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The round goby was noted for the first time in the Gulf of Gdańsk in 1990. After a few years, the round goby had invaded all suitable areas in the west part of the Gulf of Gdańsk.

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The European flounder (Platichthys flesus) is presently the third most commercially fished species in the Baltic Sea. According to the available data from the Polish Fisheries Monitoring Centre and reports from fishermen, the fitness of the species in the Baltic Sea has declined in the last years. The aim of this study was to obtain the information on the current genetic condition of the European founder and to check if the observed decline in the fitness of the species in the Baltic Sea is associated with the adverse alterations in its gene pool. For this purpose, 150 fish were collected from three locations across middle part of the Polish seacoast, i.e., Mechelinki, Władysławowo (in 2018) and Słupsk Bank (in 2020) and were genetically screened with application of ten microsatellite DNA markers. Moreover, the correlation analyses between biological and genetic characteristics in the sampled fish were carried out. The examined European flounder specimens displayed a high level of genetic diversity (PIC = 0.887–0.905, I = 2.697–2.709). Lack of significant genetic differentiation (Fst = 0.003, P > 0.05) within all the examined fish was observed, indicating that the European flounder in the sampled area constitute single genetic cluster. Significant deficiency of heterozygotes (Fis = 0.079–0.130, P < 0.05) and signs of past bottleneck(s) under Infinite Allele Model (IAM) mutation model and by Garza-Williamson index (M-value) were also detected in the examined fish. The estimated effective population size (Ne) among the sampled fish groups varied from 712 (Słupsk Bank) to 10012 (Władysławowo and Mechelinki). The obtained results in the present study suggest that European flounder in the southern Baltic Sea have recently experienced a significant census size reduction. The correlation analysis between genetic and morphological characteristics did not detect signs for directional selection and the density-dependent adaptive changes in gene pool of the examined fish.