Large variations in natural mortality rate of marine fish and its effect on multispecies management; the mackerel icefish (Champsocephalus gunnari) a practical example

1. Bottom trawl surveys on the South Georgia shelf indicate episodic declines in the abundance of the mackerel icefish Champsocephalus gunnari that, since 1990, are not directly attributable to commercial fishing.

2. The greatest effect has been observed in years when krill are known to have been scarce on the South Georgia shelf. 3. It is thought that C. gunnari survivorship is closely related to, but indirectly influenced by, krill availability.

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.