Vertical density distributions of fish: a balance between environmental and physiological limitation

Abstract

Data (trawl, acoustic, CTD) from scientific surveys along the Norwegian coast, in the North Sea, in the Barents Sea, west of the British Isles, and in the Irminger Sea are used. The vertical density distributions of blue whiting, cod, haddock, redfish, saithe, capelin, and herring are described in relation to environmental conditions and physiological limitations. The first four surveys mainly cover banks and shelf areas shallower than 500 m. The last two surveys, aimed at blue whiting and redfish, mainly cover shelf edge and deep-sea areas with depths from 200 to 1300 m and from 440 to 3000 m. In regard to cod some information from data-storage tags is used. For physoclists the relative vertical profile of each acoustic sample i.e. acoustic-area backscattering coefficient (sA), is expressed in terms of the relative pressure reduction level from seabed up to surface. Thus relative vertical profiles with different bottom depths are normalized and are made compatible for a discussion in terms of the free vertical range (FVR). This restriction to rapid vertical movement is evident in the physoclist species studied. For samples in the shelf area, the profiles show that blue whiting, haddock, saithe, cod, and redfish are mainly distributed within the bottom half of the water column. Some fish adapt to pelagic living especially in areas with high acoustic density and where the bottom is deep. Here a pelagically living fish is defined as an individual fish having a current free vertical range that does not include the seabed. For demersal fish, day and night relative vertical profiles are corrected for unequal day and night losses in the bottom acoustic dead zone, which is the zone near the seabed where echoes from fish cannot be discriminated from the sea bottom echo. Day and night samples are separated by the sun’s passing 5 below the horizon. In most years evidence of diurnal vertical migration is found for all investigated species. In many cases of demersal fish there is a higher relative acoustic density (sA-values) in the mid-range of the bottom half of the water column in the daytime as opposed to the night-time. At night there is a degree of separation, one group of fish descends to aggregate near the seabed and another ascends. Inter-annual variations in the diel movement from different parts of the stock are discussed in relation to the inter-annual variations in age composition of the stock. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.