Population structure of cod and plaice in the Baltic Sea

Background and Objective

It is crucial for the catch advice to determine the proportion of mixing of cod of eastern and western origin reliably in the area (the western Baltic management area) and in the catches of the commercial fishery. The mixing proportions of cod in SD24 have direct impact on the catch estimates, especially for the Western Baltic cod stock.

The latest ICES benchmark of the cod stocks in the Baltic Sea (ICES 2015) determined that the stock assessment for the first time should take into account the significant mixing of Eastern and Western Baltic cod in the Arkona Sea (ICES Sub-Division (SD) 24).

While this new approach will significantly reduce the uncertainty attached to previous stock assessments, the data sources used to assess the spatio-temporal dynamics of mixing suffers from several sources of uncertainty. For instance, existing data on mixing proportions based on genetic analyses were only available from very few years, and mixing proportions based on otolith shape analyses were available for seven selected years (covering the time period 1996-2014). There are no data prior to 1994, while it is known that the stock was at a record high in the 1980s and this period is thus highly relevant for the determination of reference points. Genetic samples did not originate from commercial catches but from scientific samples, and thus they might not be representative for the commercial catch composition. Spatio-temporal coverage was very limited, and the results on the use of methods alternative to genetics (e.g. analyses of otolith ring structures) were preliminary. Therefore, we will use molecular markers (Single Nucleotide Polymorphisms (SNPs)) to discriminate between the two stocks in the Baltic Sea unambiguously.

The ICES benchmark of Baltic plaice (ICES 2015 WKPLE) concluded that “the stock entity of plaice in SD 21-23 and in SD 24-32 is less well defined and available studies are inconclusive. WKPLE reviewed arguments to include SD 21-23 and SD 24-32 in one stock and had not sufficient arguments to suggest a deviation from the present perception of two stocks, a Kattegat-Belt stock (SD21-23) and a Baltic stock (SD24-32). Examination of a combined 21-23 and 24-32 assessment was initiated but further work is required”. However, the “present perception” (two stocks) originated from a vague conclusion of an earlier ICES workshop (ICES 2013 WKPESTO), which analyzed virtually very limited data on stock identification of Baltic plaice. As a conclusion, the null hypothesis that there is only one plaice stock in the Baltic Sea, has never been firmly rejected. We want to test the null mhypothesis using genetics.

Target Group

EU, ICES, HELCOM, fisheries

Approach

Cod from commercial and scientific catches sampled in the mixing area will be assigned to their population of origin using single nucleotide polymorphisms (SNPs). The genetic assignment of individuals to different stocks will be compared to their otolith structures with the goal to test the assignment power of otoliths to infer the source populations of Baltic cod. The combination of different methods will allow us to determine present and historical mixing proportions in the Arkona Sea.

We will also use SNPs to analyze the population structure of plaice in the Baltic Sea.

Results

For cod, a genetically validated method of stock separation based on otolith shape analysis has been established. Since 2019, the mixing ratios in the cod catches of the German commercial fishery and the fishery research cruises from the Arkona Sea area have been determined, submitted to ICES and are used for the stock assessment each year. Furthermore, a 40-year time series of mixing ratios of eastern and western Baltic cod in the Arkona Sea was developed, which provided the basis for the 2019 ICES cod benchmark so that the assessment time series now extends back to the 1980s for western Baltic cod and could even be extrapolated to the 1940s for eastern Baltic cod.

Analysis of plaice genetics showed that plaice in the North Sea and Baltic Sea are closely related and that there is no genetic structure within the Baltic Sea that justifies maintaining two separate stocks.