Search

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]

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 abundance, biomass, size distribution, and taxonomic composition of bacterial and protistan (heterotrophic and autotrophic nanoflagellates and ciliates) communities were investigated in six lakes of Masurian Lake District (north-eastern Poland) differing in trophic state. Samples were taken from the trophogenic water layer during summer stratification periods in the years of 2004 and 2006. Image analysis techniques with fluorescent in situ hybridization (FISH) as well as [3H]-methyl-thymidine incorporation methods were applied to analyze differences in the composition and activity of bacterial communities. The greatest differences in trophic parameters were found between the humic lake and remaining non-humic ones. The same bacterial and heterotrophic nanoflagellate (HNF) cell size classes dominated in all the studied lakes. However, distinct increases in the contributions of large bacterial (>1.0 microm) and HNF (>10 microm) cells were observed in eutrophic lakes. The bacterial community was dominated by the beta-Proteobacteria group, which accounted for 27% of total DAPI counts. Ciliate communities were largely composed of Oligotrichida. Positive correlations between bacteria and protists, as well as between nanoflagellates (both heterotrophic and autotrophic) and ciliates, suggest that concentrations of food sources may be important in determining the abundance of protists in the studied lakes.

The effects of inorganic nutrients (N, P) enrichment of mesotrophic lake water on changes in bacterial and protistan (heterotrophic nanoflagellates and ciliates) communities compositions were studied in the mesocosm experiment. Phosphorus (PO4(3-)) and nitrogen (NH4+) alone and in combination were added to three types of experimental mesocosms. Mesocosms results suggested that simultaneous addition of P and N stimulated phytoplankton growth and production rates of bacterial biomass its turnover rate. Strong positive correlations between chlorophyll a and bacterial secondary production rates suggested that bacteria were mainly controlled by organic substrates released in course ofphytoplankton photosynthesis. Both nutrients increased distinctly protistan biomass and resulted in the shift in ciliate community composition from algivorous to large omnivorous species. The response of bacterial numbers and biomass to nutrients addition was less evident. However, intensive grazing caused their dynamic changes. Fluorescence in situ hybridization (FISH) revealed only small changes in bacterial taxonomic composition. There was an apparent shift in dominance from Cytophaga-Flavobacterium to the Alphaproteobacteria group in the mesocosm with simultaneous addition of P and N, which positively related to increased abundance of bacterivorous protists. Experiment demonstrated that inorganic N and P nutrients directly influenced the bottom-down control of microbial communities, which had a crucial effect on morphological diversity of bacteria.

Effects of mesotrophic lake water enrichment with organic phosphorus and nitrogen substrates (DNA and model protein, bovine serum albumin--BSA) on dynamics and diversity of natural microbial communities (bacteria, heterotrophic nanoflagellates, ciliates) were studied in mesocosm experiments. Simultaneous enrichment with DNA and BSA strongly increased the abundance and biomass of all studied groups of microorganisms and induced changes in their morphological and taxonomic structure. The increased participation of large heterotrophic nanoflagellates cells (larger than 10 microm) in their total numbers and shifts in taxonomic and trophic structure of the ciliates, from algivorous to small bacterivorous, species were observed. Grazing caused changes in bacterial size distribution in all enriched mesocosms. Large (10-50 microm) filamentous bacteria significantly contributed to the total bacterial numbers and biomass. Pronounced increase in populations of beta- and gamma-Proteobacteria was found in lake water enriched with organic P and N sources, whereas alpha-Proteobacteria did not change markedly in the studied mesocosms. DNA additions stimulated the rates of bacterial secondary production. BSA shortened the rates of bacterial biomass turnover in lake water. Relatively high and constant (approximately 30%) percentage contribution of active bacteria (MEM+) in two mesocosms enriched with DNA and DNA+BSA suggested the important role of nucleic acids as a source of phosphorus for bacterial growth, activity and production. Numerous and statistically significant correlations between bacteria and protists indicated the direct and selective predator-prey relationship.