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Mass fish kills caused by wastewater treatment plant (WWTP) failure are uncommon. In this study, we analyzed the recovery of the fish community in the Czarna Hańcza River after a fish kill caused by a WWTP incident in Suwałki in 2018. The research focused on brown trout, the dominant fish species in this river. The incident lasted about six hours and accounted for 7% of the mean river’s flow during the accident. The fish population recovered quickly, and ten months after the fish kill, the number of fish species and their abundance were similar to the pre-incident state, but the species structure was different. In subsequent years, fish species richness decreased conspicuously, while their numbers remained at a high, relatively constant level. Species that are more resistant to difficult environmental conditions, such as white bream and European perch, returned to the river first. Concurrently with the improvement of environmental conditions, more demanding species, including brown trout, began to dominate in the river. Fulton’s condition factors decreased significantly as the river self-purified and the density of the brown trout population increased (P < 0.05). Our results indicated that the brown trout population was able to recover relatively quick, presumably because of stocking. Other species that were not stocked required more time to recover.

The contents of 174 stomachs of great cormorants (Phalacrocorax carbo) inhabiting northeastern Poland were analyzed. Fish and hard parts of fish were found in most of the stomachs. (87.4%). Moreover, nematode and tapeworm parasites were observed in 76.4% and 4.6% of stomachs, respectively. Only 1.7% of stomachs were entirely empty. The fish identified in the stomachs were represented by 14 species. The most common species (72.2% frequency) in the diet of cormorants was European perch (Perca fluviatilis L.). Roach, Rutilus rutilus (L.) was the second most abundant prey (30.8%), followed by bream, Abramis brama (L.) (22.6%), bleak, Alburnus alburnus (L.) (16.5%), and vendace, Coregonus albula (L.) (13.5%), while nine other species occurred sporadically (frequencies < 7%). The number of fish species per stomach ranged from 1 to 5 (most often one species), while the number of individuals varied between 1 and 43 (most often 5–10 species). The length (SL) and weight of prey ranged from 4.0 to 62.0 cm and from 0.4 to 721.5 g, respectively. The most specimens were in the size range of 7–9 cm. The smallest specimens were perch and bitterling, Rhodeus amarus (Bloch), while the largest were pike and European eel, Anguilla anguilla (L.). Statistical analysis revealed significant differences in the length and weight of the fish consumed among the most abundant species.

In a commercial fishery, some fish are classified as low-value, but their classification varies in different countries. The aim of this study was to determine the abundance, contribution, and dominance of low-value fish species, such as Abramis brama < 1000 g, Alburnus alburnus, Blicca bjoerkna, Gymnocephalus cernua, Perca fluviatilis < 100 g, Rutilus rutilus < 200 g, and Scardinius erythrophthalmus < 200 g, in 145 Polish lakes of different areas, depths, and trophic statuses situated in the northern and central parts of Poland in 2021. Perca fluviatilis and R. rutilus were the most frequent low-value species (100% and 99%, respectively). The contribution of all low-value fish to the total biomass of caught fish was relatively high, ranging from 37% in the mesotrophic lake to 100% in the eutrophic lake (mean of 77 ± 14%). Lakes in which the contribution of low-value species exceeded 90% were relatively numerous (24 lakes, 17% of the studied lakes). Among a total of about 437.5 thousand low-value fish, 261 thousand specimens (60%) had a body weight of below 10 g. All low-value fish species, except for P. fluviatilis and S. erythrophthalmus, were related to the studied environmental variables. The relative biomass of these species increased with increasing lake productivity, while it decreased with the increasing maximum and mean depth of the studied lakes. The high contribution of low-value fish to the total biomass in many lakes indicates the need for the constant monitoring of the abundance and structure of fish communities and the use of appropriate actions (biomanipulation and stocking with piscivorous fish species) to improve the ecological condition of lakes. [ABSTRACT FROM AUTHOR]

