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In recent years, the topmouth gudgeon (Pseudorasbora parva) has been one of the most invasive fishes in Europe. Pseudorasbora parva can potentially affect ecosystems, fish communities, and particular fish species. Electrofishing was carried out over a five-year period at three study sites in the Ciemięga River (eastern Poland) before and after P. parva had been found in the river. Changes in the occurrence, abundance, and density of native fish species after the appearance of the invader were determined. Changes in the species’ richness were calculated, and correlations were estimated between the occurrence and density of P. parva and particular fish species and richness indices. The presence of P. parva has not affected the density of native species but coincides with a significant increase in estimated species richness and the total density of fish. There was a significant relationship between the presence of this invasive species and the fish community’s composition, though the PERMANOVA result was unclear with regards to site-specific effects. Moreover, tench and bleak were associated positively, whereas Eurasian perch and sunbleak were associated negatively with the occurrence of P. parva. Pseudorasbora parva density was highly correlated with Cyprinidae density (excluding P. parva), species richness, and the Margalef diversity index. Thise study has shown that the presence of predatory fish in the river (Salmo trutta L.) may reduce the numbers of invasive P. parva.

The brown bullhead is a fish native to North America that became an invasive species after being introduced into the waters of Europe and other regions. Studies on its sexual cycle and fecundity were conducted on a population from an oxbow lake of the central Vistula River in Poland. The fish ranged in age from 1 + to 9+. The average body length (SL) was 14.4 ± 3.4 cm. Individuals as young as 1 + were already mature. Females lay eggs multiple times from mid-April to mid-June, with absolute fecundity averaging 3227 oocytes and relative fecundity at 46 oocytes g −1 . The highest mean GSI of 1.9 during spawning was recorded in June. The reproductive tract of males takes the shape of lobes and consists of a paired cranial region formed by testes and a caudal region of undefined function. In males, semi-cystic spermatogenesis occurs, with secondary spermatocytes leaving cysts. Males overwinter with tubules filled with spermatids and initiate spermatozoa formation. The highest average GSI of 0.49 was recorded in early April. The brown bullhead observed in the new habitat was characterized by multiple egg laying, earlier maturation of individuals, and spawning in the earlier part of the calendar year, i.e. mid-April to mid-June, compared to its native habitat.

Managing invasive species such as the Chinese sleeper (Perccottus glenii) supports the goals of sustainable development by preserving native biodiversity. This study investigated the population structure, growth, and ecological impact of P. glenii in a small, anthropogenically altered tributary of the Vistula River (central Poland). Electrofishing surveys conducted between 2017 and 2023 assessed sex ratio, age structure, body size, condition (Fulton’s index), and growth parameters, as well as changes in the local fish community. The sex ratio was nearly balanced (♀:♂ = 1.00:0.99), and average standard length and weight were 6.54 cm/9.11 g (females) and 6.36 cm/7.69 g (males). Dominant individuals were from age group of 2+ years. The Fulton condition factor ranged from 2.54 to 2.58, while positive algometric growth was observed for both sexes. The von Bertalanffy growth model parameters (L∞ = 175.37 mm, k = 0.104, t0 = −1.711) revealed slower growth compared to other Eurasian populations. In the individual months of the study, changes in the sex structure, length, weight, and age of the fish were observed. This seasonality may have resulted from physiological changes (including fish growth and reproductive processes), their migration, and environmental changes, such as food availability and hydrochemical parameters, occurring during this period. Additionally, over the study period, the abundance and density of P. glenii increased significantly, coinciding with a marked decline in native fish species. These findings highlight the adaptability of this invasive fish and emphasise the need for targeted management strategies in degraded freshwater ecosystems.

Heavy metals are introduced into water due to anthropogenic activities and can significantly affect an entire ecosystem. Due to their close integration with the water environment, fish are a sensitive indicator of contamination. In addition, fish is an important element in human diets, therefore, monitoring the concentrations of metallic contaminants in their meat is particularly important for food safety. This study aimed to assess the pollution of water ecosystems with selected toxic heavy metals in lakes Dratów, Czarne Sosnowickie, and Syczyńskie. The concentration of Pb, Cd, and Hg in water, sediment, and freshwater fish muscle tissue was determined, and a food safety assessment was performed. The analysis of water and sediments showed that the sediments were characterised by a significantly higher concentration of heavy metals. Presumably, this ecosystem element plays an important role in the uptake of heavy metal contaminants by fish whose levels were higher in planktonophagous and benthophagous fish species as compared to predatory fish. The food safety assessment showed that amounts of heavy metals in the muscle tissue posed no threat to the health of consumers ingesting that fish species, neither individually (THQ) nor collectively (TTHQ).

