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ABSTRACT. The aim of the study was to determine changes in the seasonal abundance of stone moroko, Pseudorasbora parva (Temminck and Schlegel), and to identify the role this alien species plays in the littoral zone of a heated lake. This study also aimed at defining the habitat preferences of this species and describing the relations between the relative abundance of stone moroko and the occurrence of submerged vegetation. A significant dependence was determined of the degree to which the lake bottom is covered with macrophytes and the occurrence of stone moroko. The fish caught belonged to 14 species and 2 families. Stone moroko preferred habitats that were abundantly overgrown with submerged vegetation and avoided areas devoid of macrophytes. Thus, as the bottom cover increased, so did the relative number of this fish. In light of habitat availability and its food preferences, the abundant occurrence of this species poses a serious threat to the endemic ichthyofauna.

The article presents the principles of the program to restore the Atlantic sturgeon Acipenser oxyrinchus in Poland and the results obtained to date. Since 2004, various life cycle stages (fertilized eggs, hatchlings, fingerlings) obtained through the controlled reproduction of wild spawners caught in the St John River in Canada have been imported. Broodstocks are held in three facilities, two of which were built specifically for the Atlantic sturgeon restoration programme. From each group of the initial imports from Canada, 50-100 individuals are being held for further rearing. Additionally about fifty individuals from the 1997 and 2001 year classes were transferred from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (Berlin, Germany). These are also the offspring of earlier imports from Canada. Juvenile sturgeons are used for experimental stocking and studies aimed at increasing rearing effectiveness. Rearing early life cycle stages of Atlantic sturgeon was a difficult task, the final results of which remained unpredictable in early trials. The highest losses of larvae are incurred when the fish begin exogenous feeding. Feeding studies focus on the selection of appropriate diets and feeding strategies. The latest results indicate that during the initial feeding period, the fish can be fed once daily. Studies on sturgeon migration were conducted in the Drwęca River, and the results indicated that migration rates vary significantly among juvenile sturgeon, with the mean migration rate in spring being lower than in autumn. Currently, the Polish efforts to restore the Atlantic sturgeon focusses on (i) building broodstocks at specialized facilities, (ii) improving hatching and early rearing techniques, (iii) performing experimental stocking with hatchery-reared juveniles, and (iv) acquiring knowledge on the basic elements of the Atlantic sturgeon life history under local conditions. Fertilized eggs of A. o. oxyrinchus will still have to be imported from Canada for several years before self-sufficient production from raised ex-situ brood stocks will become available. The formation of a Polish ex-situ founder population is imbedded in an overall restoration approach in close cooperation with the neighbouring country having shared sturgeon waters and this is in line with the respective international recommendations for sturgeon species conservations.

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The Konin lakes, heated by power stations and invaded by alien organisms, are a natural laboratory in which we can study the impact of climate change on the native communities of aquatic organisms. The aim of our study was to assess the impact of water heating and the occupation of the littoral zone of the lake by invasive species Vallisneria spiralis on changes in the species structure of rotifer communities of plankton, epiphyton and psammon. The archival material was used from the years: 1970–1975, 1978 and 1983, and compared with the results of studies conducted in Licheń and Ślesin Lakes in the years 2010–2011 and 2017–2018. It has been shown that the heating of waters of the studied lakes, combined with the shortening of their retention time, as well as the invasions of alien species, have caused significant changes in the taxonomic and trophic structure of plankton rotifers. In inhabiting Vallisneria bed epiphytic rotifer communities, the share of alien species did not increase, but relatively high densities of uncommon sessile species still persist. Psammon communities in the lakes are dominated by monogonont species relatively common in this habitat in nonheated lakes, but they are nearly devoid of bdelloids, which are abundant in psammon of Masurian lakes.