Research Document - 2002/128

Update assessment of Silvergray rockfish (Sebastes brevispinis).

By Stanley, R.D., and Olsen, N.

Abstract

This document summarizes a re-assessment of silvergray rockfish (Sebastes brevispinis) in British Columbia waters and provides yield recommendations for the 2003/2004-2006/2007 fishing years. The catch at age analysis of the previous 2000 assessment was updated with two additional years of catch and proportion at age data, and a 2001 survey biomass estimate for a portion of the Area 3CD stock off the west coast of Vancouver Island. The assessment model for the three more northerly stocks was tuned to the last six years of commercial CPUE, unlike the "un-tuned" analyses conducted previously. The principal objective of this review was to examine whether the previous work had underestimated the stocks. The bias was presumed to have arisen because of an "over-reliance" on a sparse and possibly biased array of age samples for the three northern stocks.

This review indicates terminal biomass estimates about 10% higher for the 5AB stock in Queen Charlotte Sound and 50% higher for the 5CD stock in Hecate Strait. No difference was observed for the 5E stock off the west coast of the Queen Charlotte Islands. Analysis of the 3CD stock tends to indicate a lower biomass owing to the 2001 low survey estimate from the U.S. triennial survey. Some modelling scenarios for this stock predicted a terminal biomass for 2001 of less than half of what was reported in the earlier assessment.

The large uncertainty in the analyses effectively precludes the opportunity of detecting a statistically different stock status. The changes in estimates of current biomass result more from a re-scaling of the biomass trends rather than changes over the last two years. Nevertheless, the document recommends updating the management quotas based on the results from this review and provides a summary of harvest options.

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