Distribution of Themisto (Amphipoda) spp in the Barents Sea and predator-prey interactions

Themisio abyssorum and Themisto libellula were the dominant amphipod specie s observed in the central and the northern Barents Sea during 1984-1996. T abyssorum was predominant in the subarctic waters, T. libellula in the Arct ic waters. A third species, Themisto compressa, was rare and was restricted to the Atlantic waters. Our study showed peak abundances of T. abyssorum a nd T libellula in summer and in early autumn. High abundances were usually associated with Polar Front waters. 7: libellula has a more near surface di stribution than T abyssorum. Followed by a decrease in the capelin stock fr om 1985-1987 there was an increase in the abundance of Themisto abyssorum a nd T libellula, probably due to the reduced grazing pressure from capelin ( Mallotus villosus). In the mid-1980s and 1990s when the capelin stuck was a t extremely low levels, cod (Gadus morhua) switched from capelin to alterna tive prey such as amphipods and krill. Detailed analysis of amphipods in th e cod stomachs from 1984-1999, showed that cud fed mainly on Themisto spp.. especially on 7. libellula. Themisto species were consumed by most age gro ups of cod. With an increase in the capelin stock from 1987-1991, a corresp onding decrease in the abundance of Themisto abyssorum and T. libellula was observed. During 1993 1996 when the capelin stock again was at low levels, the abundance of these two amphipod species increased. The increase in abu ndance was less pronounced in the mid-1990s for T. libellula probably owing to higher grazing pressure from cod and other predators such as harp seal (Phoca groenlandica) and arctic sea birds. The stock size of cod in the mid -1990s was about twice the amount as in the mid-1980s. These results illust rate strong predator-prey interactions between macrozooplankton species as amphipods and capelin and cod in the Barents Sea. The amphipod populations in the Barents Sea appear to be to a large extent controlled by predation.