2012 South coast rock lobster operating models-some initial results

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University of Cape Town

Initial updated assessments are developed for the South Coast rock lobster resource incorporating the recently revised estimates of somatic growth rate by area developed by OLRAC, and fitting to area-specific CPUE and scientific catch-at-length data. The initial focus is on a model structure which splits the resource into areas A1E, A1W and A2+3. A problem arises because the slow growth rate estimate for A1E results in an unrealistically high estimate of preexploitation biomass, and some approaches to circumvent this are explored. The pack-category-based catch-at-length data are not consistent with these results and it is suggested that they be excluded from future model fits. Allowing selectivity at length to vary over time makes little difference to stock trajectory estimates, but scarcely improves non-random patterns in the catchat-length residuals, so that the investigation of alternative functional forms for this variation is suggested. An alternative model structure based on areas A1, A2 and A3 yields an estimate of the current depletion of the spawning biomass which is appreciably less than in previous assessments which were based on this structure; the reasons for this need to be identified.