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

The ICES Working Group in Elasmobranch Fishes (WGEF) has not significantly changed thebasis of assessment for most of its stocks in the last ten years. Over that time-period, new andexisting 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 ofdata, methods and advice basis were used when assessing skate and ray stocks. North Sea stockswere 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 categoriesused in advice setting and variety of catch. In each case, the number of available surveys waslooked at in detail. Different methods of combining surveys were presented, and the effects onthe 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 combiningsurveys. 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 wereagreed.

Data issues

Some issues were discovered during the workshop. In particular, there are differences betweensurvey data held on DATRAS and survey data held by national institutions. There are also apparenterrors 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 therelative levels of the stock, the overall trends remain unchanged. This is considered positive asit implies that new surveys can be used in future advice without having to disregard previousimpressions of stock status. In most cases, comparisons between utilising different levels wouldnot change the advice in recent years, as changes in relative values are greater than the 20% ICESUncertainty 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 foursample stocks will be replicated for remaining stocks by WGEF and used as an advice basis forthe next advice rounds. Further workshops may follow a similar process for Celtic Seas and Biscayand Iberian skate and ray stocks (2022) and catsharks and other sharks (2023).