Assessment of Petrale Sole in British Columbia in 2024

Abstract

This stock assessment evaluated a single BC coastwide population. Petrale Sole is primarily harvested by the multi-species groundfish trawl fishery. A two-sex statistical catch-at-age model was used to assess Petrale Sole stock status. The model included catch data (from 1938 to 2023), and was fit to three fishery-independent survey indices (from 2003 to 2023) and age data from the fishery and fishery-independent surveys (from 2004 to 2019). Natural mortality (M), which had to be fixed, was identified as a key uncertainty in this stock assessment, leading to the development of an ensemble model based on three separate models with low, medium, and high M fixed estimates. This approach allowed some uncertainty in M to be incorporated into estimated stock status and harvest advice. Maximum sustainable yield (MSY)-based reference points were recommended for characterizing stock status and informing harvest decisions. A limit reference point (LRP) at 0.4 female spawning biomass at maximum sustainable yield (BMSY), a candidate upper stock reference (USR) at 0.8BMSY, and a candidate removal reference (RR) at fishing mortality at maximum sustainable yield (FMSY) were used, consistent with the provisional recommendations in the Precautionary Approach (PA) policy. Stock status relative to the unfished female spawning biomass (B0) was also presented in the assessment document. The stock at the beginning of 2024 was estimated to be in the Healthy zone, with female spawning biomass at the beginning of 2024 (B2024) estimated to be 3.01 (95% credible interval: 1.62,5.60) times the female spawning biomass at maximum sustainable yield, BMSY. Also, B2024 was estimated to be 0.96 (0.55,1.63) times B0. B2024 is above the LRP and candidate USR with a greater than 99% probability. Additionally, fishing mortality in 2023 was below the candidate RR with a greater than 99% probability. The Petrale Sole stock is predicted to decline over the next 10 years, even under a no-catch scenario; however, the stock is expected to remain above the LRP and candidate USR with a very high probability (greater than 99%) at catch levels up to 1,500 t/y (compared to the average coastwide 2019–2023 catches of 630 t). Catches up to 1,250 t/y were predicted to keep the harvest rate below the maximum removal reference (FMSY) in 10 years with a very high probability (greater than 98%). Relationships between environmental conditions, recruitment deviations, and average body condition were explored. These indicated that productivity may have increased with warmer conditions, although there was also a suggestion that further warming could become detrimental. Model uncertainties were explored with sensitivity analyses which varied model assumptions and data inputs. Sensitivity analyses showed that most uncertainties considered had a relatively small impact, with B2024 estimated to be above BMSY and F2023 estimated to be below FMSY in all cases. Assumed values for M resulted in the largest differences in estimated stock status, supporting the choice for a base ensemble model. Limited age composition data were identified as a key source of uncertainty in the assessment, with the lack of age data after 2019 creating high uncertainty in recent recruitment estimates. It is recommended that formal stock assessments occur every 5 years, but could be up to 10 years given the current high estimated stock status. During intervening years, abundance trends can be tracked using indices from the Groundfish Multispecies Synoptic Bottom Trawl Surveys.