Stock assessment of Namibian orange roughy populations using an age-structured production model and all available indices of abundance from 1994 to 2002, and making allowance for annually variable aggregation of the stocks

Brandão, Anabela; Butterworth, Doug S

Stock assessment of Namibian orange roughy populations using an age-structured production model and all available indices of abundance from 1994 to 2002, and making allowance for annually variable aggregation of the stocks

Working Paper

2003

Publisher

University of Cape Town

Department

Marine Resource Assessment and Management Group

Faculty

Faculty of Science

Abstract

Updated assessments of the four orange roughy aggregations off Namibia, based upon a maximum penalised likelihood approach which uses all available indices of abundance, are presented, and projections under constant catch levels reported. Earlier methodology is extended to reflect the proportion of a stock present at the fishing aggregation each year governed by a beta distribution. This new methodology is argued to provide the most reliable assessment of the resource. It suggests that Johnies, Frankies and Rix are all presently at some 60% of their preexploitation level, but that Hotspot is perhaps much more depleted. Overall, medium term sustainable yields would seem to be in the 2 500 – 3 500 ton range. However, variable aggregation levels from year to year would lead to difficulties in making a catch of this size every year.

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