WINTER DISTRIBUTION AND MIGRATION OF THE SOUND-SCATTERING LAYERS, ZOOPLANKTON AND MICRONEKTON IN MASFJORDEN, WESTERN NORWAY

Sound scattering layers (SSLs) and vertical distribution of zooplankton and micronekton in Masfjorden, western Norway, were studied in January 1988. Two characteristic SSLs were found in the deepest fjord basin (bottom depth 490 m) while single scattering layers were observed in shallower basins (depths <300 m). The SSLs performed diurnal vertical displacements, exhibiting migrational speeds of up to 90 m h-1. The adjustment of the SSLs' day depth was directly proportional to the rate of change in the logarithm of surface light and inversely proportional to the extinction coefficient of the water. Large horizontal differences in biomass in the fjord are related to a combination of advection and changes in migrational behaviour. Feeding activity of the micronekton was higher during light hours. Euphausiids of the deepest basin were mainly feeding on detritus while those in the innermost part of the fjord seemed to be preying upon copepods. The observed vertical distribution of the zooplankton and micronekton were examined in relation to mode of feeding (visual vs tactile), trophic level (omnivory vs carnivory), size (small vs large), and food type and availability.