Proceedings of the Pacific Region Science Advisory Process for Outside Stocks of Lingcod (Ophiodon elongatus) and the Inside Population of Yelloweye Rockfish (Sebastes ruberrimus) in British Columbia, Canada; April 7-8, 2011
Chairperson: A.R. Kronlund
A regional advisory process meeting was held April 7-8, 2011 in Nanaimo, British Columbia (BC) to conduct science peer reviews of the status of four outside lingcod (Ophiodon elongatus) stocks and the inside population of yelloweye rockfish (Sebastes ruberrimus). The science review was conducted in response to requests from DFO Fisheries and Aquaculture Management (FAM) for advice regarding the current stock status and appropriate fishery reference points for the stocks. In addition, an evaluation of the impacts of varying harvest levels on future population trends was requested.
Lingcod in British Columbia are assessed and managed as five separate units based on DFO Statistical Areas; one inside stock in the Strait of Georgia and four outside stocks. The four outside stocks include southwest Vancouver Island (Area 3C), northwest Vancouver Island (Area 3D), Queen Charlotte Sound (Areas 5A and 5B), and Hecate Strait and the west coast of Haida Gwaii (Areas 5C, 5D, and 5E). Data inputs included annual catch from all commercial sectors and recreational fisheries beginning in 1927. At least three abundance indices were available for each area, drawn from commercial trawl fishery catch per unit effort (CPUE), the U.S. National Marine Fisheries Service (NMFS) triennial trawl survey, shrimp trawl surveys, the Hecate Strait multi-species assemblage survey and the multi-species synoptic trawl surveys.
A Bayesian surplus production (BSP) model was applied to assess lingcod stock status within each of the four stock areas. As a result of contradictory trends between fishery-dependent and survey indices, a technological efficiency parameter was introduced to adjust commercial trawl catch rates for time-dependent changes in efficiency. Stock-specific parameter estimates for the intrinsic rate of increase, r, and carrying capacity, K, were used to calculate management parameters such as maximum sustainable yield (MSY), the optimum fishing mortality rate at MSY (FMSY), and the optimal stock size at MSY (BMSY). Stock-specific prior probability distributions were supplied for estimated parameters. Limit and upper stock reference points were set at 0.4BMSY and 0.8BMSY, respectively, with a candidate target reference point of BMSY. Even with informative priors, the stock assessment and projection results was imprecise. However, under the reference case model configuration, it appeared to be unlikely that the exploitable biomass in 2009 for any of the stocks was depleted below the limit reference point of 0.4BMSY. Application of a variety of harvest policy options spanning the current range of catches all resulted in higher than a 50% probability of maintaining stocks at or above BMSY up to 20 years into the future. Stock status and projection results were relatively insensitive to alternative priors for r. However, stock status and projection results for Areas 3C, 5AB, and 5CDE were sensitive to the choice of alternative Bayesian priors for the “tech” parameter that determined time-varying adjustment to catchability.
Yelloweye rockfish in British Columbia are assessed and managed as separate “inside” and “outside” units. This review considered an assessment of the inside population of yelloweye rockfish, which is primarily located in protected waters to the east of Vancouver Island in Area 4B. Both inside and outside populations of yelloweye rockfish have been designated as Species of Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). Population dynamics were modeled using a Bayesian surplus production model. The model was fitted to (i) reconstructions of historical catches, (ii) four standardized commercial catch per unit effort (CPUE) series that covered four different periods in the history of the fishery; and (iii) eight fishery independent longline survey indices that varied in spatial coverage within the assessment area. A suite of sensitivity tests was conducted to evaluate the effects of uncertainty in key model parameters, the magnitude of the commercial catch and the influence of different indices.
Advice to fishery managers was based on the BSP model reconstructions of stock status in 2009 and associated projections of the stock trajectory under a range of constant annual harvest policies calculated at intervals over an 80-year time horizon. The 2009 exploitable stock biomass of the inside population of yelloweye rockfish was estimated using the reference case model to be 780 t (with standard deviation, SD=390 t), or 12% of the initial biomass in 1918. The probability of the exploitable biomass exceeding the limit reference point was estimated to be P(B2009 > 0.4BMSY) = 0.05. All sensitivity test results were similar and indicated a high probability that the exploitable biomass of yelloweye rockfish was less than the limit reference point of 0.4BMSY in 2009. For the fixed annual catch levels considered in the projections, the probability of stock recovery to levels above the limit reference point ranged from 0.12 to 0.14 over a 5-year time horizon and increased to about 0.4 to 0.7 over a 40-year time horizon.
In order to allow investigations into non-fishery factors that may affect stock status, the BSP model was extended to incorporate changes in the level of predation of yelloweye rockfish by pinnipeds. The predation form of the model (PBSP) included mortality due to harbour seals (Phoca vitulina), Steller sea lions (Eumetopias jubatus), and California sea lions (Zalophus californianus) implemented using a Type I functional response relationship where the amount consumed per predator increases linearly with rockfish density up to a maximum. This model extension represents the first application of predator-prey interactions involving marine mammals to yelloweye rockfish assessment. Future development of this model depends on obtaining improved estimates of the proportion of yelloweye rockfish in pinniped diets and the consideration of alternative forms of the predator-prey functional response. Evaluation of future stock status depends as well on the development of plausible scenarios for the abundances of the predator species. The identification of BMSY-based fishery reference points for the PBSP model remains problematic because multiple equlibria conditions exist that depend on the magnitude of pinniped predation.
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