Understanding variation in ecology and physiology of marine migratory species.

Abstract

As the world faces the threats of multiple compounded and worsening crises, scientists are racing to gather the knowledge necessary to safeguard entire ecosystems and species. Technological advances are continuously facilitating more in-depth studies to understand the mechanisms driving species functioning and variations among and within populations, communities, and individuals. New methods are providing insights into difficult to access environments and species such as the open ocean. The following thesis uses theoretical and empirical approaches to understand the drivers of variation in ecological niches of large marine predators. One of the methods commonly used to study a species resource use is stable isotope analysis (SIA). However, the relationship between variation in stable isotope values in the tissues of consumers and their diet is often misconstrued or over-simplified. Here, we lay out the underlying factors that influence stable isotope ratios and how these can be accounted for when designing an ecological study. I then review new advances in stable isotope technologies and how compound-specific SIA can be used to ask questions about the life-history of a broad range of species. Having shown the value in combining biochemical methods with other disciplines, I apply this approach to study the ecology of large marine vertebrates in Ireland. By using fatty acid analysis in combination with reproductive hormone analysis, biologging, morphometrics and observations, I investigate the ecology and physiology of blue sharks, Prionace glauca, in Ireland. This population of predominantly female individuals in varying stages of maturity show indications of a seasonal change in resource use between the June and November ? possibly indicating opportunistic foraging on abundant gelatinous or planktonic prey. Finally, I present the results from our tagging efforts on mature female porbeagle sharks, Lamna nasus, caught in Donegal (Ireland) in April 2022. As I have followed these two individuals over the past nine months, both have crossed many environmental and jurisdictional boundaries but have displayed high inter-individual variation. I argue the need for increased and continued cross-country collaboration in the Northeast Atlantic to study and manage this critically endangered species. The work carried out here emphasises the value in moving our field from uni- and multidisciplinary approaches towards more holistic interdisciplinary approaches, particularly when our aim is the sustainable preservation of large oceanic migrators.