Research Document - 2011/129

Stock Assessment for the inside population of yelloweye rockfish (Sebastes ruberrimus) in British Columbia, Canada for 2010

By K.L. Yamanaka, M.K. McAllister, P.F. Olesiuk, M.-P. Etienne, S.G. Obradovich, and R. Haigh

Abstract

A new stock assessment is presented for the inside population of yelloweye rockfish(Sebastes ruberrimus) in British Columbia, Canada for 2010. A Bayesian state space surplus production (BSP) model is used due to the lack of sufficient age data for a fully age structured stock assessment. The model presented requires a time series of historical annual catch biomass from all fisheries (reconstructed from 1918 to 2009) and is fitted to stock trend data derived from commercial catch and effort data as well as two research survey catch indices. Prior probability distributions are required for key parameters including the maximum intrinsic rate of increase, r, initial stock size relative to unfished stock size, carrying capacity and constants of proportionality for the abundance indices. A reference case assessment model based solely on mortality from fisheries selects the most credible set of model inputs and projects results which are evaluated for a variety of model settings.

The BSP model fits the stock trend data fairly well and predicts that the stock declined in the 1980s when fishery catches increased substantially. Management advice is based on output from a reference case BSP model run which estimates that the inside yelloweye rockfish biomass in 2009 is 780 tonnes (CV 0.46) which is 12% (CV 0.43) of the initial biomass in 1918.  This is below the fisheries Limit Reference Point (LRP), 0.4 B_MSY, consistent with the Canadian Precautionary Approach and there is a 90% probability that the stock is within the Critical Zone (DFO 2006). The fishing mortality rate in 2009 was above F_MSY and the total catch (15 t) is estimated to be 78% (CV 0.66) of the replacement yield.

The model predicts the stock biomass to increase over time under harvest policies similar to 2009. Projecting the stock biomass over a five year time horizon, there is low probability (<14%) that the stock biomass will exceed 0.4 B_MSY regardless of the harvest policy. With a total fishing mortality (TAC) of 15 t, over a 40 and 80 year time horizon the probability that the stock biomass exceeds 0.4 B_MSY ranges from 44% to 56%, respectively.

Since the early 1990’s, abundance indices have continued to decline despite dramatic reductions in fishing mortality.  This disparity may indicate the possibility of influences other than fishing on the population that cannot be accounted for in the reference case BSP model. Other possible explanations, outside of fishing effort, could be a lag in population response to the reduction in fishing effort and/or recruitment declines or failure. Another potential influence is investigated, for illustrative purposes only. This novel approach incorporates predation by pinnipeds and sets a framework for possible future analyses but, given current uncertainties in pinniped consumption, is not considered here for management advice. Should future stock biomass continue to decline, under current low fishery catches, other models should be explored to examine factors other than the fishery that could influence the inside yelloweye rockfish stock.

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