Spatial variation in size distribution of two freshwater species, European perch (Perca fluviatilis L.) and roach (Rutilus rutilus L.), in the shallow coastal area of the NE Baltic Sea 2007

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Patterns of distribution and abundance of different size groups of European perch (Perca fluviatilis L.) and roach (Rutilus rutilus L.) in the different zones of shallow elongated brackish-water Matsalu Bay (NE Baltic Sea) were investigated. A horizontal salinity gradient from shallow (1 m) freshwater (0-2 ppt) inner part to deeper (3 m) brackish-water (4-6 ppt) outer part occurs in the bay, and therefore provides diverse environmental conditions for various size groups of fish with different habitat optimum. The study revealed the habitat selection pattern to be quite regular over the study period (mid-summer fish monitoring 1995-2007). Perch was abundant all over the bay, whereas larger and older size classes showed preference for shallower and warmer areas of the inner part. On the contrary to perch, both abundance and size of roach increased significantly towards deeper areas of the outermost zone of the bay. The possible reasons and tendencies behind the spatial variability and gradients are discussed.