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Workshop on the use of surveys for stock assessment and reference points for rays and skates (WKSKATE; outputs from 2020 meeting)

ICES
2021

Record created 13/08/2021 | Last updated 13/08/2021
ICES Scientific Reports volume 3, issue 23
DOI: https://doi.org/10.17895/ices.pub.7948

Abstract

The ICES Working Group in Elasmobranch Fishes (WGEF) has not significantly changed the basis of assessment for most of its stocks in the last ten years. Over that time-period, new and existing surveys have begun or expanded. ICES has changed its assessment methods for other, primarily demersal stocks. WKSKATE was convened to determine if the best possible sources of data, methods and advice basis were used when assessing skate and ray stocks. North Sea stocks were prioritised, but stocks in Biscay and Iberia were also examined. For the North Sea, four example stocks were examined. These were • Raja brachyura in 4a and 6 • Amblyraja radiata in 2, 4 and 3a • Raja clavata in 4, 3a and 7d and • Raja brachyura in 4c and 7d These stocks were chosen to reflect the variety in surveys available, the different ICES categories used in advice setting and variety of catch. In each case, the number of available surveys was looked at in detail. Different methods of combining surveys were presented, and the effects on the perception of stock status on the inclusion or exclusion of particular surveys were compared. Methods examined The group examined stock assessments using different surveys, and different methods of combining surveys. Raising methods were also discussed; Swept-area indices were examined in detail, with particular discussion over the raising methods used e.g raised to stock area, ICES division, or geographic area. Methods for deciding how and whether surveys should be used were agreed. Data issues Some issues were discovered during the workshop. In particular, there are differences between survey data held on DATRAS and survey data held by national institutions. There are also apparent errors in how DATRAS calculates distance, important when using swept-area calculations. National institutions also differ in their raising methodologies. Results Overall, while the effects of the addition or exclusion of different surveys tended to change the relative levels of the stock, the overall trends remain unchanged. This is considered positive as it implies that new surveys can be used in future advice without having to disregard previous impressions of stock status. In most cases, comparisons between utilising different levels would not change the advice in recent years, as changes in relative values are greater than the 20% ICES Uncertainty Cap, and so actual annual advice would be unchanged in the short term. The outputs from this workshop will be reviewed. Where appropriate, the methods for the four sample stocks will be replicated for remaining stocks by WGEF and used as an advice basis for the next advice rounds. Further workshops may follow a similar process for Celtic Seas and Biscay and Iberian skate and ray stocks (2022) and catsharks and other sharks (2023).