Nesting female hawksbill sea turtles trending smaller in the western Indian Ocean

Abstract

Decreasing body size in nesting sea turtles has been documented globally in recent decades and attributed to a combination of environmental, ecological, and demographic changes, along with concerns about potential negative consequences for reproductive output. Our study examines long-term trends in body size of nesting hawksbill turtles Eretmochelys imbricata in the Seychelles and the likely demographic factors that may be responsible. The average curved carapace length (CCL) declined significantly from 1974-2022 (0.05 cm yr-1) for females nesting at 2 neighbouring islands, Cousine and Cousin. At Cousine Island, adult growth rates were calculated, and neophyte (recruit) and remigrant nesters were distinguished from 2002-2022. Growth was significantly different from 0 (0.18 cm yr-1). Neophyte CCL declined significantly (0.19 cm yr-1), while CCL of returning remigrants increased significantly (0.12 cm yr-1). The annual number and proportion of neophytes and remigrants fluctuated throughout 2002-2022, but the proportion of neophytes significantly increased from 2013 onward, approximately 20 yr after complete protection and increased hatchling production began at Cousine. Clutch size correlated positively with CCL, and annual clutch numbers have been increasing significantly since 1992. We conclude that overall declining trends in body size likely result from declining neophyte CCLs combined with increasing proportions of neophytes over the last decade. Meanwhile, conservation measures have enabled neophytes to survive to breed repeatedly during multiple nesting seasons and, over time, to grow bigger, produce larger egg clutches, and ultimately enhance the reproductive output of the entire population.