Abundant mesopelagic fauna at oceanic high latitudes

The presence of a dense layer of organisms in the mesopelagic zone is a ubiquitous feature of the world oceans, and these organisms may constitute a major component of marine bio-mass worldwide. Many mesopelagic organisms perform light-dependent diel vertical migration. It has been hypothesised that extreme light regimes encountered at high latitudes may disturb these migration patterns and thereby limit the northern expansion of mesopelagic fauna into the Arctic. Using hydroacoustic data collected during 4 surveys conducted in the open Norwegian Sea during the summer season, we evaluated if the key features of mesopelagic fauna reported worldwide (high density and diel vertical migration) are also observed in the high latitudes of the Northeast Atlantic. The results confirm that the high-latitude Northeast Atlantic hosts a high density of mesopelagic fauna which performs daily migration patterns similar to those reported in other regions. They also support the limiting effect of photoperiod on its potential biomass. These results stress the need for thorough studies on the abundance, biodiversity and trophic ecology of the mesopelagic fauna in this region.