Report of the Workshop on mixing of western and central Baltic herring stocks (WKMixHER)

The workshop on mixing of western and central Baltic herring stocks (WKMixHER), chaired by Valerio Bartolino, Sweden, and Jan Horbowy, Poland, met in Gdynia the 11–13 September 2018. The meeting was attended by altogether 12 participants, including experts from Germany, Denmark, Latvia, Poland, Lithuanian, Norway and Sweden covering expertise from herring biology, biological data collection and analysis, genetics, otolith shape analysis and stock assessment.
The aims of WKMixHER were to review recent research and available methods to discriminate western Baltic spring spawning herring and central Baltic herring in mixed catches, evaluate potential implication of mixing for the assessment, develop a coordinated plan to collect and analyse relevant data to quantify the mixing.
Different methods for herring stock identification were reviewed at the workshop based on old and on-going analyses carried on samples from the western and central Baltic Sea.

Analyses presented at the meeting suggested that interpretation of the ToRs for this work-shop is actually conditioned by a better understanding of the central Baltic herring stock structure. The central Baltic herring is known to be dominated by a northern and a southern component, but for the first time at the workshop it was shown how the latter actually shares numerous characters with the adjacent western Baltic herring stock. Its growth and otolith shape are more similar to those of herring of western origin than to fish from the northern component. Based on only growth, a high proportion of fast growing herring is found in SD25 and especially in the westernmost rectangles but it remains unclear if those fish are part of the southern component of the central Baltic or if they are the results of extensive mixing with the western Baltic herring.

Preliminary analyses suggest a progressive genetic differentiation along the entire southern Baltic coasts from SD24 to SD26 rather than a clear cut division between different assessment units. Soon it became clear that the problem of separating the Central Baltic herring stock from the western Baltic spring spawning herring stock is related to understand if the southern component should be considered together with the western Baltic herring, maintained with the central Baltic herring, or if it should be considered sepa-rately. Depending on the task, the methodologies reviewed for stock identification could be promising or insufficient.
Clarify the stock structure of the central Baltic herring was not the objective of this workshop, but the issue steered the proposal for a two years coordinated plan of sampling which was delineated at the workshop with the objective to validate herring assessment units and look for operational methods to separate them in mixed catches.