VERTICAL MIGRATORY BEHAVIOR OF THE EUPHAUSIID, MEGANYCTIPHANES-NORVEGICA, AND ITS DISPERSION IN THE KATTEGAT CHANNEL

The euphausiid Meganyctiphanes norvegica (Northern Krill) is predomina ntly an oceanic species common to the North Atlantic and adjacent seas . In the Kattegat the species concentrates in a series of depressions in the Kattegat Channel east of the island of Laser which represent ha vens of marine conditions beneath the low salinity Baltic outflow. The vertical migratory behaviour of Meganyctiphanes results in it encount ering considerable contrasts in physical conditions during its diurnal cycle. This behaviour and the resulting physical experience of the an imals were investigated by means of a combination of net sampling and ADCP transects across the Alkor Deep (131 m) in summer (18-19/7/96) an d winter (9-10/3/97). In both summer and winter the krill tended to co ncentrate within the basin during daytime and to disperse in the upper layers at night. The period of dispersion was longer in winter (17.00 -05.00 h) than in summer (23.00-02.00 h). The complex layering of diff erent wind-induced and tidal current systems acted to advect the krill away from the basin during their upward phase at night. ADCP measurem ents showed that in summer the krill would be advected SSW at an avera ge rate of 3 cm s(-1), but that in winter they would be advected WSW a t 3.2 cm s(-1). Calculations show that the krill would be capable of s wimming against such currents without increasing their standard metabo lism and that their distribution in the vicinity of the basin seems to be determined more by biotic than by abiotic factors.