Science Advisory Report  2016/016

Stock assessment of the coastwide population of Shortspine Thornyhead (Sebastolobus alascanus) for British Columbia, Canada in 2015

Summary

• Shortspine Thornyhead was caught in amounts less than 100 t by the commercial trawl fishery up to the late 1980s, followed by increasing catches into the 1990s, when catches reached 958 t. Although there is some directed fishing on this species, it is most often caught along with other groundfish species in the commercial trawl fishery. Species separation with its congener S. altivelis (Longspine Thornyhead) did not occur in catch records until 1996 with the introduction of 100% observer coverage.
• The coastwide stock was assessed using a delay-difference model fit to five fishery-independent surveys, a catch per unit of effort (CPUE) time series derived from commercial catch and effort data, and an annual time series of mean weights derived from unsorted commercial catch samples.
• A model-averaged decision table is presented using the provisional reference points from  Fisheries and Oceans Canada’s Fishery Decision-making Framework incorporating the Precautionary Approach: a limit reference point (LRP) of 0.4B_MSY, an upper stock reference (USR) of 0.8B_MSY, and a reference harvest rate of u_MSY. The decision table provides estimates of stock status and harvest rates that result from three-year projections at a range of constant annual catches from 0 to 1000 t.
• The estimated stock biomass trajectory has remained above the estimates of the stock status reference points throughout the history of the fishery.  Estimated current stock status (beginning year biomass in 2016) has a 0.97 probability of being above the USR and a 1.0 probability of being above the LRP. The probability that U₂₀₁₅ exceeded u_MSY is 0.72.
• The stock is expected to decline if annual harvests of 600 t/year (the 2010-2014 average catch) are removed in each of the next three years. The probability that the stock will stay above the USR at the end of the next three years is 0.76. The probability that the stock will remain above the LRP after three years is 0.88.
• An assumption of the analysis is that Shortspine Thornyhead is a single coastwide stock.  Therefore, harvest advice on a smaller scale is not provided. Advice at a smaller spatial scale would require definition of spatially specific objectives for the stock and fishery.
• Uncertainties associated with growth, natural mortality, and knife-edge selectivity were bounded by adopting a model average approach across a range of hypotheses for these assumptions. There is a contradiction in the model data, with the annual mean weight index showing an increasing trend, while biomass indices show no trend. Additionally, it is not known if the lack of large, old fish in the data is a sampling issue or a feature of the BC Shortspine Thornyhead population.

This Science Advisory Report is from the December 10-11 and 18, 2015 regional peer review on Shortspine Thornyhead (Sebastolobus alascanus) Stock Assessment for the Pacific Coast of Canada in 2015. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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