Benchmark session for the assessment of sprat in GSA 29
27 November, 2018 -28 November, 2018
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EXECUTIVE SUMMARY The Subregional Group on Stock Assessment for the Black Sea (SGSABS) benchmark session for the assessment of sprat in GSA 29 was held at in Constanta, Romania, on 27–28 November 2018. The objective of the meeting was to perform a full analysis and review of the information and methods used to provide advice on the status of the stock, focusing on the consideration of old and new data sources as well as old and new (or improved) assessment models and assumptions. The session was attended by a total of 36 experts, including experts from the region, experts on the species and/or stock assessment models discussed, as well as an external reviewer. The session investigated all available input data and carried out an analysis of the performance of three different stock assessment models: state-space assessment model (SAM), Integrated Catch-at-age Analysis (ICA) and Extended survivor analysis (XSA), all were tested with different assumptions and/or input data series. ICA with fully selected age of two and all XSA models run produced very similar results overall, especially for the more recent part of the time series. SAM provided a lower perception of spawning stock biomass (SSB) and a higher perception of F, although the final year of XSA was within the confidence bounds of the SAM. The historical part of the quantities estimated by SAM was very different from ICA and XSA; this discrepancy is ascribable to the different modeling approach. All models showed a cyclical pattern in both recruitment and SSB. XSA and ICA also showed overall decreasing trends in these quantities, coupled with an increasing trend in F, which SAM did not show. For practical reasons related to operational constraints making the running of ICA models virtually impossible, it was agreed not to use ICA in the future. Pending further work and, given the expert perception on the stock (i.e. a decrease in SSB and length-structure of the stock), the Group agreed with the overall results of XSA towards providing precautionary advice of not increasing fishing effort for the Black Sea sprat stock, temporarily considering its status as uncertain, while further investigating methodological and data-related issues. Considering the trend in the past years, the partial reduction in stock size and catches may be a consequence not only of fishing mortality but also of the environment and the cyclical nature of the stock.