the Zapatilla Cays, the shortest intervals were for 62.5 and 77.1 cm turtles from Secretary and a 72.6 cm turtle at Zapatilla. These values approach the weight- ed average size at departure from Bermuda (70.6 cm SCL).

THE LITERATURE: Green turtles arrive at benthic foraging grounds in the West Atlantic (not including the two Panama sites) at minimum sizes ranging from 20.8 cm (St. Lucie Power Plant, Florida) to 34.6 cm (Cedar Key, Florida), with an average for 22 sites of 26.0 ± 3.4 cm (table 10). Reich et al. (2007) used stable isotopes to detect this habitat shift in the diet of C. mydas smaller than 36 cm in the Bahamas. For three Pacific sites, average size of arrival at benthic foraging grounds is about 10 cm larger (37.8 ± 1.00).

Departure size is more difficult to summarize because at some sites occasional ‘‘adults’’ are reported, but their maturity status was not verified. However, for most sites the largest individuals reported are from about 67–81 cm SCL. Thus, results from Bermuda are typical for the West Atlantic green turtle benthic developmental stage. The Panama results clearly differ and must represent only a latter part of this stage for the green turtles that occur there.

Size at departure for C. mydas from Inagua, Bahamas, is approximately 10 cm less than it is for C. mydas departing from Bermuda (compare figs. 5 A and 24 A). This could be due to the relative proximity of adult foraging range for the Inagua turtles.