The ecological assessment of rivers under the Water Framework Directive (WFD) requires the use of biological quality elements (BQEs) across defined abiotic types of rivers. However, limited evidence exists on how well biological indices perform across multiple typological classes, particularly under the influence of complex, overlapping stressors. This study evaluated the diagnostic performance of four biological indices (IO—diatoms, MIR—macrophytes, MMI_PL—benthic macroinvertebrates, and EFI + PL—fish) in 16 river sites in southern Poland. These were classified into four abiotic types (5, 6, 12, and 17) and subjected to varying levels of human pressure. Biological, physical and chemical, and hydromorphological data were collected along environmental gradients including conductivity, nutrient enrichment, and habitat modification. Statistical analyses were used to evaluate patterns in community composition and index responsiveness. The IO and MMI_PL indices were the most consistent and sensitive in distinguishing between reference and degraded river conditions. MIR and EFI + PL were more variable, especially in lowland rivers, and showed stronger associations with habitat structure and oxygen levels. Conductivity emerged as a key driver of biological responses across all BQEs, with clear taxonomical shifts observed. The results support the need to consider both typological context and local environmental variation in ecological classification. The findings underscore the need for typology-aware, pressure-specific biomonitoring strategies that combine multiple organism groups and integrate continuous environmental variables. Such approaches can enhance the ecological realism and diagnostic accuracy of river assessment systems, supporting more effective water resource management across diverse hydroecological contexts.

Epiphytic algae are an important group of organisms involved in primary productivity, nutrient cycling, and energy transfer in littoral food webs. However, multifactorial studies conducted on epiphyton on the same substrate across a spectrum of environmental parameters are very limited. Here, we present first complex field study on the role of abiotic and biotic factors in shaping the species richness, diversity, abundance, and biomass composition of epiphyton on common reed in four shallow lakes with different trophic status, water and fish management, and the abundance and structure of potential algal grazers: littoral crustaceans and fish. The obtained results revealed that the algal taxa richness was the lowest in the hypertrophic lake and the highest in the meso-eutrophic lake. Epiphyton abundance (predominated by pennate diatoms) and biomass were found to be the highest in eutrophic water bodies. Biomass consisted primarily of diatoms, but we found the seasonal predominance of filamentous Chlorophyta (in the eutrophic lakes) and Chlorophyceae (in the meso-eutrophic lake). Redundancy analysis (RDA) showed that the frequency of water level fluctuations was the most significant variable in the composition of epiphyton. RDA also revealed the importance of grazing pressure of fish. Thus, in hydrologically transformed lakes, man-made factors may be of great relevance in the development of epiphytic algae. Les algues épiphytiques constituent un groupe important d'organismes impliqués dans la productivité primaire, le cycle des éléments nutritifs et le transfert d'énergie dans les réseaux trophiques littoraux. Toutefois, les études multifactorielles menées sur l'épiphyton sur le même substrat portant sur un éventail de paramètres environnementaux sont très limitées. Nous présentons ici une première étude de terrain complexe sur le rôle des facteurs abiotiques et biotiques dans la formation de la richesse, de la diversité, de l'abondance et de la composition de la biomasse de l'épiphyton sur les roseaux communs dans quatre lacs peu profonds ayant différents statuts trophiques, gestion de l'eau et des poissons, ainsi que l'abondance et la structure des brouteurs d'algues potentiels: crustacés littoraux et poissons. Les résultats obtenus ont révélé que la richesse taxonomique des algues était la plus faible dans le lac hyper-eutrophe et la plus élevée dans le lac méso-eutrophe. L'abondance de l'épiphyton (dominée par les diatomées pennées) et la biomasse étaient les plus élevées dans les plans d'eau eutrophes. La biomasse se composait principalement de diatomées, mais nous avons trouvé la dominance saisonnière de Chlorophyta filamenteux (dans les lacs eutrophes) et de Chlorophyceae (dans le lac méso-eutrophique). L'analyse de redondance a montré que la fréquence des fluctuations du niveau d'eau était la variable la plus importante dans la composition de l'épiphyton. La RDA a également révélé l'importance de la pression exercée par le broutage des poissons. Ainsi, dans les lacs transformés par l'hydrologie, les facteurs anthropiques peuvent jouer un rôle important dans le développement des algues épiphytes.

