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.
The European flounder (Platichthys flesus), which is closely related to the recently discovered Baltic flounder (Platichthys solemdali), is currently the third most commercially fished species in the Baltic Sea. According to the available data from the Polish Fisheries Monitoring Center and fishermen’s observations, the body condition indices of the species in the Baltic Sea have declined in recent years. The aim of the present study was to obtain information on the current patterns of genetic variability and the population structure of the European flounder and to verify whether the Baltic flounder is present in the southern Baltic Sea. Moreover, we aimed to verify whether the observed decline in the body condition indices of the species in the Baltic Sea might be associated with adaptive alterations in its gene pool due to increased fishing pressure. For this purpose, 190 fish were collected from four locations along the central coastline of Poland, i.e., Mechelinki, Władysławowo, the Vistula Lagoon in 2018, and the Słupsk Bank in 2020. The fish were morphologically analyzed and then genetically screened by the application of nineteen microsatellite DNA and two diagnostic SNP markers. The examined European flounder specimens displayed a high level of genetic diversity (PIC = 0.832–0.903, I = 2.579–2.768). A lack of significant genetic differentiation (Fst = 0.004, p > 0.05) was observed in all the examined fish, indicating that the European flounder in the sampled area constitutes a single genetic cluster. A significant deficiency in heterozygotes (Fis = 0.093, p < 0.05) and overall deviations from Hardy–Weinberg expectations (H-WE) were only detected in fish sampled from the Słupsk Bank. The estimated effective population size (Ne) among the sampled fish groups varied from 712 (Słupsk Bank) to 10,115 (Władysławowo and Mechelinki). However, the recorded values of the Garza–Williamson indicator (M = 0.574–0.600) and the lack of significant (p > 0.05) differences in Heq > He under the SMM model did not support the species’ population size changes in the past. The applied SNP markers did not detect the presence of the Baltic flounder among the fish sampled from the studied area. The analysis of an association between biological traits and patterns of genetic diversity did not detect any signs of directional selection or density-dependent adaptive changes in the gene pool of the examined fish that might be caused by increased fishing pressure.
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