The effects of clutch size and nest depth on nest success and hatchling vigour of flatback sea turtles, Natator depressus

Koch, Andrea

Flatback sea turtle (Natator depressus) eggs were incubated in artificial nests at a hatchery on a nesting beach at Bare Sand Island, Northern Territory, Australia. Eggs were collected from natural nests and reburied at the hatchery in experimental groups of 10, 30, and 50 eggs at depths of 20, 35, and 50 cm. All combinations of clutch sizes and nest depths were investigated with four replicates of each. Hatchlings were collected on emergence and their size, mass and scalation were measured. Locomotor performance trials were conducted on hatchlings emerging from each nest. Hatching and emergence success, incubation durations, nest temperatures and sand temperatures were measured. The sex ratios of hatchlings produced from treatment nests were estimated using temperature and incubation duration data. Histological examination of the gonads of hatchlings and full-term embryos found dead in the nest provided a further indication of the sex of hatchlings produced from nests. Hatchling production from reduced clutch sizes was investigated to determine the potential impact from harvesting portions of clutches by Aboriginal people. In addition, the study aimed to determine the impact of splitting clutches for artificial incubation of N. depressus is required for conservation purposes. The effects of reduced nest depths on embryogenesis were investigated to determine the impact of wind erosion and unstable dune formations on the viability of clutches. Neither clutch size nor nest depth were found to significantly affect either hatching or emergence success. Emergence of hatchlings often occurred asynchronously, and clutches emerged over an average of 2.7 days. Hatchlings from both larger and shallower nests emerged with less synchrony than smaller and deeper clutches. Smaller hatchlings were produced from both larger clutches and shallower nests. Hatchlings from shallower nest had a lower mass relative to their carapace length. The running speed of hatchllings was slowest for clutches of 30 eggs. Hatchling running speed increased with increasing nest depth. Incubation duration were longer than previously reported for N. depressus, and increased with smaller clutches. During the study, ambient temperatures were at their seasonal minimum. Smaller clutches experienced lower nest temperatures than larger clutches. The variation in sand temperature decreased with increasing depth, but the mean temperature did not change with depth. The gender of hatchlings produced were estimated to be predominantly male, but the effects of clutch size and nest depth on hatchling gender could not be quantified. This study found that reduced clutch sizes were not beneficial to the embryogenesis of N. depressus. Smaller clutches are likely to produce a greater percentage of male hatchlings due to the lower temperatures. This may be detrimental to the population size, especially from beaches already producing predominantly male hatchlings (including Bare Sand Island). The practice of splitting clutches, either during Aboriginal harvest or for artificial incubation would thus be unadvisable. Nests at 20 cm depth appear detrimental to embryogenesis, producing less vigorous hatchlings with a potentially reduced period of hatchling frenzy. Nests at shallower depths also had greater hatchling mortality in the nest. As a conservation strategy, it may be advisable to relocate nests where there is a significant risk of sand reduction. Alterations to natural nest conditions were generally unfavourable to embryogenesis. Any change to the natural conditions experienced may, therefore, require management. Abstract provided by author.