The aim of the study was to determine changes in the abundance of Ameiurus nebulosus and native fish in four lakes after 4–7 years. It was hypothesised that A. nebulosus would be the dominant species of fish in various types of lakes. The study was conducted in the summer of 2014 and 2018–2021, using Nordic multi-mesh gillnets. We indicated that relative numbers and biomass of A. nebulosus decreased in the oligo-mesotrophic lake, while increased in lakes of higher trophic conditions. Body weight and total length of A. nebulosus fluctuated from 3.2 to 283.3 g and 64 to 267 mm, respectively. The coefficient of condition of this species varied widely from 0.2 to 1.8 with the highest and lowest values in the meso-eutrophic lake. Ameiurus nebulosus accounted for 0.2–34.1% of the total numbers and 2.4–36.5% of the total biomass of the fish community. In the mesotrophic and meso-eutrophic lakes, A. nebulosus was a dominant component of the fish community after 7 years. The percentage contribution of A. nebulosus to the total numbers of fish decreased with the increasing trophic status of the studied lakes. Our study showed that A. nebulosus could be one of the factors responsible for a decrease in the number of native fish species and a change in the taxonomic structure of native fish in lakes of different trophic status (from oligo-mesotrophy to eutrophy).

The freshwater habitats of semi-aquatic carnivores in Europe have undergone substantial alterations due to regulation and construction of artificial watercourses. We compared seasonal estimates of otter Lutra lutra food composition with concomitantly collected data on fish availability and otter holt location in a strongly human-modified ecosystem comprising the upper stretches of the Wieprz-Krzna Canal, the longest artificial waterway in Poland, and an associated regulated river. Concrete lining of the canal, extreme water level variation and regular vegetation clearance to enhance water flow substantially limited otter food and shelter resources, yet the canal offered attractive habitats for some fish and hibernating amphibians, while otter resting sites were available at the closely adjacent river. The taxonomic composition of fish in the otter diet, reconstructed based on faecal analysis, was largely reflected in electrofishing catches. Fish communities in our study area were numerically dominated by small-sized species and otters fed on very small fish (median seasonal total lengths: 46-54 mm). Either the smaller fish in the population were taken or prey sizes did not differ from those available. The significant contribution of amphibians and waterbirds to otter diet (seasonally 41-75% in terms of biomass) indicated poor fish supplies for otters in the canal-river system. High trophic diversity, compared to other otter studies in temperate climatic conditions, indicates that otters facing unstable food conditions exhibit great flexibility in their diet, which may facilitate colonisation of anthropogenic, depauperate environments. Otter occurrence provides little opportunity for conflict with human economic interests in ecosystems that are highly managed for purposes other than fish production. Since anthropogenic habitats are typically poor in some resources, such as shelter structures, their suitability for otters can be enhanced by ensuring good connectivity with nearby less-disturbed habitats, where the necessary resources can be supplemented.

Amur sleeper ( Perccottus glenii ) is currently one of the most invasive fish species in Europe. This inspired our research to identify potential channels of emigration in two rivers, the Vistula and the Wieprz. Amur sleeper occurrence, abundance, population size structure and impact on indicators of diversity were examined in oxbow lakes of both rivers. Amur sleeper was recorded for the first time in floodplain areas located between two rivers, which may serve as main travel corridors. Amur sleeper was noted in four out of six oxbow lakes in each river. Its relative abundance varied depending on the oxbow lake, ranging from 0.002 to 1.60 ind. m −2 in the basin of the Vistula River and from 0.04 to 0.35 ind. m −2 in the basin of the Wieprz River. Percentage share in the dominance structure ranged from 2.42 to 100 % and from 40 to over 67 % in the oxbow lakes of the Vistula River and Wieprz River, respectively. The size structure of the Amur sleeper populations was dominated by young individuals (Tl < 50 mm), which is characteristic of developing populations. The presence of Amur sleeper did not significantly affect biodiversity indicators. Its occurrence was related to the amount of vegetative cover.