Evaluation of a management decision rule for a New Zealand rock lobster substock

Abstract

The performance of a proposed management ‘decision rule’–an algorithm that specifies management actions when specified criteria are met–was evaluated by exploring the impact of different choices for three decision-rule parameters on the ability of the rule to achieve management objectives. To do this, forward simulations from an age-structured assessment model for the substock of red rock lobster (Jasus edwardsii) off southern New Zealand were used. The size of this substock is currently estimated to be about one-third of B_MSY (the level of vulnerable biomass that would produce maximum sustainable yield), and the management goal is to rebuild it to B_MSY. A target rebuilding trajectory to B_MSY was generated by allowing the model population to rebuild under the current catch regime with constant recruitment. The decision rule is based on comparison of observed catch per unit effort (CPUE) to predicted CPUE.

Process and observation errors, each with a coefficient of variation of 20%, were introduced, and five values were used for each of the three parameters of the rule. The decision rule was effective in removing all instances of failure to rebuild and all excessively slow rebuilding trajectories. The decision rule was also applied to an arbitrarily depressed starting substock size, and the conclusions were the same.