Diets of five important predatory mesopelagic fishes of the central North Pacific

The diets of 5 large predatory mesopelagic fishes-Alepisaurus ferox (longnosed lancetfish), Gempylus serpens (snake mackerel), Lepidocybium flavobrunneum (Smith's escolar), and Lampris spp. (big-eye and small-eye opah, or moonfish)-from the central North Pacific Ocean (around Hawaii) were examined (n = 430, all species combined), most for the first time. Recent analysis of fishery data has shown that many of these species have been undergoing decadal increases in abundance, suggesting system-wide changes. A. ferox diet was numerically dominated by hyperiid amphipods from 3 genera (Phrosina, Phronima, Platyscelus; 37% N), pelagic polychaete worms, mesopelagic fishes (including young A. ferox size classes), and cephalopods. G. serpens fed primarily on epipelagic fishes (exocoetids, molids) and ommastrephid squids. Diets of the 2 Lampris species were the most similar to one another, consisting of large numbers and frequent occurrences of the onychoteuthid squid Walvisteuthis youngorum and a diverse assemblage of epipelagic and mesopelagic fishes. More than 90% of the L. flavobruneum stomachs were without food items; small numbers of prey identified included the ommastrephid squid Sthenoteuthis oualaniensis, aristeid shrimps, and unidentified fishes. The diet descriptions support the idea that these predatory fishes carve out unique ecological niches in the pelagic environment by exploiting unique components of micronekton communities across epipelagic and mesopelagic depth zones. Adult size classes of tunas and billfishes occupying a shared vertical habitat do not appear to compete for prey resources to any great extent, perhaps allowing for successful partitioning of limited prey resources within an oligotrophic gyre ecosystem.