Spatial density distributions of fish, a balance between environmental and physiological limitation

Data (trawl, acoustic, CTD) from four scientific survey series , i.e. along the Norwegian coast, in the North Sea, and in the Barents Sea summer and winter, are used to describe and discuss the spatial density distributions of blue whiting, cod, haddock, redfish, saithe, capelin, and herring, in banks and shelf sea of depth mainly < 500 m, in relation to environmental conditions (depth, change of pressure, temperature, salinity, daylight, and physiological limitations). For cod, also information from data storage tags (DST) is used. For the demersal physoclists the relative vertical profiles are defined in terms of relative pressure reduction with reference to the pressure at the bottom. Thus vertical profiles with different bottom depth can be expressed in terms of free vertical range (FVR) and compared on a physiological basis. This restriction to rapid vertical movement is evident in the studied physoclist species. The acoustic sA-values show that blue whiting, haddock, saithe, cod, and redfish are mainly distributed within the bottom half of the water column, but also that they adapt to pelagic living. Haddock and blue whiting are more often found to distribute higher into the water column than saithe, cod, and redfish. Pelagic living is seen especially in areas with high acoustic sA-values and where the bottom is deeper than 200m. Day and night vertical profiles in terms of FVR are corrected for unequal day and night losses in the bottom acoustic dead zone. In most years, evidence of diurnal vertical migration for all seven species are found when day and night are clearly distinguishable. In many cases of demersal fish there is higher relative acoustic density in the mid-range of the bottom half of the water column at daytime than nighttime. At nighttime there is a degree of separation, one group of fish descends to aggregate near the seabed and another ascends