Long-term changes in adult size of green turtles at Aldabra Atoll and implications for clutch size, sexual dimorphism and growth rates

Abstract

Around the world, declines in the mean size of nesting sea turtles have been reported with concerns of a concomitant decrease in the reproductive output of populations. Here, we explore this possibility using long-term observations of green turtles (Chelonia mydas) at Aldabra Atoll, Seychelles. Based on > 4500 individual measurements over 21 years (1996–2016), we found the curved carapace length of nesting females declined by about 0.64 cm per decade, from 111.43 to 110.08 cm. For 391 individuals that were measured more than once with measurement interval of 2.8–19 years apart, the mean growth rate was 0.14 cm year−1. Comparisons between the size of adult females and males were based on 23 and 14 weight measurements, 107 and 33 carapace length measurements and 103 and 33 carapace width measurements, respectively, taken during 1981–1983. Adult females were larger than males, with the sexual dimorphism index, i.e. the ratio of size of the larger sex to the smaller, being 1.09 and 1.10 for carapace length and width, respectively, and 1.25 for weight. Smaller females tended to lay fewer eggs per clutch but the decrease in female mean size was accompanied by increases in numbers of turtles nesting annually, such that the estimated total numbers of eggs per year increased from 1.3 million to 2.0 million between 1996 and 2016. Therefore, a decrease in mean size of nesting females has not compromised egg production for this population.

Growth rate and projected age at sexual maturity for immature hawksbill turtles and green turtles foraging in the remote marine protected area of Aldabra Atoll, Seychelles

“…The approach used by Colman et al (2015) and Bellini et al (2019) was modified for this study by applying a small growth rate to fill the data gaps for larger immature/small mature turtles (67-109 cm CCL for green and 73-87 cm CCL for hawksbill turtles). Turtles in the smaller end of these ranges in other studies have been found to have spikes in growth (as discussed above), but then once sexual maturity is reached, annual growth appears to become very low (Omeyer et al 2017;Mortimer et al 2022). Coupled with the small sample size in this study of larger immatures/small mature turtles, this approach could be overestimating age and needs further investigation.…”

“…Published adult growth rates in peer-reviewed literature from nesters at Aldabra (for green turtles) and rookeries in the Seychelles (for hawksbill turtles) were used in the model to estimate age at sexual maturity. For green turtles, we used the mean nesting size (108.9 cm CCLn-t) from Aldabra collected during 1995-2016 (Mortimer et al 2022) and a female adult growth rate of 0.14 cm year −1 from Aldabra nesters (Mortimer et al 2022) in the model. For hawksbill turtles, we used the mean nesting size (86.4 cm CCLn-t) from Cousine Island, Seychelles (Gane et al 2020) and an adult growth rate of 0.17 cm year −1 for females > 80 cm CCL (Coral Sea, Queensland, Australia; Bell and Pike 2012).…”

“…Furthermore, growth rates/age of sexual maturity are likely to differ between males and females, as has been suggested for both species (e.g., Chaloupka and Limpus 1997;Limpus and Chaloupka 1997;Bell et al 2005;Goshe et al 2010;Murakawa and Snover 2018;Turner Tomaszewicz et al 2022a. Nesting females were found to be larger than adult males at Aldabra (Mortimer et al 2022). understanding sex differences in growth and/or age of sexual maturity should be prioritized.…”

“…Rather they may simply be growing slower and reaching adulthood at the same age, but at a smaller size. Aldabra nesting green turtles have seen a significant decline in mean carapace length from 1996 to 2016(Mortimer et al 2022), possibly due to an increase in the numbers of new nesters(Hays et al 2022a) leading to an age structure shift(Piacenza et al 2016) at the same time that nesting numbers have increased(Pritchard et al 2022).Mortimer et al (…”

Growth rate and projected age at sexual maturity for immature hawksbill turtles and green turtles foraging in the remote marine protected area of Aldabra Atoll

“…For green turtles, we used the mean nesting size (108.9 cm CCLn-t) on Aldabra from 1995-2016 (n = 4635; SIF unpubl. data, same dataset from Mortimer et al 2022, and a female adult growth rate of 0.14 cm year-1 from Aldabra nesters (Mortimer et al 2022) in the model. For hawksbill turtles, we used the mean nesting size (86.4 cm CCLn-t) from Cousine Island, Seychelles (Gane et al 2020), and an adult growth rate of 0.17 cm year-1 for females > 80 cm CCL (Coral Sea, Queensland, Australia; Bell and Pike 2012).…”