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 : 44-49

Chelonia mydas

BERMUDA: Carr (1980) interpreted the work of Mowbry and Caldwell (1958) as evidence that C. mydas in Bermuda was itinerant. Expansion of the sampling regime at Bermuda since 1992 has shown that immature green turtles are present throughout the year. There was a minimal seasonal pattern with increased catch rates in spring relative to fall (fig. 7). Nonparametric analysis of variance detected significant differences in catch per set of the net by month, but Dunn’s pairwise comparison detected only four pairs of months that were significantly different. April was different from September, October, and December, and May was different from December. Turtles were caught during all months and with equal likelihood across the range of temperatures at which sampling took place (17 ° –30 ° C; fig. 8). Spearman rank correlation detected no correlation between temperature and capture rate.

Establishing year-round occurrence of green turtles in Bermuda introduces the possibility that individual turtles are continuously resident for extended periods. Residency is suggested by recapture records. Over 24 % (n 5 609) of turtles captured in Bermuda were recaptured one or more times (806 recaptures), up to a maximum of six. As of 2005, 159 C. mydas have been recaptured over intervals

TABLE 13

Summary of foraging ground data for Lepidochelys kempii from the literature

of five or more years (fig. 9). Each sampling session yielded evidence of more of these long-term residents. The longest record through 2005 is F3414, first captured on 23 July 1976 at the grass flats at Outside Daniel’s Head; this turtle was recaptured over 14 years later, on 14 August 1990, on the same flats.

The great distance from the Bermuda Platform to other possible foraging grounds for green turtles (at least 1000 km to North Carolina) makes transient use of the habitat in Bermuda unlikely. If turtles are resident in Bermuda, however, one might expect a higher recapture rate and longer intervals of observation. A partial explanation for the patterns observed is the vagaries of sampling. A core group of sites (,10) has been sampled frequently since the early years of the project, but the complete suite of sampling locations (. 40) includes sites that are no longer sampled, sites that were sampled only once, and sites that have been added recently. The regular netting sites are purposefully sampled only once or twice per year to minimize disturbance to the turtles. Even at small, discrete sampling sites, such as single grass flats surrounded by deeper water, not all turtles are captured during a sampling event; some sites on the west end of the island encompass as much as 80 ha and may be sampled annually with a single setting of the net. Larger-scale factors that affect the maximum length of time individual turtles are observed in Bermuda include tagging methodology, tag loss, and predation. In the early years of the project only a single tag was applied. Starting in 1985 all turtles were double tagged. Beginning in 2001, PIT tags were added to all turtles under 30 cm SCL and in 2005, to all turtles regardless of size. These measures are expected to increase the recapture rate.

Although direct evidence indicates that individual green turtles may stay in Bermuda waters for as long as 14 years, there is circumstantial evidence that some may reside longer. A preliminary estimate of average annual growth for C. mydas in Bermuda, calculated from growth intervals for 71 different individuals (26.2–65 cm avg. SCL during interval) recaptured after one year (365 ± 30 days), was 2.51 ± 1.29 cm /yr. Growth rates in C. mydas vary with size, year, sex, and habitat ( Bjorndal and Bolten, 1988; Limpus and Chaloupka, 1997; Kubis et al., 2009). They also vary by ocean basin, with turtles from Pacific sites having lower rates of growth than those of the same size in the Caribbean. The preliminary estimate of growth rate in Bermuda is somewhat lower than other Caribbean sites but not exceptionally so. Given the northerly location of the site, and the likelihood of seasonal growth, the estimate of 2.51 cm /yr falls within expectations. Using this rate, it can be predicted that individual C. mydas remaining in Bermuda from the smallest (approximately 25 cm SCL) to largest (approximately 75 cm) size class, may be resident as long as 20 yrs. Continued monitoring at Bermuda will allow the direct testing of this prediction.

Recapture data also suggest that site fidelity is well developed in green turtles in Bermuda. Recaptures are typically made within relatively small areas of habitat (i.e., on the same grass flat), and often over long periods of time (see table 14 for examples). Table 6 shows that 88.3 % of all recaptures occurred on the same grass flat where the original capture was made. The site of release did not appear to affect the site of subsequent capture, with 90.2 % of turtles released where they were captured being subsequently recaptured at that site, compared to 81.8 % of turtles released at a different site. Many of the turtles in the latter category were held for one to three days for laparoscopy and were subsequently released at sites 10–20 km from their capture site. They appeared to be able to home effectively (see also Ireland, 1979, 1980).

