The history of brown trout Salmo trutta L. stocking has long tradition in the European Union and other countries. Hundreds of hatchery facilities on continent have artificial broodstocks used for enhancement of neighbouring and also geographically far river basins. These practices have substantial effect on wild brown trout populations. To illuminate this phenomenon, eleven hatchery stocks and wild populations from northern Poland and Carpathian region were analysed using 13 microsatellite markers. Obtained results revealed high genetic diversity between studied stocks and clear differentiation between northern and southern populations and hybridization between these two major clads. As a recommendation, the principle of treating regions as metapopulations should be applied, which, in the case of Poland, means using the division of the northern and southern genetic lines that were revealed in the present study.

Fisheries Institute in Olsztyn, Poland Citation: W¹s-Barcz A., Bernaoe R., Wenne R. 2017 -The genetic approach for assessing sea trout stock enhancement efficiency -An example from the Vistula River -Arch. Abstract. Many countries in the Baltic Sea basin have initiated enhancement programs for Baltic migratory sea trout, Salmo trutta L., to compensate for losses stemming from anthropogenic pressure that has resulted in the declining population abundance of this species. Regular stock enhancement has been conducted in Poland since the 1960s. Currently, over one million sea trout smolts are released into Polish rivers annually. In most Baltic countries, including Poland, stock enhancement depends on hatcheries producing material using spawners caught in native rivers. However, increasing difficulty obtaining spawners in recent years in Poland has meant that stock enhancement performed in the Vistula has been done largely with material obtained from broodstocks. Simultaneously, there is a lack of information regarding the proportion of wild and cultured sea trout in this river basin. This paper is a review of methods applied to identify individuals from natural and artificial sea trout spawning in rivers, and it proposes using genetic techniques as an alternative to traditional marking methods. A set of 13 microsatellite loci are proposed that are characterized by high selectivity. Using negative controls while simulating the assignment of parental pairs revealed that the number of loci in the set was highly significant and should not be reduced. This method could be useful in the proposed assessment of the proportions of wild and cultured fish in Polish rivers.

The age and growth of Vistula River migratory sea trout, Salmo trutta L., caught in commercial catches in the Vistula in 2017-2018 were analyzed. The mean length of the sea trout caught was 61.4 cm in 2017 and 62.8 cm in 2018. It was confirmed that the fish caught reached an average of 56.5 cm following the first year at sea, 69 cm after the second year, and 82 cm after the third. The dominant age class among the fish in the catches was a sea age 1+. No older age classes than 3+ were noted. The age at smoltification of the sea trout examined in most instances was 2+. The results obtained were compared and discussed in light of historical results. The condition coefficient calculated was lower than that of the 1960s.

The longest Baltic River, the Vistula, historically hosted numerous sea trout populations. However, dam construction in the twenty-first century drastically reduced the spawning migration rate. Reduced natural reproduction has resulted in a population collapse and, consequentially, a substantial reduction in catches. In response, like other Baltic countries, Poland has initiated an intensive stocking program, mainly involving smolt. Initially, stocking was conducted primarily with offspring of sea trout caught during spawning migration. Currently, due to difficulties in obtaining fish, most stocking involves fish from breeding stocks. Therefore, determining the proportion of fish derived from stocking has become an important issue. Experiments based on traditional tagging did not provide sufficient material for analysis; hence, we decided to use genetic methods based on analysis of relatedness. In this study, we performed parentage-based tagging and an analysis of the origins of parent animals used for artificial spawning in 2013, and offspring returning to the Vistula in subsequent years. We based the analysis on three different algorithms and compared the results, showing that the presented methods were effective for estimating mass stocking success. The study also indicated that a certain level of natural reproduction in the Vistula continues to occur. The proportion of sea trout from spawning in 2013 in Vistula sea trout catches from 2017 to 2018 was approximately 30%.

