Arrowtooth Flounder Assessment 2021

Arrowtooth Flounder (Atheresthes stomias) is a commercially important flatfish species in British Columbia (BC), which occurs along the BC coast. In the past decade, markets have been established for Arrowtooth Flounder fillets frozen at sea. All areas of Canada’s Pacific coast, excluding waters between Vancouver Island and the BC mainland, were assessed using a two-fleet, two-sex, catch-at-age model implemented in a Bayesian framework to quantify uncertainty of estimated quantities. The model, which assumes a single coastwide stock, was fit to commercial and survey data from 1996 to 2021. Catch data prior to 1996 were not used due to unknown levels of releases at sea before the introduction of at-sea observers. Reference points based on maximum sustainable yield (MSY) were strongly impacted by the relationship between the estimates of maturity and commercial age selectivity, unrealistically suggesting that all vulnerable biomass could be taken without repercussions to the stock. Consequently, MSY reference points were not recommended for this stock. Advice presented to management is relative to reference points based on 0.2B0 (Limit Reference Point; LRP) and 0.4B0 (proposed Upper Stock Reference; USR), where B0 is the unfished spawning biomass (males plus females) at the mean recruitment level. Probabilities using an alternative USR of 0.35B0 are also presented. The median spawning biomass (with 5th and 95th percentiles of the Bayesian results) at the beginning of 2022 (B2022) was estimated to be 0.37 (0.26, 0.51) of B0. The base model had an estimated probability close to zero of being below the LRP (P(B2022 < 0.2B0)) and a probability of 0.66 of being below the proposed USR (P(B2022 < 0.4B0)). Given the median relative spawning biomass for 2022 (0.37B0), the stock is currently considered to be in the cautious zone relative to the LRP of 0.2B0 and proposed USR of 0.4B0. Advice to management is presented in the form of a decision table, which reports the probability of the biomass for 2023-2026 being below the reference points 0.2B0, 0.35B0, and 0.4B0, as well as the probability of the biomass declining. Projections of the stock across a range of constant annual catches of 0 to 15 kt in 1 kt increments from 2022-2026 are provided. The reference removal rate (U0.4B0 = 0.105 of vulnerable population annually), which is equivalent to an annual removal of approximately 4.4 kt, was estimated by projecting the stock for 50 years and finding the constant catch where the stock reached 0.4B0 assuming continuing low recruitment as was estimated over the period from 2010 to 2019. The size of catches and discards prior to 1996, the lack of random-stratified surveys prior to 2005 that together cover the entire coast, the estimation of maturity and selectivity curves, the assumed magnitude of recruitment variability, and the estimation of B0 are major sources of uncertainty in this assessment that make it challenging to estimate the size and productivity of the stock. Given that the stock is estimated to be below 0.4B0 (proposed USR) in the base model, and that survey indices are declining coincident with declining estimated spawning biomass and recruitment, it is recommended that the current assessment be updated in two years when one additional survey has been run in each area of the coast. In the longer term, it is suggested that conducting this assessment using a management procedure approach could provide management advice that more explicitly handles the uncertainties identified in this system.

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