Inter-specific variations in distribution, abundance and possible life-cycle patterns of Themisto spp.: (Amphipoda) in the Barents Sea

Amphipods from the central and northern areas of the Barents Sea were studied by a series of MOCNESS profiles between 1990 and 1997. Themisto abyssorum, though dominant in warmer Atlantic water, was also present in Arctic water. In contrast, T. libellula was a typical Arctic species and penetrated very little into the Atlantic water masses. T. compressa was seldom found in the studied area. Linear regression analysis showed a statistically significant negative relationship between abundance of T. libellula and the variability in the Atlantic inflow. The abundance of this species seems to be, to a large extent, determined by the amount of Arctic water in the Barents Sea. The sub-Arctic species, T. abyssorum, only has a 1-year life-cycle, with the peak in release of young (2-3 mm) occurring in May and June. A few individuals may survive to be older. The Arctic species, T. libellula, seems to release the young earlier and the length-frequency distributions seem to indicate a 2-year life-span. The spring phytoplankton, which usually blooms during April in the Barents Sea, followed by high abundance of Calanus spp. and krill species, are regarded as important factors that influence the release of the amphipods' young.