Application of yield models to the New Zealand rock lobster fishery

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Abstract
Two general types of yield model, stock production and dynamic pool (yield‐per‐recruit), were used to analyse available data for the New Zealand rock lobster, Jasus edwardsii (Hutton). An exponential stock production model was applied to commercial catch and effort data from 1945 to 1975 for the North and South Islands combined. The results suggest a maximum sustainable yield of 4227 t with an estimated optimal level of fishing effort of 784 vessels, about 60% of the 1975 number. Empirical and von Bertalanffy growth equations were derived from limited tag‐recapture data from the Gisborne area. Both equations indicate that males grow faster than females. The empirical growth equation, based on moult increment and frequency as functions of size, is considered the more realistic. Two yield‐per‐recruit models provided different results, especially at low levels of natural and fishing mortality. One model (Beverton‐Holt) incorporated the von Bertalanffy growth equation and the other (empirical model) an empirical growth equation; the latter model is considered more realistic. The empirical model suggests that a reduction in fishing mortality below the estimated present values will not result in a substantial increase in yield‐per‐recruit. No strong evidence for changing the minimum size at first capture emerged from the results of the empirical model. A need is evident for more data, spanning a greater size range, to provide better estimates of growth and natural mortality, because the yield‐per‐recruit models were found to be sensitive to both.

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