Large scale distribution of acoustical scattering layers at the Norwegian continental shelf and the eastern Norwegian Sea

Vertical and horizontal distributions of fish and micronekton were mapped in a zigzag pattern from Lofoten, northern Norway to Stadt, southwestern Norway, in March 1992. Pearlsides (Maurolicus muelleri) were present whenever depth was sufficient for daytime requirements (i.e. > 150-200 m). They performed diel vertical migrations, characterized by ascent at dusk, followed by nocturnal descent, and a dawn ascent before downward migration to day depth. Nocturnal descent among pearlside has previously been explained by preference for warm water to speed up digestion, but this could not explain the results in This study as relevant vertical temperature gradients were minor. A deep scattering layer consisting of mesopelagic fish, pelagic shrimps and other macroplankton was present outside the shelf. This layer was located beneath ca 300 m by day, while the tendency to carry out diel vertical migrations varied strongly between water masses. Blue whiting (Micromesistius poutassou) associated with the layer by day consistently swam into upper waters by night. Dense concentrations of blue whiting at the shelf break, however, remained near the bottom both day and night. Herring (Clupea harengus) were confilled to the shelf, staying near the bottom by day and ascending into upper waters by night. Norway pour (Trisopterus esmarkii) lived near the bottom by day, while degree of diel vertical migration appeared to differ among locations.