Some alien invasive fish species, such as, for example, the brown bullhead Ameiurus nebulosus can be the dominant component of the native fish community. The aim of this study was to determine the abundance and size structure of A. nebulosus and its contribution to the ichthyofauna of mesotrophic Lake Jegocinek (Masurian Lake District, north-eastern Poland). In August 2020, a total of 723 individuals of A. nebulosus were caught by benthic and pelagic Nordic multi-mesh gillnets. This alien species probably originated from an illegal release into the lake. Total length and body weight of A. nebulosus ranged from 77 to 248 mm (mean of 146 ± 55 mm) and from 4.2 to 241.3 g (mean of 57.0 ± 54.9 g), respectively. The population of A. nebulosus was mainly represented by two size groups. The first group was composed of small-sized specimens (below 100 mm and < 10.0 g), while the second group consisted of large individuals (above 180 mm and 70.0 - 110.0 g). This indicates that the population in this lake has a multigenerational structure. Our study showed that A. nebulosus accounted for 19% of the total numbers and 21% of the total biomass of all caught fish. The total length and body weight did not differ significantly between the littoral and pelagic zones. Relative numbers (NPUE) were lower in the littoral than in the pelagic zone, while relative biomass (WPUE) was very similar in both zones. Although the values of the coefficient of condition were almost identical in the littoral and pelagic zones, it seems that the near-shore waters offer more favourable environmental conditions for the growth of A. nebulosus than deeper open waters of a lake. [ABSTRACT FROM AUTHOR]

Global warming, eutrophication, fisheries overexploitation, species invasions, and habitat loss are the major threats to freshwater biodiversity. The aim of this study was to determine the species richness and diversity of fish in 535 Polish lakes of different morphometry (area of more than 50 ha and a maximum depth ranging from 0.4 to 108.5 m) and trophic status (from oligotrophy to hypereutrophy). A total of 39 fish species were found in the studied lakes, among which eight species were alien invasive. The Shannon diversity index varied between 0 and 2.04.The most common and frequent species were Rutilus rutilus (99.8% frequency) and Perca fluviatilis (99.6% frequency). Ten fish species, including five alien ones, were characterized by a very low frequency (<1%). The number of fish species in single lakes ranged from 1 to 19. In most of the studied lakes, 11 and 12 species (104 and 108 lakes, respectively) were caught. The richest taxonomic composition (19 species) was recorded in the meso-eutrophic lake with an area of 80 ha and a maximum depth of 34.4 m, slightly poorer (18 species) in the hypereutrophic lake with an area of 168 ha and a maximum depth of 2.7 m. The poorest taxonomic composition (one species) was found in a lake with low conductivity and circumneutral pH. Statistical analysis showed that the number of fish species decreased with increasing eutrophication conditions of the studied lakes, while it increased with an increasing area and the maximum depth of these lakes. [ABSTRACT FROM AUTHOR]

Some alien invasive fish species, such as, for example, the brown bullhead Ameiurus nebulosus can be the dominant component of the native fish community. The aim of this study was to determine the abundance and size structure of A. nebulosus and its contribution to the ichthyofauna of mesotrophic Lake Jegocinek (Masurian Lake District, north-eastern Poland). In August 2020, a total of 723 individuals of A. nebulosus were caught by benthic and pelagic Nordic multi-mesh gillnets. This alien species probably originated from an illegal release into the lake. Total length and body weight of A. nebulosus ranged from 77 to 248 mm (mean of 146 ± 55 mm) and from 4.2 to 241.3 g (mean of 57.0 ± 54.9 g), respectively. The population of A. nebulosus was mainly represented by two size groups. The first group was composed of small-sized specimens (below 100 mm and < 10.0 g), while the second group consisted of large individuals (above 180 mm and 70.0 - 110.0 g). This indicates that the population in this lake has a multigenerational structure. Our study showed that A. nebulosus accounted for 19% of the total numbers and 21% of the total biomass of all caught fish. The total length and body weight did not differ significantly between the littoral and pelagic zones. Relative numbers (NPUE) were lower in the littoral than in the pelagic zone, while relative biomass (WPUE) was very similar in both zones. Although the values of the coefficient of condition were almost identical in the littoral and pelagic zones, it seems that the near-shore waters offer more favourable environmental conditions for the growth of A. nebulosus than deeper open waters of a lake. [ABSTRACT FROM AUTHOR]

