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

Abstract

Themisto abyssorum and Themisto libellula were the dominant amphipod species observed in the central and the northern Barents Sea during 1984-1996. T. abyssorum was predominant in the subarctic waters, T. libellula in the Arctic waters. A third species, Themisto compressa, was rare and was restricted to the Atlantic waters. Our study showed peak abundances of T. abyssorum and T. libellula in summer and in early autumn. High abundances were usually associated with Polar Front waters. T. libellula has a more near surface distribution than T. abyssorum. Followed by a decrease in the capelin stock from 1985-1987 there was an increase in the abundance of Themisto abyssorum and T. libellula, probably due to the reduced grazing pressure from capelin (Mallotus villosus). In the mid-1980s and 1990s when the capelin stock was at extremely low levels, cod (Gadus morhua) switched from capelin to alternative prey such as amphipods and krill. Detailed analysis of amphipods in the cod stomachs from 1984-1999, showed that cod fed mainly on Themisto spp., especially on T. libellula. Themisto species were consumed by most age groups of cod. With an increase in the capelin stock from 1987-1991, a corresponding 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 abundance 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 illustrate 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.