Trophic Ecology of a Dominant Mesopredatory Fish Family (Alepisauridae) in the Western North Atlantic Revealed by Stomach Content and Stable Isotope Analysis

The mesopelagic zone of the ocean is gaining increased notice due to its potential for commercial value and the lack of knowledge of its ecosystems. Lancetfishes, Alepisaurus spp., are large mesopelagic predators and common bycatch species in commercial fishing operations that potentially play a role in connecting epipelagic and mesopelagic waters and may be a source of carbon from near the surface to the mesopelagic. While little is known about the behavior and biology of Lancetfishes, they share many food sources, and are themselves a food source for important commercial fisheries species such as tuna and swordfish. This study examines the diet and trophic ecology of both species of Lancetfish, using stomach content analysis from both Longnose Lancetfish, Alepisaurus ferox, and Shortnose Lancetfish, A. brevirostris caught opportunistically as bycatch on commercial fishing vessels, as well as carbon and nitrogen stable isotope analysis on white dorsal muscle of both species of Lancetfish and their important prey species. In addition, morphological descriptions relating to diet are made to compare species and size classes of Lancetfish. Both species were found to have a diverse diet made up of a variety of taxa from both epipelagic and mesopelagic waters. Fish were the most important prey group for both species and all size classes of Lancetfishes. Size, rather than species or season, defined differences in diet and trophic position. Stable isotope values revealed the highest trophic positions were occupied by large Lancetfish (A. ferox over 130 cm) (δ15N=10.2) and the closely related omosudid hammerjaws (δ15N=8.9), while the lowest levels were occupied by heteropods (δ15N=4.8). In stable isotope analysis as well as stomach content analysis, individual size instead of species was the differentiating factor in diets of Lancetfishes