Distinguishing between sea turtle foraging areas using stable isotopes from commensal barnacle shells

Ryan M Pearson,Michael K Gagan,Jason P Van De Merwe,Colin J Limpus,Rod M Connolly

doi:10.1038/s41598-019-42983-4

Abstract

Understanding the movement behaviour of marine megafauna within and between habitats is valuable for informing conservation management, particularly for threatened species. Stable isotope analyses of soft-tissues have been used to understand these parameters in sea turtles, usually relying on concurrent satellite telemetry at high cost. Barnacles that grow on sea turtles have been shown to offer a source of isotopic history that reflects the temperature and salinity of the water in which the host animal has been. We used a novel method that combines barnacle growth rates and stable isotope analysis of barnacle shells (δ18O and δ13C) as predictors of home area for foraging sea turtles. We showed high success rates in assigning turtles to foraging areas in Queensland, Australia, based on isotope ratios from the shells of the barnacles that were attached to them (86–94% when areas were separated by >400 km). This method could be used to understand foraging distribution, migration distances and the habitat use of nesting turtles throughout the world, benefiting conservation and management of these threatened species and may be applied to other taxa that carry hitchhiking barnacles through oceans or estuaries.

Highlights

Understanding the distribution, migratory pathways, and habitat use of marine fauna is valuable to inform management decisions, especially for threatened species
The largest previous study that assessed turtle movement using barnacle isotopes analysed 70 samples from six barnacles from six turtles[32] The only other study on sea turtle barnacles assessed a total of twelve barnacles from four turtles, though only nine barnacles from three turtles were included in their analyses due to sample yield being insufficient for mass spectrometry[30]
We show that isotope ratios (δ13C and δ18O) of the calcareous shells from barnacles commensal on sea turtles can be used to distinguish between foraging areas with success varying at different spatial scales

Summary

Introduction

Understanding the distribution, migratory pathways, and habitat use of marine fauna is valuable to inform management decisions, especially for threatened species. SIA studies on sea turtles have almost entirely focused on analysis of soft-tissues[25], in which isotope ratios are affected by diet choices of the individual This type of study generally combines a relatively high-volume of satellite telemetry (e.g. 50+ turtles) with SIA of soft-tissues to build a reference library (isoscape or calibration data) of isotopic signals (e.g.12,26). These reference libraries are used to assign foraging areas to individuals that were not tracked. Loggerhead turtles are known to be diet generalists at the population level, but specialists as individuals[27] This suggests that within a foraging area, individual diet choices may result in high variation in isotope ratios and lower ability to identify differences between regions. This phenomenon is partly supported by recent findings that foraging areas with higher prey diversity show higher variation in soft-tissue isotope ratios[28]

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