Variations in the diet composition and feeding intensity of mackerel icefish Champsocephalus gunnari at South Georgia (Antarctic)

The diet composition and feeding intensity of mackerel icefish Champsocephalus gunnari around Shag Rocks and the mainland of South Georgia was analyzed from ca 8700 stomachs collected in January/February 1985, January/February 1991 and January 1992. Main prey items were krill Euphausia superba, the amphipod hyperiid Themisto gaudichaudii, mysids (primarily Antarctomysis maxima), and in 1985 also Thysanoessa species. The proportion of krill and T gaudichaudii in the diet varied considerably among the 3 years, whereas the proportion of mysids in the diet remained fairly constant, Krill appears to be the preferred food. In years of krill shortage, such as in 1991, krill was replaced by T gaudichaudii. The occurrence of krill in the diet in 1991 was among the lowest within a 28 yr period of investigation. Variation in food composition among sampling sites was high. This high variation appears to be primarily associated with differences in prey availability, but much less with prey size selectivity. Feeding intensity varied considerably among seasons. It was highest in 1992. The proportion of empty stomachs was uncommonly high and stomach content weight was uncommonly low in January 1991, a period when energy-rich food was needed for the final maturation of gonads. At the same time, an unusually high proportion of sexually mature fish showed no signs of the gonad development necessary for spawning in that season. It was hypothesized that as in some other non-Antarctic fish species the shortage of suitable food may have forced the fish to sacrifice gonad maturation in order to maintain body size.

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