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    The utility of surface plots in the development of the CCAMLR Decision Rule, its interpretation, and the rationalisation of current management and fishery metrics

    Request Meeting Document
    Document Number:
    C. Darby and T. Earl
    Submitted By:
    Ms Georgia Robson (United Kingdom)
    Approved By:
    Dr Chris Darby

    The CCAMLR Scientific Committee has noted that there is a need to consider developments to the CCAMLR Decision Rule, including the potential utility of target and limit exploitation rate objectives to increase its robustness. WG-SAM has been tasked with this in its future workplan. 

    The CCAMLR Decision Rule as applied to toothfish requires that catches are set, over the long-term, such that, on average, the spawning stock biomass realises half of its unfished state over a 35-year projection period. In addition, the risk of the biomass being reduced to below one fifth of its unfished state, should be low. These constraints take into account the biological parameters of the stock, such as growth rates, maturity, and expected recruitment, as well as the expected selectivity of the fishing fleet.

    In this paper we consider graphical approaches, Beverton and Holt equilibrium yield and spawning biomass surfaces and Kobe plots, used in many fisheries fora. We show how exploration of CCAMLR Decision Rule modifications, applying alternative objectives, such as target, and limit exploitation rates could be presented to managers. Metrics from the longline fishery in Subarea 48.3 are used to demonstrate how the two graphical approaches allow the current and historic stock status to be illustrated against several management and fishery metrics simultaneously, providing a simplistic and effective reporting tool for managers. The examples highlight the trade-offs between fishing pressure, selectivity and stock status that are inherent in the dynamics of the fishery but are also adapted for within the application of the CCAMLR Decision Rule.

    We recommend that graphical summaries are used to simplify the information provided for managers to highlight why a range of harvesting strategies can each achieve the current targets and limits; for example, applying a low fishing effort across a wide range of ages, or a higher fishing effort across a narrower range of ages.