IDENTIFYING PATTERNS IN DIET OF MACKEREL ICEFISH (CHAMPSOCEPHALUS GUNNARI) AT SOUTH GEORGIA USING BOOTSTRAPPED CONFIDENCE INTERVALS OF A DIETARY INDEX

Ontogenetic, inter-annual and regional variations in diet were investigated in mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomachs from 2239 icefish (130-560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance (% IRIDC) of prey categories. Diet varied significantly between years and age classes but there was little regional difference in diet. In general diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003-2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length-weight relationships) were significantly lower than in the other summer seasons. The poor krill season was followed by a large reduction (> 80%) in the estimated annual biomass of C. gunnari the following year (2005). This may have been a result of mortality of age 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.