Diets and standing stocks of benthopelagic fishes at two bathymetrically different midoceanic localities in the northeast Atlantic

Abstract

Deep-sea benthopelagic fishes were sampled by trawl and free-fall longline at two latitudinally and bathymetrically separated stations in the northeastern Atlantic. The catch structure and the contents of the fish stomachs were analysed. The northern, 3000 m deep station in the Iceland Basin exceeded the deeper (4500 m) site in terms of demersal fish biomass and abundance by factors of 5 and 10, respectively. Synaphobranchidae of the species Histiobranchus bathybius and several species of Macrouridae contributed to the samples, the latter dominating with Coryphaenoides (Nematonurus) armatus most abundant in the north and with Coryphaenoides (Chalinura) leptolepis in the south. Diet composition was studied for numerical abundance and wet weight of prey organisms. In the Iceland Basin, where two size classes of C. (N.) armatuscould be analysed separately, an ontogenetic change in diet was noted. Individuals shorter than 30 cm in length fed mainly on benthic and epibenthic Crustacea whereas the larger specimens showed a high proportion of fish and Cephalopoda in their stomachs. At the southern site (BIOTRANS station in the West European Basin) Amphipoda, fish and Cephalopoda were the main food source. Coryphaenoides (Chalinura) leptolepisfed on Malacostraca and fish at the northern site, but on Polychaeta and Cephalopoda at the southern station. Stomachs of H. bathybius contained food only in the Iceland Basin. Their diet consisted mainly of Malacostraca and Cephalopoda. Despite their state of digestion, most Cephalopoda could be identified by means of their beaks, which also allowed a recalculation of their original size. The taxonomic composition and the size structure led to the assumption that most Cephalopoda were epi- or mesopelagic and had been preyed upon near the bottom as carcasses. The possible trophic position of the predominant benthopelagic fishes in the benthic boundary layer is discussed.