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 c. 8,700 stomachs collected in January/February 1985, January/February 1991 and January 1992. Main prey items were krill (Euphausia superba), the hyperiid Themisto gaudichaudii, mysids (primarily Antarctomysis maxima) and in 1985 also Thysanoessa species. The proportion of krill and Themisto in the diet varied considerably between the three seasons, whereas the proportion of mysids in the diet remained fairly constant. Krill which has a high energy content appears to be the prefered diet. In years of krill shortage, such as in 1991, krill is replace d by the ubiquitous Themisto gaudichaudii. The occurrence of krill in the diet in 1991 was one of the lowest within a 28 year period of investigation. Variation in food composition between sampling sites was high. This high variation can be primarily attributed to differences in prey availability, but was much less influenced by prey size selectivity due to differences in length composition of fish between sampling sites. Feeding intensity varied considerably between seasons. Both, the proportion of empty stomachs was uncommonly high and stomach content weight was uncommonly low in 1991 in a period when energy-rich food is needed for the final maturation of the gonads. It was highest in 1992. At the same time, an unusually high proportion of sexually mature fish showed no signs of the gonad development necessary leading up to spawning in that season. It was hypothesized that like in other, non-Antarctic fish species the shortage of suitable food, such as krill, may have forced the fish to sacrifice gonad maturation in order to maintain body size.