UC Santa Cruz

Industrial fisheries threaten oceanic sharks and rays. Effective management and conservation responses require solutions that incorporate biological, ecological, and policy considerations. In this dissertation, I use an interdisciplinary approach to measure and mitigate fisheries impacts for data-poor but highly threatened pelagic elasmobranchs. First, I describe the quantity, composition, and potential impact of industrial tuna fishing on 22 elasmobranch species using catch data reported by global tuna fisheries and a catch estimation approach. I show that publicly accessible catch rates indicate that these fisheries report ~2.9 million individual sharks and rays per year, but that estimated catches are as much as five times higher. Importantly, these impacts are understudied: only 20% (15 of 76) of the examined populations had been formally assessed and of these, roughly 30% are overexploited. Given these findings, in the second chapter I focus on one particularly vulnerable group: manta and devil rays (collectively, Mobulids). I analyzed the genetic population structure of three species in the eastern Pacific Ocean and found weak but statistically significant population genetic structure among all four species. These findings suggest that Mobulids exhibit metapopulation structure and local selection occurring regionally in the eastern Pacific, and support the identification of unique regional units for conservation and fisheries management. Finally, and as a way forward, I assess the policy response of five large tuna Regional Fisheries Management bodies responsible for managing and mitigating to elasmobranch bycatch. Using a mitigation hierarchy framework, I show that current policies are not adequate to sufficiently reduce oceanic shark and ray bycatch, and make management, data, and transparency recommendations that are likely to reduce the impacts of bycatch on threatened sharks and rays.