Temporal variations in the feeding habits and trophic levels of three deep-sea demersal fishes from the western Mediterranean Sea, based on stomach contents and stable isotope analyses

Temporal variations in feeding habits and trophic levels of 3 deep-sea fishes: Hoplostethus mediterraneus, Hymenocephalus italicus and Nezumia aequalis, were investigated off the Archipelago of Cabrera (Algerian Basin). These species are representative of the 3 main trophic guilds found at bathyal depths: plankton feeders, suprabenthos feeders and epi-endobenthos feeders. Samples were collected during 6 trawl surveys at bi-monthly intervals from August 2003 to June 2004, between 550 and 750 m depths. Stomach content and stable isotope analyses were performed to detect temporal variations in diet. Because fish were immediately fixed in 10% formalin, a parallel experiment comparing isotope composition in fixed and freshly frozen tissues was carried out. H. mediterraneus mainly preyed on isopods throughout the sampling period and to a lesser extent on amphipods and mysids, although mixing models revealed high consumption of mesopelagic fish in winter and spring. H. italicus fed on mysids and amphipods that live in the benthic boundary layer, while N. aequalis exhibited a more benthic diet composed of benthopelagic and benthic prey (e.g. tanaids and polychaetes). Temporal variation was observed in the diet of H. mediterraneus, with February being the period of change in feeding habits. Temporal changes were also evident, though weaker than in H. mediterraneus, in the diets of the 2 macrourids, H. italicus and N. aequalis. Patterns of temporal change in fish diets follow temporal variations in prey, which are stronger in mesopelagic than in suprabenthic or benthic fauna. Temporal variations in tissue delta N-15 were detected in the 3 species, with the lowest values occurring in September and the highest in April to June. In general, different time lags were detected between the isotopic turnover time of consumers and that of their diets (prey) at a monthly time scale, showing that assimilation times depend on species behaviour and metabolism. Assimilation times are brief in H. mediterraneus but last up to 2 mo in the benthic feeder N. aequalis. The experiment comparing fixed versus frozen tissues showed that formalin did not affect delta N-15 values after 6, 12 and 24 mo of preservation, while delta C-13 signatures were depleted to different extents in the 3 species compared with controls.