Research Document 2022/010

Yellowmouth Rockfish (Sebastes reedi) stock assessment for British Columbia in 2021

By Starr, P.J. and Haigh, R.

Abstract

Yellowmouth Rockfish (Sebastes reedi, YMR) ranges from the Gulf of Alaska southward to northern California near San Francisco. In British Columbia (BC), the apparent area of highest concentration occurs in Queen Charlotte Sound with isolated hotspots west of Haida Gwaii and at the northern end of Vancouver Island. This species occurs along the west coast of Vancouver Island, but its density appears to be low south of Brooks Peninsula.

In 2010, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed the coastal population of YMR in British Columbia as ‘Threatened’, based on an analysis of survey indices and the threat from commercial fishing. As a result, the species was considered for legal listing under the Species at Risk Act (SARA). In a 2011 stock assessment (also acting as a recovery potential assessment), Edwards et al. (2012a) put forward two base case runs (‘Estimate M’ and M=0.047), which both estimated that the B₂₀₁₁ stock status was well above the upper stock reference level for a healthy stock in the Fisheries and Oceans Canada (DFO) Sustainable Fisheries Framework (DFO 2009). In 2017, a decision was made not to list Yellowmouth Rockfish under Schedule 1 of the SARA. In 2019, Bill-C-68 was enacted to amend the Fisheries Act with the Fish Stocks provisions, prompting a national review of the approximately 180 stocks using Sustainability Surveys with the aim to include the majority of those stocks in regulation over the next five years. Yellowmouth Rockfish is one of 18 groundfish stocks in the Pacific Region being considered for inclusion. The purpose of this YMR stock assessment is to evaluate the current stock status and provide advice suitable for input to a sustainable fisheries management plan.

This stock assessment evaluates a BC coastwide population harvested by a single fishery dominated by bottom trawl. Midwater trawl catches of YMR were combined with bottom trawl for the purposes of this stock assessment. YMR catches by capture methods other than trawl were negligible, averaging less than 1% over the period 1996 to 2020. Analyses of biology and distribution did not support separate regional stocks for YMR. A single coastwide stock was also assumed by Edwards et al. (2012a).

We use an annual catch-at-age model tuned to four fishery-independent trawl survey series, a bottom trawl Commercial catch per unit effort (CPUE) series, annual estimates of commercial catch since 1935, and age composition data from survey series (25 years of data from four surveys) and the commercial fishery (28 years of data). The model starts from an assumed equilibrium state in 1935, the survey data cover the period 1967 to 2020 (although not all years are represented) and the CPUE series provides an annual index from 1996 to 2020.

A two-sex model was implemented in a Bayesian framework, using the Markov Chain Monte Carlo (MCMC) ‘No U-Turn Sampling’ procedure, to estimate models which fixed natural mortality to one of five levels (0.04, 0.045, 0.05, 0.055, 0.06), spanning a range that was considered plausible and which returned acceptable MCMC diagnostics. The parameters estimated by these models included average recruitment over the period 1950–2012, and selectivity for the four surveys and the commercial trawl fleet. The survey and CPUE scaling coefficients (q) were determined analytically. These five model runs were combined into a composite base case which explored the major axis of parameter uncertainty in this stock assessment. Fourteen sensitivity analyses were performed relative to the central run (M=0.05) of the composite base case to test the effect of alternative model assumptions.

The composite base case estimated the YMR female spawning population biomass at the end of 2021 to be 0.69 (0.44, 1.08) relative to B₀ and to be 2.4 (1.5, 3.7) relative to B_MSY. This latter result suggests that the YMR spawning population currently lies well in the Healthy zone (with a probability >0.99). Projections predicted that the stock will remain in the Healthy zone up to the end of 2031 at all evaluated catch levels up to 3000 t/y. However, these projections also predicted that the stock would decline at catch levels greater than 500 t/y, under the assumption that recruitment will be average over that time period. None of the fourteen sensitivity analyses changed this conclusion. These analyses included, estimating M, higher and lower pre-1996 catch histories, higher and lower recruitment standard deviation (σ_R) assumptions, dropping the CPUE series and substituting an alternative CPUE series, omitting ageing error, restricting the period over which recruitments were estimated to 1970–2012, and upweighting the age frequency data for the Queen Charlotte Sound synoptic survey. The most pessimistic sensitivity run was the one which omitted the ageing error, an option that we consider unrealistic.

Research Document 2022/010 (PDF, 31,781 KB)

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