Fish communities on the slope of the eastern Norwegian Sea

Bottom trawl data from the depth interval 300-2050 m on the slope off Norway were analysed to study distribution of fish biomass and the associated species assemblages. It is hypothesised that the pronounced hydrographic front between the warm Atlantic Watermass and the cold Norwegian Sea Deepwater, combined with a declining food supply with depth and distance from the shelf, structure the assemblages on the continental slope of the eastern Norwegian Sea. From a high level at the shelf edge and upper slope, the fish biomass declined abruptly at about 720 m when crossing the 0 degrees C-isotherm associated with the hydrographic front. Beneath the frontal zone, the fish biomass was only 11% of that above it and decreased further with increasing depth. In upper slope waters, the fish biomass increased with increasing latitude. Arctic species such as Lycodes frigidus and Paraliparis bathybius were dominant in the deep cold slope waters. The high fish biomass associated with the steep temperature gradient at 600-720 m was mainly attributable to a few large and/or abundant boreo-arctic species, i.e. Greenland halibut Reinhardtius hippoglossoides, redfishes Sebastes mentella and S. marinus, roughhead grenadier Macrourus berglax, and Raja hyperborea. In the shallower and warmer upper slope waters a species assemblage occurred which resembled that associated with adjacent deep shelf areas strongly influenced by the Atlantic Watermass.