Intraspecific variability in flatback turtle habitat use: δ15N as an indicator of foraging locations

Abstract

Identifying migration routes and key habitats is critical for the management and conservation of migratory species. Tracking and stable isotope analysis (SIA), particularly of carbon (δ13C) and nitrogen (δ15N), are often used to study animal movements, with SIA particularly useful when animals move through isotopic gradients. Marine turtles are typically highly migratory, moving between nesting and foraging grounds often located 100s-1000s km apart. The flatback turtle Natator depressus is endemic to the Australian continental shelf. Satellite tracking (n = 44) and SIA (n = 33) of females nesting in eastern Queensland, Australia, were used to identify main foraging areas, describe intraspecific variation in the location of foraging areas, and determine if δ13C and/or δ15N values can be used to identify foraging regions. Although foraging grounds were widely dispersed, tracking identified 3 main foraging regions. SIA agreed with tracking, indicating foraging site fidelity. Generalized linear models and linear discriminant analysis (LDA) were used to estimate how well δ13C/δ15N and nesting sites can perform as indicators of broad foraging regions and to predict foraging regions for turtles with no tracking data. δ15N was a strong predictor of the foraging region. LDA correctly classified the foraging region of 94% of individuals and was suitable to predict foraging regions of untracked individuals. A strong negative linear relationship between turtle δ15N and foraging latitude indicates the presence of a δ15N isoscape along the eastern Queensland coast. This is the first demonstration of an isoscape for the region, which should be useful for studying and monitoring the habitat use of flatback turtles and other migratory species.