Research Document - 2003/082

Assessment of Green Sea Urchin (Strongylocentrotus droebachiensis) Stocks in Bristish Columbia, 2003

By R.I. Perry, Z. Zhang, and B.J. Waddell

Abstract

This paper (i) provides an analysis of the green sea urchin fishery in British Columbia by updating information from the 2001 and 2002 fishing seasons; (ii) provides analyses and recommendations for limit and target reference points for the 2003-2004 and 2004-2005 fishing seasons; and (iii) presents results from fishery-independent surveys of green sea urchins conducted in B.C., Canada. Reference points are determined using biomass dynamic models applied to the core stocks in the B.C. south coast: Queen Charlotte Strait (Pacific Fisheries Management Areas 11-13) and the Gulf Islands (PFMA 17-20,28). Two methods are used to determine the parameters of these models: a linear approximation to the dynamic Schaefer model and a time series fitting method. For both core stocks, both models produce similar (i.e. overlapping 95% confidence intervals) estimates of the maximum sustainable yields (MSY). The time series fitting method produces a lower MSY with narrower confidence intervals for the smaller stock (Gulf Islands region), and it is recommended as the more conservative method for calculating reference points. The calculated MSYs are recommended as limit reference points. A Bayesian approach to a state-space model is developed to incorporate uncertainties in both the observations from the fishery and in the specification of the surplus production model, and to include information from the fishery-independent surveys. The result of this model is a probability distribution for MSY which can be used to assist with the choice of target reference points (TRPs) for this fishery. Target reference points for both core regions in the south coast combined are in the range of 101.5 to 203.1 t. Target reference points in this range have a low probability (1.4 - 12.7%) of including the actual MSY (the limit reference point), according to the Bayesian model developed here. Fishery-independent surveys have been conducted annually since 1995 at index sites in Area 12 (Queen Charlotte Strait) and indicate that the biomass of green urchins in this area in 2001 and 2002 was among the highest observed for legal-sized urchins.

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