Population structure of Caretta caretta in the Mediterranean Sea: from nesting beaches to foraging grounds
With more than 40% of the world’s population living in coastal areas, oceans have become an indispensable resource for humankind but many coastal and marine regions are suffering from multiple anthropogenic threats. Pollution, coastal tourism, agricultural practices and development of ports threaten marine species worldwide and have caused the decline of many species, some of them currently on the verge of extinction. Bycatch, the unintentional catching of non-targeted species during fishing operations has been described as one of the most important threats causing the decline of many species, especially of large marine vertebrates: sharks, sea turtles, birds and marine mammals. This is because large marine vertebrates are the most vulnerable group to bycatch due to their life-cycle characteristics, presenting a long lifespan, late age at maturity and low reproductive output. However, conservation needs for marine megafauna are particularly difficult to assess as these species usually occur in remote oceanic habitats, are distributed across entire oceans and often have complex life cycles involving migrations that cover thousands of kilometres. Thus, because sea turtle life stages may occur in different areas, a detailed knowledge on the life cycle, distribution and habitat use of these species is highly relevant as populations might face different anthropogenic impacts depending on the areas used. Accordingly, these questions have been addressed for the loggerhead turtle (Caretta caretta) in this thesis through different approaches such as population genetics, stable isotope analysis, growth models and fisheries impact assessments. The results presented in this thesis have revealed deeper structuring in nesting and foraging grounds for loggerhead turtles than previously thought. An earlier colonisation of the Mediterranean has been discovered, fine-scale rookery contributions to Mediterranean foraging grounds have been unveiled, the habitat use of nesting females has been discovered and new management units have been described within the Mediterranean Sea. The effects that foraging ground use have on loggerheads biology has also been approached showing that individual patterns of habitat use may have a remarkable effect on fitness (clutch size) and growth rates. This, in turn, may be driven by surface water circulation patterns of the basin and the trajectory followed by juvenile turtles during their developmental migration. Finally, this thesis has highlighted the importance of regional studies to understand the consequences of fisheries bycatch as the actual impact will depend on the origin of the turtles incidentally caught.