Age-structured production model assessments and projections including updated parameters to model the intermittent aggregation of Namibian orange roughy

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 and reflects the proportion of a stock present at the fishing aggregation each year, are presented, and projections under constant catch levels reported. Further abundance data now available for Frankies allow the parameters of the beta distribution used to reflect the proportion aggregating to spawn each year to be updated. This leads to more optimistic results for stock status, slightly so for Frankies and Rix for which estimates of the ratio of current to preexploration levels increase from the 60%’s to the 70%’s, but substantially so for Johnies which is no longer estimated to be heavily depleted. With no new standardised CPUE values available, Hotspot remains estimated as highly depleted. The range of variable aggregation levels estimated are not qualitatively incompatible with information for Australian orange roughy aggregations. Changing from the earlier to the updated beta distribution parameters increases medium term annual sustainable yield estimates for the resource as a whole from 1 850 to 4 400 tons, though the latter figure needs to be considered with caution as it is particularly dependent on the assumption that the beta distribution estimated for proportions aggregating at Frankies applies also to Johnies. Broadly speaking, MSY estimates at 1 600 to 3 500 tons are some 5–60% more than estimated a year previously. However, the varying proportions of abundance present at aggregations from year to year would lead to difficulties in making a catch of this size every year.