An updated assessment of the South African kingklip resource including some sensitivity tests
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2008
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University of Cape Town
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Abstract
The deterministic Age-Structured Production Model (ASPM) assessment of the kingklip resource by Mori and Butterworth (2002) is updated to take account of further catch and abundance survey data that have since become available for the years 2000 to 2007. The assessment is conducted for the West coast and the South coast separately, and compared to a retrospective model fit up to 1999. The model is fit to two different sources of data: the trawl abundance surveys, and the CPUE series for both trawl and longline catches. When steepness h is set to 0.5, the spawning biomass of the West coast component shows a recovery since 1992; however, that of the South coast component shows a continuing decrease since 1995. The spawning biomass relative to its unexploited equilibrium level (Bsp/Ksp) for the South coast is 0.12 for the base case scenario (h = 0.5). This is a consequence of the continued high catches by the trawl fishery over the last decade on the South coast, though these have declined more recently. These analyses suggest that, if the South coast component of the resource is an isolated stock, then annual South coast catches by the trawl fishery would need to be reduced to below 1 000 tons at least for sustainability. Several sensitivity tests were conducted for the South coast base case scenario, however there is no substantial impact on the estimated status of this component of the kingklip resource. The resource is assessed as a single stock through the crude approach of summarising survey estimates for the two coasts; this unexpectedly leads to estimated status which is better than that for either coast considered separately. This result suggests the need for further investigations using both more elaborate modelling assumptions and further data, and some specific suggestions are made in this regard.
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Brandao, A., & Butterworth, D. S. (2008). An updated assessment of the South African Kingklip resource including some sensitivity tests. Marine and Coastal Management document. WG-Dem: K, 2.