Somatic growth rates of hawksbill turtles Eretmochelys imbricata in a northern Great Barrier Reef foraging area

Abstract

ABSTRACT: An understanding of the rate at which long-lived species grow is essential for determining important life history parameters, including assessments of foraging habitat quality, the time taken to reach sexual maturity, and the age at which maturity is reached. The hawksbill sea turtle Eretmochelys imbricata is a Critically Endangered species that is typically associated with coral reefs throughout the tropical oceans. Little is known about its life history through most age-classes, especially in the western Pacific, one of the last strongholds for the species. We undertook annual surveys of a foraging population of hawksbill turtles on 13 reefs, within the far northern section of the Great Barrier Reef (GBR), from 1997 to 2008. We obtained growth rate data from 128 turtles that were recaptured once (n = 103), twice (n = 23), or 3 times (n = 2). The curved carapace length of recaptured turtles ranged from 61.3 to 91.4 cm (x= 79.8 ± 7.26 cm, mean ± SD). Recapture intervals spanned between 11 and 120 mo (mean: 59 mo; median: 72 mo). We show that mean annual growth rates did not differ between sexes and had a non-monotonic growth pattern, reaching peak growth for both sexes within the 65 to 70 cm curved carapace length size-class. Growth slowed thereafter at a linear rate, to negligible growth upon reaching an adult size of >80 cm. Hawksbill turtles grew faster in the southern GBR, compared to those found in the far north of the reef. Understanding spatial and temporal differences in growth rates, and thus age at maturity, is an important step towards resolving life history differences among sea turtle populations.