Review of candidate stock assessment frameworks for the Northwest Atlantic Fisheries Organization subarea 0+1 (offshore) Greenland halibut stock

Abstract

The North Atlantic Fisheries Organization (NAFO) Subarea (SA) 0+1 (offshore) Greenland Halibut (GH-0+1) stock had been assessed using an index-based approach based on surveys up to 2017, when the standardized vessel and gear were decommissioned. A new survey time series started in 2022 with a new vessel and gear but will require at least 3–4 data points before an empirical index can be re-established. In the interim, analytical methods are sought to support the provision of Science advice and to evaluate the feasibility of transitioning to a conventional assessment framework over the longer term. Fishery-dependent and fishery-independent data relevant to this stock were compiled and used to inform various aspects of population and distribution models. Additionally, a list of potential factors affecting catchability was provided. Exploratory modeling work was conducted using an age-structured population dynamics model (Spatially Integrated Statistical Catch-at-Length; SISCAL) and a spatiotemporal stock distribution model (species distribution model in Template Model Builder; sdmTMB). Both approaches showed potential; however, further development will be required for these models to be considered for the provision of Science advice for this stock. The exploratory modeling work using SISCAL highlighted some key uncertainties—including stock size, somatic growth and harvest rate—that could be addressed through additional data collection (e.g., additional surveys, tagging). Given its potential to provide Science advice in the near term (i.e., for the next NAFO Scientific Council (SC) assessment scheduled in 2024), it is recommended that a model based survey index calibration method be further investigated. This approach should be tested empirically using a retrospective analysis of the previous survey index, and/or via analysis of existing comparative fishing data from other time periods and regions. Concurrently, it is recommended that an age-structured stock assessment continues to be developed, aiming to provide harvest advice and an evaluation of stock status relative to reference points for the NAFO SC assessment in 2026. If an acceptable age-structured stock assessment cannot be developed, an index-based management procedure that is simulation tested in a Management Strategy Evaluation (MSE) is considered to be a suitable alternative to provide harvest advice over the longer term.