Spatial structure of body size of European flounder (Platichthys flesus L.) in the Baltic Sea

- European flounder shows a spatial gradient in length distributions with decreasing body sizes towards north-east in the Baltic Sea Proper.
- In the south-west, European flounders grow fast, and mature earlier compared to the north-east.
- Results suggest several connected European flounder stocks in the Baltic Sea Proper.

The spatial structure of fish species is important for stock identification and management. The European flounder (Platichthys flesus L.) shows morphological differences across the Baltic Sea Proper. However, it is not known whether flounders cluster into several distinct areas based on morphological characters, indicating discrete sub-populations, or whether they show continuous morphological variation along space indicating a more continuous population structure. Here, we study the spatial structure of body length and length-at-age distributions of the European flounder (Platichthys flesus L.) across the Baltic Sea Proper (International Council for the Exploration of the Sea (ICES) subdivisions 25-28) using high spatial resolution data (ICES rectangles) from fishery independent surveys 2008-2014. Our results are in agreement with genetic data suggesting a continuous gradient of decreasing body length from south-west to north-east. Further, we observed distance decay in the spatial synchrony of temporal changes in the length distributions, such that the temporal trends were correlated among adjacent ICES rectangles but independent across the whole study area. Length-at-age and maturity patterns that were calculated for each subdivision also showed a consistent spatial difference where SD 28 was significantly different from SD 25 and 26. Our results indicate that the European flounder in the Baltic Sea consists of several loosely defined sub-populations, which may warrant a reconsideration of assessment models, management targets and regulations across subdivisions.