2022 assessment of Atlantic Halibut on the Scotian Shelf and Southern Grand Banks (NAFO Divisions 3NOPS4VWX5Zc)

Abstract

The new Spatially Integrated Statistical Catch-At-Length (SISCAL) age- and sex-structured model was used to assess the stock status. Closed-loop simulation was used to evaluate the impact of a suite of harvest strategies on the biomass/population trends and landings. A Richards-Schnute growth model, which accounts for gear size-selectivity and length stratified sampling, was fit to all available age-length data and used to inform the length based assessment model. The 2021 spawning stock biomass (SSB) estimate is at a time series high of 20.6 kt. Total and legal-sized biomass are near record levels at 54.1 kt and 46.5 kt, respectively. While the stock is in the healthy zone, it is likely to decline in the near term under either proposed harvest control rule (HCR). The model estimate of recruitment was higher from 2002–2013 leading to the stock increasing well above the biomass at maximum sustainable yield (BMSY); however, since then, recruitment has been closer to the long-term mean. Recent exploitation rates have remained stable providing evidence of success of the constant fishing mortality (F) HCR adopted in 2015. Estimates of natural mortality (M) in the model 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. Based on the new assessment model the limit reference point (LRP) was defined as 0.4 SSBMSY=5.3 kt and a proposed upper stock reference (USR) was defined as 0.8 SSBMSY=10.6 kt. Closed loop simulations were used to evaluate a range of candidate management procedures (MPs). Both tested HCRs met the conservation objectives for this stock. The increased precision of the random survey design allowed MPs to perform better than those based on the fixed survey portion of the Industry-DFO Halibut Longline Survey with respect to TAC stability and conservation measures. Simulations over two generations of the release of live Halibut >125 pounds (170 cm) did not improve stock performance, and in some cases led to increased probability of falling below the USR. There was also no indication that increasing the minimum legal size to 86 cm would impact stock performance as measured by the probability of falling below reference points or projected catch. Using the 3-year mean of the stratified random survey the TAC for the 2022 fishing season would be 4,040 t under the ramped HCR and 4,807 t under the articulated HCR.