Modélisation de la population de l'aiguillat commun du Pacifque (Squalus suckleyi) pour les eaux extérieures de la Colombie-Britannique en 2024

Abstract

Pacific Spiny Dogfish (Squalus suckleyi) is a long-lived shark with late maturation and low fecundity, distributed from Alaska to southern Baja California. In British Columbia (BC), Canada, two stocks are assessed: an inside stock in the Strait of Georgia and Johnstone Strait, and an outside stock in remaining coastal areas. This assessment focuses on the outside stock. This stock has been commercially fished since the 1870s with a large vitamin A liver fishery in the 1940s and no targeted fishery since 2011. Discards have exceeded landings over the past decade. This assessment updates the 2010 analysis using a two-sex, age-structured population dynamics model fitted to fishery and survey catch, indices of abundance, and length composition data. Due to the species’ life history, spawning output (number of pups) is used to characterize stock status. This assessment uses literature-informed discard mortality rates under low, base, and high assumptions. The model explores uncertainties in natural mortality (M), discard mortality, index representativeness, stock-recruit curve shape, and potential increases in M. One base model, 15 sensitivity models with constant M, and five increasing-M models were evaluated. All models estimated a steep decline in spawning output in the 1940s due to the vitamin A fishery, followed by recovery driven by maturation of juvenile cohorts, and a slower decline through 2010 due to fishing and senescence. Estimated 2023 depletion (S/S₀) was robust across most assumptions. Some increasing-M models fit steep recent declines in the Synoptic trawl index but implied a stock that would be unable to replace itself if the higher M were to persist. An increase in M is plausible, but how this change is captured in the model requires future research. A Limit Reference Point (LRP) of 0.2 S/S₀ and candidate Upper Stock Reference (USR) of 0.4 S/S₀ are proposed, with F/F0._4S0proposed as a Removal Reference. All models estimated the stock to be below the LRP in 2023 with high probability (>0.95), placing it in the Critical Zone. Projected spawning output remains below the LRP through 2028, even under zero catch. Across scenarios, dead catch levels (landed Dogfish plus those assumed to die from discard mortality) associated with ≥ 95% probability of F < F0._4S0 ranged from 0 to 250 t. Average recent dead catch ranged from 160 to 423 t depending on discard mortality assumption. The known limited productivity of Dogfish, the estimated population size, and steep declines in two of three survey indices suggest that catches need to be lower than the current total allowable catch of 12,000 t to increase the spawning output and achieve a high probability of F < F0._4S0. Reassessment is recommended within five years, with monitoring of survey indices in the interim.