Research Document - 2000/173

Silvergray rockfish (Sebastes brevispinis) assessment for 2000 and recommended yield options for 2001/2002

By Stanley, R.D., Kronlund, A.R.

Abstract

This document summarises the available information on the stock status of silvergray rockfish (Sebastes brevispinis) in British Columbia waters and provides yield recommendations for the 2001/2002 fishing year. It also summarises biological and historical fishery information so that future researchers will be able to use this document as the starting point for their assessment work on silvergray rockfish.

The available biological data are analyzed to provide recommended harvests. We provide harvest recommendations for silvergray rockfish based on three alternative harvest strategies (F=0.5*M, F=0.75*M and F=1.0*M). These estimates are not directly comparable to the previous low-risk and high- risk yield options. The F=0.75*M and F=1.0*M estimates are attempts to estimate a midpoint harvest option as opposed to a "bracketing" of possible harvests.

Our best estimate of M is 0.06. Data for each of four stocks, corresponding to PMFC Areas 3CD, 5AB, 5CD and 5E, were examined using catch-at-age analysis. Each stock analysis examined three general cases, where Case 1 examined the impacts of tuning with commercial CPUE and/or survey estimates; Case 2 fitted proportion-at-age data only, with variable recruitment and; Case 3, which was similar to Case 2, but forced recruitment to be constant to mimic simple catch curve analysis.

For Area 3CD, quota recommendations are based on the model tuned to a U. S. triennial survey which surveyed part of the area. The three recommended harvest levels were 152, 228, and 296 t corresponding to three target levels of F. Previous documents provided a "low-risk" and "high-risk" recommendation of 150-425 t. Quota recommendations for the remaining stocks 5AB, 5CD and 5E, are based on Case 2 model runs which fits ageing data, allows variable recruitment but lack a survey tuning index. The recommended options for 5AB were 214, 319 and 422 t, as compared with the previous range of 350-700 t. For 5CD, the recommended options were 146, 217, and 288 t as compared with a previous range of 125-400 t and for 5E the recommended options were 137, 204, and 270 t as compared with the previous range of 175-300 t.

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