Nonparametric regression modelling of green sea turtle growth rates (southern Great Barrier Reef)

Abstract

Somatic growth rates recorded between 1974 and 1991 for green sea turtles Chelorl~a rnydas resident in the southern Great Barrier Reef (sGBR) foraging grounds of the sGBR genetic stock were modelled using nonparametrlc regression. The sampling deslgn in this long-term mark-recapture program was mixed longitudinal and ~ncluded growth records for female and male turtles 1-anglng between 39 and 116 cm CCL (curved carapace length). Distinct sex-specific growth patterns were found, adults displayed constant negligible growth while inter-annual variability in immature female growth attributable to major oceanographic events was evident. Females grew faster than the males from ca 62 to 65 cm CCL with divergent growth patterns following distinct juvenile growth spurts. The expected female size-specific growth rate function was nonmonotonic, rising from recruitment size (>35 cm CCL) to maximum growth of 2.1 cm yr.' at ca 60 cm CCL before declining to negligible growth approaching sexual maturlty at ca 95 to 100 cm CCL. The male size-specific growth rate function was nonmonotonic rising from the sdme recruitment size to maxlmum groiuth of 2 .2 cm yr.' at ca 63 cm CCL before declining faster than females to negligible grotzlth approachlnq sexual maturity at a significantly smaller size, 595 cm CCL Assumlng a post-hatchling pelagic phase of 5 yr, the mean age at sexual maturity for each sex was estimated at 2-40 yr using numer~cal Integration of the expected slze-specific growth rate functions. Expc.cted age-specific growth rate functions for each sex derlved by numerical differentiat~on showed that the juvenile growth s p u ~ t for females resident in the sGBR foraglng grounds occurs at around 9 yr at large since recruitment (60 cm CCL. 214 yr old) and for males at ca 11 yr a t large (63 cm CCL, 216 yr old). Juvenile growth spurts, sex-specific growth, slow size-specific growth rates, inter-annual growth variability of immatures and many decades to sexual maturity are growth characteristics for green sea turtles resident in the sGBR foraging grounds of the sGBR genetic stock. K E Y WORDS: Somatic growth . Green sea turtles . Great Barrier Reef Nonparametrlc regression