Research Document - 2009/013

Stock assessment for Canary rockfish (Sebastes pinniger) in British Columbia waters

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

Abstract

The status of the B.C. population of canary rockfish (Sebastes pinniger) is assessed as one coastwide stock. This analysis uses a catch at age model tuned to five fishery-independent surveys, age composition data from the commercial fishery, and estimates of catch from 1940. The model was started from an equilibrium state in 1940 while the available fishery-independent survey data span a period from 1967 to 2007, although not all intervening years are represented. There is one age sample from 1978 while the remaining samples cover 1990 to 2004.

The stock assessment specifically investigated the following factors: 1) the effect of including the proportion-at-age data from the commercial fishery; 2) the impact of deterministic or stochastic recruitment; 3) the impact of estimating or fixing the commercial selectivity; and 4) the impact of steepness of values 0.55 and 0.70. Six model runs covered the above uncertainty options. A Bayesian approach, based on the Markov Chain Monte Carlo (MCMC) algorithm, was used to estimate the joint posterior distributions of model parameters and to make projections for five years from 2009 to 2013 across a range of fixed catch options.

The results were consistent in indicating that the stock has declined from its original biomass levels to between 15% and 35% of B0.  It is likely that this decline has been arrested and it is even possible that the stock is presently rebuilding at recent harvest levels, which have averaged about 875 t since 1997.  What is not certain from this assessment is whether current catch levels will ensure a rebuild. Some of the runs investigated in this assessment suggest that current removals will allow a slow rebuild. The runs with lower steepness or which do not estimate the commercial selectivity suggest that this is not the case. Taken collectively, the results of this analysis indicate that to be reasonably confident of rebuilding, harvests should be lowered from current levels.  The size of the reduction is dependent on the choices of 1) a target biomass, 2) a recovery timeframe, and 3) a desired certainty of reaching the biomass in the specified timeframe. It is also affected by the assumptions related to stock productivity. Decision tables are provided to assist managers in selecting the optimal harvest option for each combination of objectives under two assumptions of stock productivity.

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