The aim of the study was to determine the optimum concentration of MS-222 for given size groups of pikeperch and water temperatures. The study considered three size groups of pikeperch (body weight [BW] 8.56, 15.72, 52.91 g), an MS-222 water solution (50, 100, and 150 mg/L), and two temperatures (20 or 23°C). It was revealed that the optimum MS-222 concentration depended largely on the size of the pikeperch and on water temperature. For fish with a BW <10 g the recommended concentration is 100 mg/L but only at 23°C. For fish with a BW of 10-40 g at an immersion temperature of 20°C the safe MS-222 concentration ranges from 100 to 150 mg/L. However, at a temperature of 23°C the recommended concentration of the anesthetic is 100 mg/L. Similarly for larger fish, that is, fish with a BW >40 g, the optimum MS-222 concentration at a water temperature of 20°C is in the range of 100-150 mg/L, but at a temperature of 23°C is it approximately 100 mg/L. Additionally, when exposure to the anesthetic is shorter (several min), a concentration of 150 mg/L is also safe for juvenile pikeperch of this size. K E Y W O R D Sanalgesia, anesthetic, phase, pikeperch, recovery, tricaine methanesulphonate

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.

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.

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

The effect of feeding frequency (one, three, and continuous feeding), feed ration (0.2, 0.5, 0.8% of total fish biomass), and feeding per se on the oxygen consumption (OC, mg O 2 kg )1 h )1 ) and ammonia excretion (AE, mg TAN kg )1 h )1 ) of juvenile tench (body weight 15-19 g) and variations in these parameters in daily cycles were examined. Fish metabolism was studied in a recirculating system (rearing tanks of 0.2 m 3 , water temperature 23°C). It was found that oxygen consumption and ammonia excretion depended significantly on feed ration. An increase of feed ration from 0.2 to 0.8% of fish biomass caused an increase of OC and AE from 126.80 mg O 2 kg )1 h )1 and 1.95 mg TAN kg )1 h )1 to 187.35 mg O 2 kg )1 h )1 and 8.80 mg TAN kg )1 h )1 (p<0.05). There was no dependence between feeding frequency and the mean rate of oxygen consumption. However, the relationship between feeding frequency and ammonia excretion by juvenile tench was statistically significant (p<0.05). Feeding frequency significantly affected daily fluctuations of AE and OC. It was found that diurnal variations in metabolic rates were strictly related to tench feeding, and the daily variations of AE were significantly higher than OC.