The contents of 174 stomachs of great cormorants (Phalacrocorax carbo) inhabiting northeastern Poland were analyzed. Fish and hard parts of fish were found in most of the stomachs. (87.4%). Moreover, nematode and tapeworm parasites were observed in 76.4% and 4.6% of stomachs, respectively. Only 1.7% of stomachs were entirely empty. The fish identified in the stomachs were represented by 14 species. The most common species (72.2% frequency) in the diet of cormorants was European perch (Perca fluviatilis L.). Roach, Rutilus rutilus (L.) was the second most abundant prey (30.8%), followed by bream, Abramis brama (L.) (22.6%), bleak, Alburnus alburnus (L.) (16.5%), and vendace, Coregonus albula (L.) (13.5%), while nine other species occurred sporadically (frequencies < 7%). The number of fish species per stomach ranged from 1 to 5 (most often one species), while the number of individuals varied between 1 and 43 (most often 5–10 species). The length (SL) and weight of prey ranged from 4.0 to 62.0 cm and from 0.4 to 721.5 g, respectively. The most specimens were in the size range of 7–9 cm. The smallest specimens were perch and bitterling, Rhodeus amarus (Bloch), while the largest were pike and European eel, Anguilla anguilla (L.). Statistical analysis revealed significant differences in the length and weight of the fish consumed among the most abundant species. [ABSTRACT FROM AUTHOR]

The invasion of aquatic environments by alien fish species is an important issue in ecological research. Alien fish species, such as Ponto-Caspian gobies, can impact native communities and ecosystems through predation, competition, parasite vectors, disturbance, and habitat modification. In this study, we present new locations of monkey goby Neogobius fluviatilis in the waters of the Great Masurian Lakes system (north-eastern Poland). Four methods (Nordic gillnets, electrofishing, fishing rod, and Vulkan crayfish trap) were used for fishing. The presence of N. fluviatilis was recorded in nine lakes of different morphometry and trophic status, one canal, and at one site of the Pisa River in 2018–2020. Total length and body weight of N. fluviatilis were rather small (42–113 mm; mean of 81 ± 19 mm and 0.3–13.7 g; mean of 6.4 ± 3.6 g, respectively). The most specimens were in the range of 80 to 100 mm. Our study documented that N. fluviatilis inhabits lakes, rivers and canals. The invasion of N. fluviatilis into the Great Masurian Lakes system occurred across the Pisa River from the Narew River. In the Great Masurian Lakes, the northernmost place in the system in which N. fluviatilis was recorded was Lake Niegocin. We may expect that N. fluviatilis can quickly spread farther, reaching lakes situated in the northern basin of the system in the near future. [ABSTRACT FROM AUTHOR]

Two short-term (24 h and 48 h) microcosm experiments using natural waters from three eutrophic lakes (Masurian Lake District, Poland) were performed to assess the impact of eukaryotic inhibitors (a combination of cycloheximide and colchicine) on the abundance of nanoflagellates and small ciliates in the <15 µm fraction. The results showed that eukaryotic inhibitors were not completely effective against either group of protists; however, they reduced their numbers considerably. At 24 h of the experiment, 41, 15, and 7% of nanoflagellate and 48, 23, and 3% of ciliate abundances were not lysed, depending on the lake from which water was taken. However, after 48 h of incubation, only below 7% of nanoflagellates and 33, 40, and 17% of ciliates were present in the treatments with inhibitors. Our results suggest that inhibitors may indirectly change bacterial growth and activity, but they do not definitively inhibit these processes. It was concluded that eukaryotic inhibitors are more effective against small nanoflagellates than larger nanoflagellates and ciliates. Concentrations of inhibitors higher than 200 and 100 mg l-1 for cycloheximide and colchicine, respectively, and an incubation time longer than 24 h also seemed to be more appropriate to achieve the complete inhibition of protists. [ABSTRACT FROM AUTHOR]

