AbstractBycatch is one of the key threats to juvenile marine turtles in the Mediterranean Sea. As fishing methods are regional or habitat specific, the susceptibility of marine turtles may differ according to inter- and intra-population variations in foraging ecology. An understanding of these variations is necessary to assess bycatch susceptibility and to implement region-specific management. To determine if foraging ecology differs with region, sex, and size of juvenile loggerhead turtles (Caretta caretta), stable isotope analysis of carbon and nitrogen was performed on 171 juveniles from a range of foraging regions across the central and eastern Mediterranean Sea. Isotope ratios differed with geographical region, likely due to baseline variations in δ13C and δ15N values. The absence of sex-specific differences suggests that within an area, all comparably sized animals likely exploit similar foraging strategies, and therefore, their susceptibility to fisheries threats will likely be similar. The isotope ratios of juveniles occupying the North East Adriatic and North Levantine basin increased with size, potentially due to increased consumption of more prey items at higher trophic levels from a more neritic source. Isotope ratios of juveniles with access to both neritic and oceanic habitats did not differ with size which is consistent with them consuming prey items from both habitats interchangeably. With foraging habitats exploited differently among size classes in a population, the susceptibility to fisheries interactions will likely differ with size; therefore, region-specific management approaches will be needed.

Understanding the spatial and foraging ecology of marine migrants is challenging, due to the vast distances travelled and the numerous habitats occupied within a dynamic seascape. Mediterranean marine turtles migrate thousands of kilometers and face numerous threats, including bycatch, in their marine realm. To help inform targeted conservation, this complex marine ecology must be better understood. This thesis focuses on Mediterranean loggerhead (Caretta caretta) and green turtles (Chelonia mydas). By complementing stable isotope analysis (SIA), satellite telemetry, and environmental observations, this thesis aims to enhance our understanding of the complexities of marine turtle spatial and foraging ecology, as well as determine how future climate conditions may influence their habitat use.

In Chapter 1, I introduce the importance of conserving marine migrants and discuss the current knowledge of marine turtle spatial and foraging ecology as well as threats faced, with particular emphasis on Mediterranean loggerhead and green turtles. By conducting an extensive review in Chapter 2, I demonstrate how SIA has been used to enhance our understanding of marine turtle ecology, as well as help inform conservation initiatives. I also highlight knowledge gaps (for example, bias in the species studied) and provide recommendations for future SIA studies (for example, following standardised protocols), and use this information to inform latter chapters. In Chapter 3, using SIA I highlight the ecological complexity of juvenile Mediterranean loggerhead turtles, demonstrating there are inter- and intra-population variations in ecology, and that region- and habitat-specific fisheries management is required. In Chapter 4, I identify the foraging grounds for two major Mediterranean loggerhead turtle populations, demonstrate foraging site fidelity over decades, show the proportion of females recruiting from each foraging region does not differ across the multi-decadal study, and suggest site-specific management would be beneficial. Finally, in Chapter 5, I show that migratory dive behaviours of loggerhead and green turtles are influenced by changes in environmental conditions (e.g. wave height and temperature) and that the species-specific migratory corridors used may be due to factors such as feeding preference and physiology, rather than species-specific environmental tolerances, suggesting dynamic and species-specific conservation is required. In Chapter 6, I summarise and discuss the findings from this thesis within the wider context. In conclusion, this thesis emphasises the complexities of marine turtle spatial ecology, shows that habitat use will likely differ under future climate scenarios, and suggests targeted and dynamic conservation is required for effective long term conservation.