DIEL VERTICAL MIGRATIONS OF MEGANYCTIPHANES-NORVEGICA IN THE KATTEGAT- COMPARISON OF NET CATCHES AND MEASUREMENTS WITH ACOUSTIC DOPPLER CURRENT PROFILERS

Diel vertical migration of a stable and well-defined population of Nor dic krill, Meganyctiphanes norvegica (Crustacea, Euphausiacea) was inv estigated during eight days in August 1989, in the Laso-Deep, East of the Danish island Laso. Net catches with a multi-net (MOCNESS) and mea surements with a moored and a shipboard Acoustic Doppler Current profi ler (ADCP) were compared; Backscattered energy as a measure for biomas s gave good correlations to the dry weight of M. norvegica and smaller zooplankton from net catches. Diel migratory patterns matched well, a s determined, parallel with both methods. Migratory vertical velocity was determined with ADCP at 2-3 cm sec(-1). The potential for the use of ADCPs for biological investigation is discussed. Vertical migration was dependent on environmental parameters. The krill did not cross a temperature barrier of 14 degrees C, although rich food sources were s ituated beyond it. Differences in salinity did not play a role. Curren ts were involved in plankton distribution. Light was an important Zeit geber (synchronizer) and determined the density of the krill aggregati ons. Feeding behaviour did not interfere with the light-induced migrat ory pattern of Nordic krill at the Laso-Deep.