Assessing resource use patterns of Mediterranean loggerhead sea turtles Caretta caretta (Linnaeus, 1758) through stable isotope analysis

M F Blasi,L Tomassini,M Gelippi,G Careddu,G Insacco,N V C Polunin

doi:10.1080/24750263.2018.1435742

M F Blasi, L Tomassini + Show 4 more

Open Access

https://doi.org/10.1080/24750263.2018.1435742

Copy DOI

Abstract

Stable isotope analysis is a useful tool for studying the ecology of marine consumers, as carbon (δ13C) and nitrogen (δ15N) isotope ratios may reflect individuals’ patterns of diet and habitat use. Knowledge of foraging strategies has significant implications for the conservation of endangered loggerhead turtles Caretta caretta (Linnaeus, 1758). In this study, δ13C and δ15N isotope data were used to assess resource use patterns of the Mediterranean loggerhead turtles (Aeolian Archipelago, Southern Italy). δ13C and δ15N values from carapace scutes of 54 loggerheads of different curved carapace length (CCL) and health status were compared with those of eight potential prey items (benthic, pelagic and fishery discards). MixSIAR results suggested that pelagic prey (from goose barnacles to planktivorous fish) comprised most of loggerheads’ diet, with small variations (i.e. benthic prey or fishery discards) depending on size (δ13C and δ15N) and health (δ15N) of individuals. δ13C variations with turtles’ size might reflect changes in dietary habitats during life stages. However, the loggerhead turtles and their main source of prey (pelagic prey) had a higher variation in values of δ15N compared to δ13C. This suggested that smaller-sized turtles might preferentially feed on pelagic prey in oceanic habitats and then, as they reach a larger size, gradually enter neritic waters, including in their diet prey sources with higher δ13C and δ15N. Some turtles foraging on longline baits/debris also displayed a marked increase in δ15N. These δ15N variations might be explained by differences in diet (trophic differences) and somatic growth rates among individuals, or dietary dilution.

Highlights

The loggerhead sea turtle (Caretta caretta) is the most abundant marine turtle species in the Mediterranean Sea, where it occurs in several areas (Luschi & Casale 2014)
The feeding ecology of the loggerhead turtle was inferred through the analysis of the stable carbon and nitrogen isotopes that accumulated in its carapace and by the comparison of these values to those for possible preys or food production sources. This is the first study that uses this methodology to investigate the influence of size and health condition in different specimens of the Mediterranean loggerhead turtle
Several studies demonstrated that the diet of the Mediterranean loggerhead turtle is highly varied in terms of prey taxa (Bjorndal 1997; Godley et al 1998; Tomás et al 2001; Revelles et al 2007a; Cardona et al 2010, 2012; Zbinden et al 2011)

Summary

Introduction

The loggerhead sea turtle (Caretta caretta) is the most abundant marine turtle species in the Mediterranean Sea, where it occurs in several areas (Luschi & Casale 2014). After attainment of a hatchling pelagic phase (Boyle & Limpus 2008; Wallace et al 2009), immature loggerheads may gradually enter feeding areas on the continental shelf (Laurent et al 1998) and stay in these habitats over several decades before sexual maturation (Musick & Limpus 1997; Bolten 2003; Casale et al 2008). Mature loggerheads move to specific mating and nesting sites during the breeding season (Bolten et al 1992; Bowen et al 1995, 2004; Boyle et al 2009) and return thereafter to the foraging and wintering areas where they spend much of their life (Casale et al 2012). The prolonged residence of loggerhead females in oceanic areas (Hatase et al 2002; Hawkes et al 2006) suggests pelagic feeding in some

Methods

Results

Conclusion