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][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][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.

Feeding habits of harp and hooded seals in drift ice waters along the east coast of Greenland in summer and winter

“…Previous investigations in the Greenland Sea during summer revealed that this amphipod species completely dominated the plankton communities in the upper Arctic water layers (Dalpadado et al 1998). Similar observations have been made both in the north-west Atlantic (Percy 1993) and in the Barents Sea, where abundance may reach a peak in early autumn (Dalpadado et al 2001;Dalpadado 2002). P. libellula is assumed to be an important link in the Arctic marine food chain between herbivorous zooplankton and fi sh, birds and mammals (Dunbar 1957;Bradstreet & Cross 1982;Mehlum & Gabrielsen 1993;Dalpadado et al 2001).…”

“…Except for the youngest stages, which may occur in the upper water layers during summer, the major hooded seal prey G. fabricii has a typical mesopelagic distribution with occurrence mainly at depths greater than 400 m (Bjørke 2001). This is in contrast to the distribution of the major food of harp seals: the observed krill and amphipod species are usually confi ned to the more upper water layers (< 200 m depth; Dalpadado et al 1998;Dalpadado et al 2001).…”

“…the same items as observed in this study. Pelagic amphipods of the genus Parathemisto include several species, but P. libellula is the only species that is confi ned exclusively to Arctic waters (Dalpadado et al 2001;Dalpadado 2002). Previous investigations in the Greenland Sea during summer revealed that this amphipod species completely dominated the plankton communities in the upper Arctic water layers (Dalpadado et al 1998).…”

“…P. libellula is assumed to be an important link in the Arctic marine food chain between herbivorous zooplankton and fi sh, birds and mammals (Dunbar 1957;Bradstreet & Cross 1982;Mehlum & Gabrielsen 1993;Dalpadado et al 2001). In the Arctic parts of the Barents Sea, Dalpadado et al (2001) suggest strong predatorprey interactions between macrozooplankton species such as P. libellula and predators such as capelin, cod and harp seals, and that the amphipod population appears to be largely controlled by predation.…”

Macrozooplankton community patterns driven by water circulation in the St. Lawrence marine system, Canada

“…At the same time, in the NW GSL, the arctic (Thysanoessa raschii, Themisto libellula, and Aglantha digitalis) and boreo-arctic (Sagitta elegans and T. abyssorum) (Richard & Haedrich 1991, Percy 1993, Dalpadado et al 1998, 2001) macrozooplankton species were 5 times more abundant in 1998 than in 2000 and 2001. In contrast, the abundance of the more temperate species Meganyctiphanes norvegica (Dalpadado et al 1998(Dalpadado et al ), did not vary significantly between 1998(Dalpadado et al , 2000(Dalpadado et al , and 2001.…”