Satellite tracking results provided some additional data on both residency and site fidelity of green turtles in Bermuda. Five turtles that were captured in the net and tracked for 33–447 days did not appear to leave the Bermuda Platform, i.e., no locations that passed the Douglas Argos filter provided evidence of departure from the Platform. Distances between the capture sites and the mean center of filtered ARGOS locations ranged from 0.1–1.9 km (fig. 10, table 7). For turtles that had been previously captured one, six, and eight years before being satellite tagged, the distances were 0.2, 3.1, and 1.7 km, respectively.

Core areas, defined by 50 % volume contours, ranged in size from 301–3829 ha for the five turtles that did not appear to leave the platform. However, the quality of the location data provided by these transmitters (lacking GPS accuracy) is less than ideal for studying home ranges of turtles. The data are used in this paper only to evaluate the hypotheses that turtles are resident and exhibit site fidelity. The data should be considered in combination with the dense recapture records like those shown in table 14. These two lines of evidence suggest that at least some immature green turtles reside in Bermuda waters continuously over extended periods of time during which they usually occupy specific sites.

SECRETARY AND THE ZAPATILLA CAYS, PANAMA: Sampling of C. mydas at Secretary and the Zapatilla Cays in Panama has been less concentrated than in Bermuda, and there is less evidence of residency and site fidelity. At Secretary, only 11 of 134 green turtles were recaptured at the study site and the longest recapture interval was one year. There was a subsistence fishery in Chiriqui Lagoon at the time of this study and local fishermen were known to set nets within the study site. This would obviously reduce the likelihood of recapturing tagged animals. However, even with this small sample, there is evidence of site fidelity. Ten of the 11 recaptures were made at the same net site as the original capture ; in one case, a recaptured immature green turtle had moved 7 km. Seven of the 11 turtles that were recaptured had been released at Secretary after the original capture, rather than at the point of capture, representing a displacement of about 6 km. Their return to the original capture site demonstrates an ability to home to a particular site.

Green turtles smaller than 46.7 cm SCL did not occur on the grass flats at the Secretary study site. That fact, along with the paucity of long-term recaptures, suggests that C. mydas may not be resident there for as long as they are in Bermuda. Experienced turtle fisherman in the Secretary area report- ed that C. mydas can be found year-round.

Regular recaptures of immature C. mydas at the Zapatilla Cays site suggest that these turtles constitute a distinct resident group separate from the migratory adults. Although sampling efforts primarily targeted adults, immature turtles were routinely captured (fig. 14B) and recaptured (12.1 % of 128 tagged immatures). As of 2005, only three of 115 adults captured near the Zapatilla Cays were seen in subsequent seasons. Recaptures of immatures occurred after 0.3, 0.5, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.0, 1.5, 1.5, 2.0, 2.0, 2.9, 4.0, and 4.5 years, suggesting that they did not correspond to an annual migration cycle. The three recaptures of adults were reproductive males recaptured during subsequent mating seasons almost exactly, l.0, 2.0, and 5.0 yrs after initial capture.

Further evidence of residency of immature green turtles at the Zapatilla Cays comes from a single netting sample taken outside of the reproductive season during January 1994 (fig. 19B). This sample was composed nearly entirely of immatures. One of 18 C. mydas was mature, and the presence of fresh corpora lutea (determined by laparoscopy) in this individual suggested that she may have been intercepted during a reproductive migration. Recapture data, along with the dominance of immatures (17 of 18 captures) during the January sample (see above), suggest that immatures are likely resident at this site year-round. Adults may be present only during the reproductive season. As at Secretary, the smallest green turtles at the Zapatilla Cays are much larger than the smallest green turtles seen in Bermuda, suggesting that the long residency times that are observed in Bermuda probably do not occur at the Zapatilla Cays.

THE LITERATURE: Fidelity to benthic developmental foraging areas by C. mydas is strongly supported by numerous studies and multiple lines of evidence. At all of the C. mydas study sites reported in table 10, recaptures were made with regularity if appropriate capture methods were used, indicating some degree of residency. The most direct evidence comes from telemetry. Hart and Fujisaki (2010: table 1) summarized telemetry data providing home range estimates for this species at nine sites in benthic developmental habitat. Recaptures for growth studies (e.g., Mendonca, 1981; Bjorndal and Bolten, 1988), population studies (e.g., Ehrhart et al., 1996), and regional studies of C. mydas on adjacent foraging grounds also provide evidence for residency and site fidelity. Two long-term studies that sampled adjacent foraging grounds support residency via reports of very few movements between adjacent sites. In Baja California, 1183 presumably immature greens were marked at four sites. Among 154 recaptures, only two had switched sites (Senko et al., 2010). Along the east coast of Florida, 6027 different green turtles were used to study variation in growth rates among four separate sites; only 19 turtles were observed to switch foraging grounds ( Kubis et al., 2009).