Towards the development of a management plan for the mackerel icefish (Champsocephalus gunnari) in Subarea 48.3

Management of the mackerel icefish (Champsocephalus gunnari) at South Georgia is complicated by the likelihood of substantial periodic variations in natural mortality rates. These may be associated with increased consumption of C. gunnari by Antarctic fur seals in years of poor krill availability. Thus natural ortality of C. gunnari may, in some years, increase by a large factor (assumed here to be 4), declining to normal levels again when krill return. This paper outlines a scheme which would use information from studies on krill and predators undertaken as part of the CCAMLR Ecosystem Monitoring Program (CEMP) to interpret or modify information from commercial fisheries and research surveys leading to estimates of stock biomass. An extension of this scheme would use predictions of coming periods of krill scarcity as early warnings of increased natural mortality of C. gunnari. Full implementation of such a scheme would require greater knowledge of quantitative aspects of food web dynamics within the South Georgia ecosystem than we possess at present. There is therefore a need for an interim approach to the setting of precautionary catch limits for this fishery. An approach based on the CCAMLR generalised yield model (GYM), with periodically varying natural mortality, provides a realistic description of perceived icefish dynamics. However, the model generates a significant probability that C. gunnari populations will be depleted even in the absence of fishing, which, given the existing CCAMLR decision rules, would preclude the fishery's ever opening. Several possible modifications to the decision rules are discussed. It is concluded that while the GYM can be used to estimate a temporary, conservative long-term yield, a new approach and set of decision rules will ultimately be required for C. gunnari.

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