Daily vertical distribution and diet of Cyclothone braueri (Gonostomatidae) in the Polcevera submarine canyon (Ligurian Sea, north-western Mediterranean)

Abstract

Despite its abundance, the role of Cyclothone braueri in intermediate mesopelagic food webs has been poorly investigated, notably its trophic behaviour in areas characterized by peculiar oceanographic and ecological features, such as submarine canyons. In this study, we investigated the diel feeding pattern of C. braueri in the Polcevera submarine canyon, in the Ligurian Sea. Stratified sampling from the surface to 1300 m was undertaken with a BIONESS every 6 h during a 24 h period. Stomach contents of 244 C. braueri specimens from 11.6 to 27.5 mm SL were examined. Calanoid copepods were the dominant taxa in the fish stomachs (98% of all copepods), with Pleuromamma abdominalis, Neocalanus gracilis, Pleuromamma gracilis and Euchaeta acuta being the main preys. Among the 20 identified copepod prey species, about 20% were typical of the epipelagic zone, while the remaining 80% usually live in mesopelagic waters. The low number of prey per stomach (average = 1.4) and high vacuity index (53%) indicated that this species had very low metabolic requirements (trophic level in this study was 3.03). Most of the full stomachs were found during the morning and afternoon in the upper mesopelagic zone (200–400 m), indicating two main periods of feeding activity per day. This study confirmed high abundance and biomass of C. braueri in Polcevera canyon compared to those of open ocean waters and raised the question of how the presence of the canyon affects migratory and trophic behaviours of this mesopelagic fish. Further analyses aimed at investigating the link between the hydrodynamic regime of the Polcevera canyon and the behaviour and trophic ecology of C. braueri are needed to address this question.