Evaluation of two-year model projections for Bay of Fundy and SFA 29W scallop fisheries

Abstract

The Bay of Fundy (BoF) Scallop Production Areas (SPAs) 1A, 1B, 3, 4, and 6, and Scallop Fishing Area 29 West (SFA 29W) are managed using total allowable catches (TAC). The annual assessments use modified versions of a state-space delay-difference population model fit to survey and commercial catch data. In 2020, the DFO Science Inshore Scallop surveys for the BoF and SFA 29W were cancelled. In the absence of survey data, two-year model projections are needed to inform the scallop fisheries in these areas for the 2020/21 season. The scallop stocks in the BoF and SFA 29W demonstrate substantial interannual variability in their productivity. There is limited temporal autocorrelation in model parameters and in surplus production beyond a 1-year lag for most areas. If used over the long-term as a basis for management decisions, harvest advice based on two-year projections could result in substantial risk of either loss in potential catch or overharvesting. Although the usefulness of 2-year projections is limited for long-term advice, for the BoF stocks (SPAs 1A, 1B, 3, 4, 6), they provide context for decision making in 2021 in the absence of 2020 survey data. For SFA 29W, two-year projections are not sufficiently reliable given challenges associated with projecting low biomasses and not recommended to inform the 2021 harvest levels for SFA 29W. For SPA 1A, for the two-year projection scenarios evaluated, the probability that the 2021 commercial biomass would be above the upper stock reference (USR) and in the healthy zone after removing 270 mt (the 2021 interim TAC) was between 0.78 and 0.87, the probability that the 2021 biomass would be above the LRP was between 0.97 and 0.98, and the expected exploitation ranged between 12% and 14%. For SPA 1B, for the two-year projection scenarios evaluated, the probability that the 2021 commercial biomass would be above the USR and in the healthy zone after removing 400 mt (the 2021 interim TAC) was between 0.67 to 0.80, the probability that the 2021 biomass would be above the LRP was between 0.96 to 0.98, and the expected exploitation ranged between 12% and 15%. For SPA 3, for the two-year projection scenarios evaluated, the probability that the 2021 commercial biomass would be above the USR and in the healthy zone after removing 200 mt (the 2021 interim TAC) was between 0.77 to 0.86, the probability that the 2021 biomass would be above the LRP was between 0.94 to 0.97, and the expected exploitation ranged between 9% and 11%. For SPA 4, for the two-year projection scenarios evaluated, the probability that the 2021 commercial biomass would be above the USR and in the healthy zone after removing 175 mt (the 2021 interim TAC) was between 0.61 to 0.70, the probability that the 2021 biomass would be above the LRP was between 0.78 to 0.84, and the expected exploitation ranged between 14% and 16%. For SPA 6, reference points are set in terms of the commercial catch rate and not the modelled biomass; therefore, an evaluation of stock status relative to the population model is not possible. For the two-year projection scenarios evaluated for 2021, assuming the total allowable catch of 210 mt is caught from the modeled area, the exploitation range is expected to be between 20% and 24%, which corresponds to an expected range of biomass change between −28% and −11%.