Research Document 2022/020
Rougheye/Blackspotted Rockfish (Sebastes aleutianus/melanostictus) stock assessment for British Columbia in 2020
By Starr, P.J. and Haigh, R.
Abstract
The Rougheye/Blackspotted Rockfish (REBS, Sebastes aleutianus/melanostictus) species complex is ubiquitous along the British Columbia (BC) coast, with trawl catches taken primarily from the depth range of 135-845 m. Fisheries using trawl gear find highest densities off NW Haida Gwaii, at the mouths of Moresby and Mitchell’s Gullies, and off the NW coast of Vancouver Island. Fisheries using hook and line gear catch REBS along the 500 m isobath with the highest densities occurring off NW Haida Gwaii. REBS prefer soft substrata in sloping areas with frequent boulders.
In April 2007, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed the REBS complex, comprising a pair of sympatric species, as ‘Special Concern’. In 2009, REBS was added to SARA’s Schedule 1 as Special Concern. Since then, no changes have been made to the status of the species complex. At the time of this assessment, the taxonomy was emerging and these species were named Rougheye Rockfish Type I and Type II, where Type I corresponded to Blackspotted Rockfish (BSR) and Type II denoted Rougheye Rockfish (RER). Although procedures for separating the two species, based on genetic analyses and various biomarkers, are available, species allocation methodologies to be applied to historical data still need to be developed. In this stock assessment, the two stocks are spatially defined by Pacific Marine Fisheries Commission areas, where REBS data from areas 5DE are called 'REBS north’ and REBS data from areas 3CD5AB are called ‘REBS south’. REBS data from area 5C were considered to be in a zone of hybridisation and consequently were omitted from the stock assessment except to proportionately distribute the 5C catch between the two species (~65-70% in favour of REBS north).
This stock assessment evaluates two stocks along the BC coast, REBS north and REBS south, which are harvested by multiple fisheries. The assessment uses an annual catch-at-age model tuned to one fishery-independent trawl survey series for REBS north and three surveys for REBS south, a bottom trawl CPUE series for both, annual estimates of commercial catch since 1935, and age composition data from survey series (spanning 1997-2016) and commercial fisheries (spanning 1978-2018). The model starts from an assumed equilibrium state in 1935. Two fisheries are modelled: one a combined bottom and midwater ‘Trawl’ fishery and an ‘Other’ fishery, which combines halibut longline, sablefish trap, lingcod/dogfish/salmon troll, rockfish hook and line, etc. The second fishery is a compromise that acknowledges other methods capturing this species while keeping the model complexity to a minimum, given the lack of good information from these additional fisheries.
For each stock, nine base model 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 to three levels (0.035, 0.045, 0.055) using three CPUE process errors (0.1, 0.2759 for REBS north or 0.2529 for REBS south, 0.4) each. Steepness of the stock-recruit function was fixed at 0.7; catchability for the surveys and CPUE, and selectivity for three of the four surveys and the commercial trawl fleet were estimated. Of the candidate component runs, nine were combined into a composite base case for REBS north and six were combined into a composite base case for REBS south. Each composite base case explored two major axes of uncertainty, namely, natural mortality M and CPUE process error, which modified the degree of fit to the CPUE biomass series. Sensitivity analyses were performed to test the effect of alternative model assumptions (e.g., wider ageing error matrices).
Stock status at the beginning of 2021 for the REBS north composite base case lies in the Healthy zone with a probability of 1, as do all nine component runs. The composite base case population trajectory from 1935 to 2021 and projected biomass to 2096, assuming a constant catch strategy of 600 t/y (just above the 5-year average catch of 548 t), indicates that the median stock biomass will remain above the USR for the next 1.5 generations (75 years). The probability envelope around the constant catch strategy will extend into the Cautious and Critical zones due to a much larger cumulative removal than that under a harvest rate strategy of 0.1/year. A phase plot of the time-evolution of spawning biomass and exploitation rate in the two modelled fisheries in MSY space suggests that the stock is in the Healthy zone, with a current position at B2021/BMSY = 2.21 (1.50, 3.15), u2020(trawl)/uMSY = 0.06 (0.02, 0.14), and u2020(other)/uMSY = 0.11 (0.03, 0.32).
Stock status at the beginning of 2021 for the REBS south composite base case lies in the Healthy zone with a probability of 0.74 and in the Cautious zone with probability 0.26. The composite base case population trajectory from 1935 to 2021 and projected biomass to 2096, assuming a constant catch strategy of 300 t/y, indicates that the median stock biomass will eventually crash at the current amount of removals (5-year average catch of 291 t). The fixed harvest rate strategy appears to offer a more sustainable catch strategy, with median projected biomass remaining above the USR for the next 1.5 generations (75 years). A phase plot of the time-evolution of spawning biomass and exploitation rate in the two modelled fisheries in MSY space suggests that the stock is in the Healthy zone, with a current position at B2021/BMSY = 1.07 (0.58, 2.61), u2020(trawl)/uMSY = 1.17 (0.19, 2.59), and u2020(other)/uMSY = 0.72 (0.13, 1.77). The Trawl fishery’s harvest rate is above that at uMSY.
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