| Summary: | Short-finned pilot whales (Globicephala macrorhynchus) generally forage on cephalopods in the mesopelagic zone, sometimes diving to over 1000m in depth in pursuit of prey. However, pilot whales and several other species of odontocetes have learned to target longline fishing vessels to depredate (prey upon) hooked fish. These odontocetes may gain energetic benefits due to the high calorie prey and relatively little effort involved in depredation. Each year, hundreds of short-finned pilot whales become hooked due to depredation attempts and even if they break free they can become injured by trailing gear. In order to understand and reduce these interactions, basic questions still need to be answered, including which segments of the population are preying upon hooked fish? Stable isotope analysis is a powerful tool used to study trophic interactions and can potentially identify pilot whales that are depredating on tuna. This requires stable isotope values of the pilot whales, tuna, and cephalopod prey of both the pilot whales and tuna to enable estimates of trophic relationships. Stable isotope values of cephalopods are lacking in the western North Atlantic. The primary cephalopod prey of short-finned pilot whales are not caught in commercial fisheries, and offshore sampling is logistically and financially difficult. Considering those challenges, this study aims to use SCA of several tuna species to determine if tuna, and which species in particular, can be used as biological samplers to obtain the same deep-water cephalopod taxa that short-finned pilot whales prey upon.
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