The structure and abundance of winter planktonic communities are limited by environmental conditions that are unfavorable to life, but do not stop biological processes. The aim of the study was to test the hypothesis that the abundance of planktonic groups of organisms in temperate lakes can be higher during winter under ice cover than during the ice‐free seasons. Physical (ice/snow cover, temperature), chemical (organic carbon, chlorophyll a, phosphorus, nitrogen) and biological parameters (phytoplankton, protozooplankton, zooplankton) were investigated in three eutrophic lakes during two consecutive years in winter, spring, summer and autumn. The results of this study showed that in all lakes chlorophyll a concentrations were usually lower in winter than in other seasons, while total nitrogen concentrations were higher in winter than in autumn. All groups of planktonic organisms were able to reach similar or even higher abundances in winter under‐ice than in other ice‐free periods, depending on the lake morphometry and the year of the study. Winter taxonomic structure of phytoplankton was similar to the spring (medium‐sized Lake Dgał Wielki with the dominance of cryptophytes), summer (small‐sized Lake Dgał Mały with cyanobacteria as a dominant group) or autumn (the shallow lake with the dominance of diatoms) structure, depending on the lake. In all lakes, the structure of the ciliate community was the most similar to that of the spring with the clear dominance of Oligotrichida. Our results show that some species among all the studied groups of organisms can lead an active and dynamic life under the ice cover. [ABSTRACT FROM AUTHOR]

The thickness and duration of ice cover are strongly influenced by global warming. The aim of this study was to determine chemical (organic carbon, total nitrogen and phosphorus concentrations) and biological (nanoflagellates, ciliates, phytoplankton, rotifers, crustaceans) parameters under the ice cover in three eutrophic lakes (Masurian Lake District, Poland), differing in their morphometry and fisheries management. All the studied groups of organisms showed high variability over a short time. Taxonomic composition of planktonic communities, except for rotifers and phytoplankton, was similar in all lakes. Nanoflagellates were dominated by autotrophic forms, while ciliates were primarily composed of small oligotrichs and prostomatids. Nano-sized diatoms and mixotrophic cryptophytes were the most important components of phytoplankton and they formed an under-ice bloom in one lake only. Rotifers were mainly represented by Keratella cochlearis, Polyarthra dolichoptera and Asplanchna priodonta. Among crustaceans, copepods clearly dominated over cladocerans. Our research suggests that winter was a very dynamic period. In the under-ice conditions, pelagic organisms were strongly dependent on each other. The shallow lake and the deeper, small lake differed significantly in nutrient and chlorophyll concentrations, ciliate and phytoplankton biomass and the ratio of autotrophic to heterotrophic biomass. These results suggest that morphometric parameters may affect planktonic organisms during the ice-covered period. [ABSTRACT FROM AUTHOR]

Cyanobacterial blooms are tightly related to increasing trophic conditions of lakes and climate warming. Abiotic and biotic parameters were studied in a shallow lake, in which the island with the largest cormorants colony in north-eastern Poland is situated. We hypothesized that the strongest cyanobacterial blooms will persist near the cormorant's island and will decrease with an increasing distance from it. Filamentous cyanobacteria (Pseudanabaena, Planktolyngbya, Limnothrix, Planktothrix) were the main phytoplankton components during summer and autumn. Their strongest blooms (up to 66 mg L−1) were recorded near the roosting area. The content of nutrients and chlorophyll a, and the biomass of phytoplankton (primarily cyanobacteria) and zooplankton, decreased gradually with the increasing distance from the island. The changes from hypertrophic to eutrophic conditions were confirmed by a decrease in values of the trophic state index from 72 (site 1) to 58 (site 5). This all suggests that cormorants might have a significant impact on the deterioration of water quality (at distance to 1.6 km) and can contribute to faster water eutrophication. Our results suggest that protection of breeding sites for many waterbirds, such as cormorants, becomes a real threat for the functioning of aquatic ecosystems due to a large load of nutrients. [ABSTRACT FROM AUTHOR]