Benchmark Workshop on three Greenland cod stocks (WKGREENCOD)
Available fisheries, scientific survey and biological data (including genetic data) was evaluated for their appropriateness to assess three Greenland cod stocks during a data evaluation workshop in December 2022. The main objective of the data evaluation workshop was to decide on assigning all available data to three stocks based on genetic analysis, and subsequently assess whether the resulting data is of sufficient quality to have age-based assessments (category 1 stocks) of all three stocks.
Genetic analysis of data was based on survey and commercial fisheries samples collected since 2000. This work was presented before the data evaluation workshop during a meeting at DTU Aqua in September 2022. The number of samples between years and areas varied, and subsequently genetic information was grouped by year classes.
The underlying assumption, based on genetic work, is that cod caught on the western and eastern Greenland shelf belong to a mix of three cod stocks: the East Greenland, West Greenland inshore and West Greenland offshore stock. This genetic classification, based on data collected from commercial catches and during scientific surveys, is the bases for the subsequent stock assessment and setting reference points.
Three separate tuning series were previously used for assessment purposes. The West Greenland gillnet survey, the Greenland shrimp survey, and the German survey. An additional survey, the Greenland halibut survey, was presented at the data evaluation workshop as an additional data source and was included in an initial attempt to combine the survey data from the latter three surveys in a statistical model (INLA) to produce a single survey index. Subsequent model runs however revealed that this particular survey introduced more uncertainty in the survey estimate and it was thus excluded in the calculation of a tuning series for the SAM assessments.
Based on the genetically split data SAM assessments were produced for initially three stocks. However, the West Greenland GRI, which was assessed by using a combined gillnet survey time series, after several configuration changes, produced an assessment which was not ideal based on the retrospective SSB pattern. It was subsequently recommended to assess the stock as two separate units – GRI south and GRI north. This decision was supported by available scientific-, survey- and fisheries data. Tagging data suggests that movement of fish between the main areas within each area, i.e. Nuuk and Sisimiut, is very limited, while treating survey data separately improves internal consistencies between ages. Furthermore, fisheries catches as well as survey indices show opposite trends in recent years, suggesting independent biological processes taking place. Separate assessments were therefore also run for the northern and southern part of the stock, and the final assessments for each of the areas was sufficiently good to be accepted as final. In this report the recommended split assessments are presented.
The estimation of references points for all three stocks proved to be challenging, given the limited length of the time series. The estimation of Blim for all three stocks was problematic, since all stocks produced relatively high recruitment at fairly low SSBs.
Estimated Blims are resultantly relatively low. This was highlighted by one of the external reviewers after the benchmark, and an alternative for calculating Btrigger was presented.
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