Survey indices

The distribution of juvenile sole and especially of juvenile plaice has changed. An offshore shift has been observed, out of the traditional nursery areas, and out of the survey area covered by the inshore survey (DFS). The shift gradually took place, over a longer time period, for 1-group fish, but since 2018 the shift is marked and also includes 0-group fish. The 2018 year class of both sole and plaice is estimated to be strong based on the offshore surveys (BTS1, BTS2 and SNS), whereas it is only slightly above average according to the inshore survey (DFS). The current situation is unprecedented and, as the survey indices are relative and not absolute estimates of abundance, it is difficult to judge which signal is a better predicter of recruitment. Also because juveniles outside of their traditional nurseries may suffer higher mortality. In the assessment procedures, the spring advice is re-evaluated in autumn based on the BTS1 and BTS2 of that year. In the current situation, a revised short-term forecast based on survey indices will lead to a substantial increase of the TAC, mainly driven by the offshore survey data. Scrutiny of the survey data and evaluation index calculation is considered necessary to be able to deal with different signals from different surveys.

De betrouwbaarheid van survey indices voor juveniele schol en tong is gedaald vanwege veranderingen in de verspreiding tussen surveygebieden. Dit heeft implicaties voor de rekruteringschattingen die gebruikt worden in de vangstadviezen. Daarom wordt een quick-scan voorgesteld om de calculatie en integratie van survey indices te verbeteren.