Abstract
Prey composition and resource partitioning were investigated among three large pelagic fish predators, yellowfin tuna (Thunnus albacares), swordfish (Xiphias gladius) and lancetfish (Alepisaurus ferox), in a poorly known oceanic area, the western Indian Ocean. The contents of 380 non-empty stomachs were analysed from specimens caught with longlines during scientific cruises carried out from 2001 to 2003. Diet data were processed by occurrence, mean proportion by number, wet weight, and mean proportion by reconstituted weight. Crustaceans, dominated by the swimming crab Charybdis smithii and the stomatopod Natosquilla investigatoris, were the major food source of lancetfish. Cannibalism was also significant for that species. Yellowfin tunas preyed upon a large diversity of mesopelagic fishes, crustaceans (C. smithii and crab larvae) and cephalopods (the ommastrephid Sthenoteuthis oualaniensis). Mesopelagic fishes (Cubiceps pauciradiatus and Diretmoides parini) and cephalopods (mainly S. oualaniensis) were the main prey of swordfish. Diet overlap between swordfish and yellowfin tuna was evidenced by high Morisita–Horn index. But the feeding habits of these three predators differed by foraging depth and prey size, with swordfish feeding at deeper depths and on larger prey than the more epipelagic lancetfish and yellowfin tuna. Using these three predators as biological samplers, the present study provides novel data on micronekton fauna that is poorly documented in the western Indian Ocean: 67 families and 84 species of prey were recovered in the stomach contents, and our results indicate the presence of large resources of pelagic crustaceans that play a primary role in the epipelagic food chain.