| dc.description.abstract | The study of animal diet has long been a fundamental area of biological sciences and has developed significantly over the centuries. An understanding of animal diet goes beyond species-specific biology providing insights into interspecific interactions and whole ecosystem functions essential for ecosystem-based management. Anthropogenic activities have caused major declines in marine vertebrate populations many of which are top predators. It is therefore vital to establish an ecological understanding of mesopredators in the oceans who may either mediate or exacerbate the cascading impacts of such declines. Elasmobranch batoids (otherwise referred to as rays) are a diverse, yet highly vulnerable group of mesopredators many of which are considered data deficient and lack comprehensive dietary assessments. Studies that do exist use stomach content analysis (SCA) or stable isotope analysis (SIA) independently of one another. In contrast, the present thesis integrates these two methods. Chapter one aimed to utilize both SCA and SIA techniques to provide the first integrative dietary assessment of the southern stingray (Hypanus americanus), and the first ever quantitative dietary study of the Caribbean whiptail ray (Styracura schmardae), two sympatric and data deficient species. Our results suggested that the diets of both species were similar in structure and composition, though Caribbean whiptail ray diet was dominated by arthropod and annelid prey, while southern stingray diet was dominated by molluscs. The broad variety of taxa identified in the stomachs of both species indicates opportunistic feeding, likely as mesopredators at a trophic level similar to other ray species in their respective families. Our integration of SCA and SIA highlights the advantages of combining the two methods, for example, the higher representation of soft-bodied prey in stable isotope mixed models (SIMM) compared to those of SCA. The focus of Chapter two was the isotopic variances between three metabolically different tissues (blood, white muscle and barb) from both the southern stingray and the Caribbean whiptail ray, highlighting how the use of multiple tissues in diet assessments may give better insight into the temporal variability of diet. This was the first quantitative comparison of SCA and SIA across tissue types in rays, the results of which suggest that method agreement is influenced by tissue type incorporated in SIMM. A limitation of our inferences, however, is the lack of data available on isotopic turnover rates in ray and elasmobranch tissues, thus we recognise the need for further literature and diet manipulation experiments. Nonetheless, the results of the present thesis provide a novel insight into the diets of these two data deficient stingray species and highlight potential avenues for future research which would improve our understanding of these ecologically significant and vulnerable animals. | en_GB |
