Trophic Ecology of Apex Predators in Pelagic Food-Webs Determined from Food Habits and Stable Isotope Analyses

The primary aim of this study is to improve our understanding of trophic relationships between apex predators and their primary prey in the northwest Atlantic using a combined approach of food habits and stable isotope analyses. Stomachs and muscle tissue were collected from 682 dolphinfish (Coryphaena hippurus), wahoo (Acanthocybium solanderi), white marlin (Tetrapturus albidus), roundscale spearfish (T. georgii), yellowfin (Thunnus albacares), albacore, (T. alalunga) and blackfin (T. atlanticus) tunas at recreational fishing tournaments in Massachusetts, New Jersey, North and South Carolina between 2007 - 2011.  Stomach content analysis found that prey fish contributed the greatest proportion by mass and frequency of occurrence to the diets of all predators landed in the South Atlantic Bight and New Jersey.  Alternatively, cephalopods were of primary importance to the diets of predators landed in northern waters off of Massachusetts.  Bullet tuna (Auxis sp.) and shortfin squid (Illex illecebrosus) were primary prey shared by multiple predators.  Other prey common among predator diets included fish from the families Scombridae, Carangidae, Balistidae, Exocoetidae, Tetraodontidae, and Clupeidae, and cephalopods from the families Ommastrephidae, and Argonautidae.  Stable isotope analysis revealed that white marlin had the highest δ¹⁵N (11.5‰) values and hence were feeding at the highest trophic level of all predators examined.  In the South Atlantic Bight, yellowfin and blackfin tuna muscles exhibited higher δ¹⁵N (10.8, and 10.7‰, respectively) values than wahoo (9.9‰) and dolphinfish (9.5‰).  However, the muscle of yellowfin (10.4‰) and albacore (10.6‰) tunas exhibited lower δ¹⁵N values than dolphinfish (10.9‰) collected in waters off of Massachusetts.  δ¹⁵N in dolphinfish exhibited a 1.4‰ increase between southern and northern waters suggesting a regional shift in foraging habits representative of an increase of approximately half a trophic level.  We hypothesize this shift is a reflection of a diet comprised of higher amounts of larger sized squid (46% by mass) in northern waters compared to a more piscivorous (94% by mass) diet composed of smaller sized prey in southern waters.  δ¹⁵N values increased significantly with body size in wahoo (R² = 0.59) and blackfin tuna (R² = 0.23), but showed no significant trend in other predators.  Wahoo exhibited the greatest range of δ¹⁵N values (3.4‰) overall, reflecting a span of approximately 1.1 trophic levels.  Results provide insight into shared resource use by these predators and will be discussed in relation to regional food habits and seasonal migration patterns.