Selection, genetic drift, and gene flow affect genetic variation within populations and genetic differences among populations. Both drift and selection tend to decrease variation within populations and increase differences among populations, whereas gene flow increases variation within populations but leads to populations being related. In brown trout (Salmo trutta L.), the most important factor in population fragmentation is disrupted river-segment connectivity. The main goal of the study was to use genetic analysis to estimate the level of gene flow among resident and migratory brown trout in potential hybridization areas located downstream of impassable barriers in one river basin in the southern Baltic Sea region. First, spawning redds were counted in the upper river basin downstream of impassable barriers. Next, samples were collected from juveniles in spawning areas located downstream of barriers and from adults downstream and upstream of barriers. Subsequently, genetic analysis was performed using a panel of 13 microsatellite loci and the Salmo trutta 5 K SNP microarray. The genetic differentiation estimated between the resident form sampled upstream of the barriers and the anadromous specimens downstream of the barriers was high and significant. Analysis revealed that gene flow occurred between the two forms in the hybridization zone investigated and that isolated resident specimens shared spawning grounds with sea trout downstream of the barriers. The brown trout population from the river system investigated was slightly, internally diversified in the area accessible to migration. Simultaneously, the isolated part of the population was very different from that in the rest of the basin. The spawning areas of the anadromous form located downstream of the barriers were in a hybridization zone and gene flow was confirmed to be unidirectional. Although they constituted a small percentage, the genotypes typical upstream of the barriers were admixed downstream of them. The lack of genotypes noted upstream of the barriers among adult anadromous individuals might indicate that migrants of upstream origin and hybrids preferred residency.

At present, four non-native gobiid species (racer goby Babka gymnotrachelus, monkey goby Neogobius fluviatilis, round goby Neogobius melanostomus and western tubenose goby Proterorhinus semilunaris) are listed as occurring in the Vistula River of Poland. In this work, the distribution and densities of gobiids in the river-mouth stretches of the Vistula tributaries flowing downstream from the Włocławek Reservoir in the Baltic Sea direction are presented. The fish were collected by electrofishing from a boat or while wading. Non-native gobiids were noted in 15 of the 18 tributaries studied. Among the 1,075 gobies caught, the western tubenose goby was the most common and occurred in 15 of the sites examined. The racer goby was registered in seven of the tributaries, while the monkey goby was recorded in only three. No round goby was caught. The highest densities of the western tubenose goby and racer goby were noted in the small stream located directly downstream from the Włocławek Reservoir. Among the species identified, the western tubenose goby seems to be more eurytopic because it was the last of these species to invade the Vistula River, but it is now the most widespread. In addition to a morphological identification of the fish, samples of each species were confirmed by an analysis of subunit I of the cytochrome oxidase (COI) barcode sequences. The genetic analysis of the COI subunit revealed that this sequence is effective for the taxonomic differentiation of the Ponto-Caspian gobies occurring in Poland. The results showed that gobiids have become a permanent element of the ichthyofauna of the Vistula and its tributaries that are available for migration Key words: invasive species, Gobiidae, Neogobius fluviatilis, Babka gymnotrachelus, Proterorhinus

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The present study describes the first record of 83 Dwarf Victorian mouthbrooder (Pseudocrenilabrus multicolor victoriae Seegers, 2000) newly discovered in Lake Naivasha, Kenya, in January 2016. Membership of the species was determined on the basis of morphometric characteristics and molecular analysis. The average length (±SD) of the fish was 6.95 ± 0.86 cm, and average weight was 5.59 ± 1.99 g. The body is normal/fusiform, the dorsal head profile convex‐shaped, the mouth is terminal with bright blue lips, and the anal fin with a reddish tip, being distinctive features of the fish. The sequences of subunit I cytochrome oxidase (COI) and cytochrome b (cyt b) genes obtained in the present study were compared with records deposited in BoldSystem and NCBI databases. Using two genetic markers enabled unambiguous determination of the species membership. The queried specimen was assigned to the Cichlidae family, as Pseudocrenilabrus multicolor victoriae (100–96.68% similarity). These results constitute the baseline study for this fish in Lake Naivasha.