Framework assessment of Atlantic Halibut on the Scotian Shelf and Southern Grand Banks (NAFO Divisions 3NOPs4VWX5Zc)

Abstract

Atlantic Halibut (Hippoglossus hippoglossus) is a large sexually dimorphic flatfish and currently the most valuable groundfish species by landed weight on the Atlantic coast. The 2014 framework for Atlantic Halibut on the Scotian Shelf and Southern Grand Banks developed a statistical catch-at-length (SCAL) model that estimated historical biomass, fishing mortality, age–1 recruitment, and biological reference points. SCAL model outputs were then used to condition, or parameterize, an age-structured operating model used to evaluate the performance of alternative TAC interim procedures. This paper describes the spatially integrated statistical catch at length (SISCAL) model adapted for Atlantic Halibut, an update of the data and software, but not the basic structure of the SCAL model which was included here for comparison. The SISCAL model estimates the 2021 spawning stock biomass to be 31.1 kt, with 95% credible interval (25, 36), the highest estimated biomass in the time series. This stock has increased from a heavily depleted state observed in the 1990s. It appears to be benefiting from a recent period of high recruitment. Estimates of total and legal-sized (greater than 81 cm since 1994) biomasses are also at record levels. Recent exploitation rates have been consistent with the target level of the constant F harvest rule adopted in 2014, providing evidence of success. Estimates of natural mortality M were allowed to vary over the time series, with recent (2014–2021) estimates of M ranging from 0.128 to 0.143, for males and from 0.120 to 0.133 for females. We used SISCAL to condition an updated operating model, estimate MSY-based reference points and test a suite of candidate management procedures using closed-loop simulations against Atlantic Halibut fishery management objectives.