Variation in standing stock of the mackerel icefish Champsocephalus gunnari at South Georgia

1. Bottom trawl surveys on the South Georgia shelf indicate episodic declin es in the abundance of the mackerel icefish Champsocephalus gunnari that, s ince 1990, are not directly attributable to commercial fishing. 2. The greatest effect has been observed in years when krill are known to h ave been scarce on the South Georgia shelf. 3. It is thought that C. gunnar i survivorship is closely related to, but indirectly influenced by, krill a vailability. 4. Examination of food chain relationships indicates that predation by fur seals is likely to play a major role in the observed changes in C, gunnari abundance. 5. Periodic shifts in food chain relationships, involving krill, C. gunnari and fur seals, are postulated that could explain the episodic declines in the abundance of C, gunnari in the absence of fishing. 6. The study demonstrates how a key predator can have an over-riding impact on a commercial fish species, highlighting the importance of the ecosystem approach to fisheries management.