Vertical migratory behaviour of the euphausiid, Meganyctiphanes norvegica, and its dispersion in the Kattegat Channel

Abstract

The euphausiid Meganyctiphanes norvegica (Northern Krill) is predominantly 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 Lsø which represent havens of marine conditions beneath the low salinity Baltic outflow. The vertical migratory behaviour of Meganyctiphanes results in it encountering considerable contrasts in physical conditions during its diurnal cycle. This behaviour and the resulting physical experience of the animals 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) and winter (9–10/3/97). In both summer and winter the krill tended to concentrate 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 different wind-induced and tidal current systems acted to advect the krill away from the basin during their upward phase at night. ADCP measurements showed that in summer the krill would be advected SSW at an average rate of 3 cm s-1, but that in winter they would be advected WSW at 3.2 cm s-1. Calculations show that the krill would be capable of swimming against such currents without increasing their standard metabolism and that their distribution in the vicinity of the basin seems to be determined more by biotic than by abiotic factors.