Research Document 2018/063

Assessment of the Arctic Surfclam (Mactromeris polynyma) Stocks on Banquereau and Grand Bank

By Hubley, B., Heaslip, S.G., and Stanley, R.R.E.

Abstract

The offshore Arctic Surfclam fishery has traditionally been managed on the basis of bank-wide estimates of biomass and Total Allowable Catch (TAC). These biomass estimates resulted from scientific surveys, the most recent of which were in 2009 (Grand Bank) and 2010 (Banquereau). This bank-wide approach does not necessarily guarantee sustainability since biomass is estimated for areas where clam densities are too low to be commercially viable. Whole-bank biomass estimates and resulting TACs could result in areas of commercial density being fished down faster than they can be replenished. In addition, updated assessments using this approach require new survey data, which is not currently available for either bank. Here we apply an updated assessment approach that attempts to mitigate these issues by generating biomass estimates that are restricted to areas of commercially viable densities identified from historical Vessel Monitoring System (VMS) positional and Catch Per Unit Effort (CPUE) data. This approach generates estimates of biomass across each bank using Bayesian surplus production models to incorporate and quantify uncertainties associated with dredge efficiency. Given the sedentary nature of clams and the cyclical nature of the spatial footprint of fishing activity, we fit surplus production models to spatial disaggregated data. Five spatial assessment areas were constructed for Banquereau that were easily navigable; encompassed large scale contiguous clam beds, were approximately equal in total biomass, and included high and low density areas. The production model was fit to the CPUE index for each spatial assessment area with parameters such as dredge efficiency estimated across areas. Model results show trends of declining catch rates for all areas over the last 5 years. The summed biomass for the fished areas of Banquereau was 475,960 t (275,592–869,243 95% Bayesian Credible Interval).

We compared estimates generated for Banquereau in 2010 to estimates generated from survey observations. Density estimates from the 2010 survey were similar to the 2010 CPUE density estimates for overlapping locations despite different vessels, gear, and statistical approach. When these density estimates were expanded to the fished area, as identified from the VMS footprint, the resulting biomass estimates for 2010 were also similar between the survey (211,136 t) and CPUE (218,262 t). Biomass estimates from the last assessment were corrected for dredge efficiency, which was estimated to be 0.45 with considerable uncertainty (95% Confidence Interval 0.21–0.86). A Bayesian surplus production model incorporated and quantified the uncertainties associated with dredge efficiency, the resulting estimates of biomass, and provided estimates of process and observation error.

Reference points were calculated from biomass estimates of the surplus production model with F_MSY estimates near 0.1; however, phase plots indicate that catch rates decline when F is greater than 0.05. Advice based on aerial expansion from the fished areas to the entire bank is more risky than only considering the fished areas; exploitation rates near estimates of F_MSY are more risky than F reference levels below F_MSY.

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