Mackerel icefish Champsocephalus gunnari in the diet of upper trophic level predators at South Georgia: implications for fisheries management

Mackerel Icefish Champsocephalus gunnari Lönnberg is an important component of the ecosystem at South Georgia where its diet is dominated by Antarctic Krill Euphausia superba, it is an important prey species for a number of upper trophic level predators, and is the target of commercial fisheries. Data on the frequency and size structure of C. gunnari in the diet of Antarctic fur seals Arctocephalus gazella and gentoo penguins Pygoscelis papua at Bird Island, South Georgia from 1991-2002 were used to examine the trophic interactions and evaluate the potential impact of predators on the C. gunnari population. C. gunnari occurred in 20 – 40 % of scats from Antarctic fur seals and comprised 48% of the diet by mass of gentoo penguins. The length-frequency distributions of C. gunnari indicated a dominant mode at 180-200 mm total length (1+ age class), with strong cohorts entering the population in 1993, 1996, 1999 and 2001. A recruitment index based on the contribution by mass of the 1+ age-class in the diet of gentoo penguins indicated a higher degree of variability in recruitment than in considered previously. Estimates of consumption of C. gunnari by Antarctic fur seals and gentoo penguins (c. 138 000 tonnes per annum) exceed standing stock estimates (17 –67 000 tonnes). A deterministic population model of cg from 1991 – 2002, using a variable mortality rate scaled by the inverse of krill abundance, replicated the fluctuations in C. gunnari shown by trawl surveys. Changes in the South Georgia ecosystem over the past 2 decades may have increased the level of predator consumption of C. gunnari providing a potential ecosystem based explanation for the lack of a recovery of pre-exploitation population size