Research Document 2024/064

Assessing the Productivity of the Georges Bank ‘a’ and Browns Bank North Offshore Scallop Stocks and the Impact of Two-Year Projection Advice

By Sameoto, J.A., Keyser, F.M., and Keith, D.M.

Abstract

Georges Bank ‘a’ (GBa) and Browns Bank North (BBn) comprise the majority of catches from the Offshore Scallop fishery in the Fisheries and Oceans Canada (DFO) Maritimes Region. Each stock is managed using a total allowable catch (TAC) and has an annual analytical assessment that uses a modified version of a state-space delay-difference population model to provide a one-year biomass projection to inform the setting of the harvest level. However, in 2020, the DFO Science Offshore Scallop survey was cancelled due to challenges associated with the COVID-19 pandemic. In the absence of survey data, two-year model projections are needed to update the stock status for 2021. The objectives of this document are to evaluate the productivity of these stocks based on the population assessment models, to derive two-year model projections to inform the final TAC decisions for 2021, and to evaluate the impact and uncertainty of the two-year model projections. For GBa, the surplus production rate estimates ranged from 0.25 to 0.27, which is consistent with previous estimates. For BBn, the surplus production rate estimates ranged from 0.13 to 0.15, which is higher than previous estimates. The scallop stocks on GBa and BBn demonstrate substantial interannual variability in their productivity such that, relative to the use of one-year projections, use of two-year projections as the basis for management decisions over the long term would result in substantial risk of either loss in potential catch or overharvesting. However, in the context of tactical one-year decision making and in the absence of 2020 survey data, these two-year projections provide context for decision making for the 2021 harvest level. For the two-year projection scenarios evaluated, the probability that the GBa 2021 fully recruited biomass would be above the upper stock reference and in the healthy zone after removing 4,000 mt (the 2021 interim TAC) is at least 0.98; this is predicted to result in an exploitation rate between 0.08 and 0.1 and an expected range of biomass change between −10% and 18%. For BBn, for the two-year projection scenarios, removing 300 mt (the 2021 interim TAC) is predicted to result in an exploitation rate between 0.12 and 0.14 and corresponds to an expected range of biomass change between −14% and 1%.

Research Document 2024/064 (PDF, 6,043 KB)

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