Research Document - 2015/070
Big Skate (Raja binoculata) and Longnose Skate (R. rhina) stock assessments for British Columbia
By J.R. King, A.M. Surry, S. Garcia, and P.J. Starr
Abstract
Big Skate (Raja binoculata) and Longnose Skate (R. rhina) are captured and landed by the commercial groundfish trawl and hook-and-line fisheries. Harvest advice was requested to assess whether current harvest levels are sustainable and compliant with the Fishery Decision-making Framework Incorporating the Precautionary Approach. This is the first detailed stock assessment undertaken for these Pacific stocks. Several methods were explored for assessing the stock status of Big Skate and Longnose Skate in order to provide harvest advice. A Bayesian surplus production model was investigated for a Big Skate case study, but produced unsatisfactory results for providing fisheries management advice and was not considered further. As such, reliable estimates of biomass could not be produced, and evaluation of current and future stock status relative to fishery and biological reference points was not possible.
As an alternative to formal stock assessment models, two data-limited approaches were investigated for a Big Skate case study. The first, Depletion-Corrected Average Catch Analysis, produced a range of potential yield estimates that were above the long-term average catch, with an upper bound that was three orders of magnitude larger than the long-term average catch. Based on these results, this approach was not investigated further. The second data-limited approach, Catch-MSY (maximum sustainable yield) Approach, produced plausible results for a Big Skate case study and was applied to Big Skate and Longnose Skate in all areas. However, results were extremely sensitive to assumptions, without consistent responses across areas or assumption combinations, and are not recommended as the sole basis of advice to managers.
In lieu of the development of decision tables, and based on life history traits (namely extremely low fecundity and low intrinsic rate of increase for these species), it is recommened that Big Skate and Longnose Skate be managed by catch limits in all areas of British Columbia. Establishing harvest yields based on mean historic catch, with consideration given to results of trend analyses of research survey abundance indices and to the ranges of maximum sustainable yield estimates identified by the Catch-MSY Approach is recommended. For Big Skate, there were no significant trends in abundance indices from surveys for all areas, and mean historical catches were below the maximum MSY estimate from the catch-MSY results. For Longnose Skate, trawl survey data indicated statistically significant declines in abundance in all areas; however, no significant trends were detected for the longline survey data. For all areas, mean historical catches exceeded the upper maximum sustainable yield estimate from the Catch-MSY Approach results.
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