Research Document 2021/039
Widow Rockfish (Sebastes entomelas) stock assessment for British Columbia in 2019
By Starr, P.J. and Haigh, R.
Abstract
Widow Rockfish (Sebastes entomelas, WWR) is ubiquitous along the British Columbia (BC) coast (at ~100-500 m depth) and occurs in high densities along the west coast of Vancouver Island (WCVI) and off the shelf edge between the top of Vancouver Island and south of Cape St. James. Shoals of WWR have been studied near Triangle Island using acoustic surveys. This species exhibits diel migration from near bottom during the day to midwater at night, feeding on shrimps, euphausiids, salps, and fish. Night time aggregations make WWR very susceptible to capture by commercial midwater trawl nets.
This species supports the fourth largest rockfish fishery in BC with an annual coastwide ‘total allowable catch’ (TAC) in 2017 of 2,358 t (98% allocated to the trawl fishery) and an average annual catch by all fisheries combined of 2001 t from 2014-2018. This stock assessment evaluates a BC coastwide population harvested by multiple fisheries aggregated into a single modelled fishery. Analyses of biology and distribution did not support separate regional stocks for WWR. A single coastwide stock was assumed for the last WWR stock assessment in 1998.
We use an annual catch-at-age model tuned to fishery-independent trawl survey series, a bottom trawl catch per unit effort (CPUE) series, annual estimates of commercial catch since 1940, and age composition data from survey series (five years of data from four surveys) and the commercial fishery (30 years of data). The model starts from an assumed equilibrium state in 1940, and the survey data cover the period 1967 to 2018 (although not all years are represented). Nine base runs using a two-sex model were implemented in a Bayesian framework (using the Markov Chain Monte Carlo procedure) under a scenario that fixed natural mortality (M) to three levels (0.07, 0.08, 0.09) and set the accumulator age (A) to three values (40, 45, 50 y) while estimating steepness of the stock-recruit function (h), catchability (q) for surveys and CPUE, and selectivity (µ) for surveys and the commercial trawl fleet. These nine runs were combined into a composite base case which explored the major axes of uncertainty in this stock assessment. Twelve sensitivity analyses were performed to test the effect of alternative model assumptions.
The composite base case suggests that low exploitation in the early years, including that by foreign fleets, coupled with several strong recruitment events (in 1961 and 1990) have sustained the population to the present. Exploitation rates were high during a period of heavy fishing by the domestic fleet extending from the mid-1980s to the mid-1990s, causing the stock size to diminish. Exploitation rates dropped with the implementation of 100% observer coverage in 1996 and the introduction of catch limits coupled with individual vessel quotas (IVQs) in 1997.
The spawning biomass (mature females only) at the beginning of 2019 is estimated to be 0.37 (0.26, 0.54) of unfished biomass (median and 5th and 95th quantiles of the Bayesian posterior distribution). This biomass is estimated to be 1.51 (0.92, 2.61) of the spawning biomass at maximum sustainable yield, BMSY.
Advice to managers is presented as decision tables that provide probabilities of exceeding limit and upper stock reference points for five-year projections across a range of constant catches. The DFO provisional ‘Precautionary Approach compliant’ reference points were used, which specify a ‘limit reference point’ (LRP) of 0.4BMSY and an ‘upper stock reference point’ (USR) of 0.8BMSY. The estimated spawning biomass at the beginning of 2019 has a probability of 1 of being above the LRP, and a probability of 0.98 of being above the USR. Five-year projections using a constant catch of 2000 t/y indicate that, in 2024, the spawning biomass has probabilities of 0.99 of remaining above the LRP, and 0.91 of remaining above the USR. Catches greater than 2250 t/y will cause u2024 to exceed the uMSY reference point with a probability of greater than 0.5.
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