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The aim of this study, conducted in accordance with both European Union and national legislation, was to evaluate the physicochemical conditions of the Rega River in northern Poland, where efforts are underway to restore its watershed for migratory fish. From 2018 to 2022, water temperature, dissolved oxygen (DO), pH, total suspended solids (TSS), conductivity, alkalinity, total hardness (TH), biochemical oxygen demand (BOD5), chemical oxygen demand (CODCr), nitrite nitrogen (NO2-N), ammonium nitrogen (NH4-N), and total phosphorus (TP) were measured at nine monitoring sites. The results showed that TSS and nitrite nitrogen (NO2-N) were the main factors threatening the suitability of the Rega River as a habitat for salmonid fishes. Additionally, maximum water temperatures occasionally exceeded the tolerance limits for these fishes. Seasonal variations in nitrogen and phosphorus forms, and the dependency of nitrogen forms on DO, indicated disruptions in natural biochemical processes due to surface runoff pollutants from the watershed. This includes fertilizer runoff from agricultural fields and municipal pollution from the towns through which the river flows. The findings underscore the need for continuous monitoring of Rega River waters to safeguard the habitat conditions required by environmentally sensitive fish species.

The aim of the study was to determine the influence of feeds with or without β-glucan-based immunomodulatory supplements (groups BF and UB or B and U, respectively; feeding period 14 and 28 days) on the welfare of brown trout. The diets tested did not influence the rearing indices. The type of feed and the feeding period were confirmed to have a significant influence on the white blood cell (WBC) count. WBC counts were lower than those in the initial sample particularly on day 14 of the experiment. Red blood cell (RBC) counts and hematocrit (HCT) were determined by feeding period, and the values of these parameters decreased the longer the tested feeds were applied. These changes were accompanied by increases in the values of mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC). Feed type determined levels of chloride ion (Cl-) and ammonia (NH3) and alkaline phosphatase (ALP) activity and feeding period affected sodium (Na+) and albumin (ALB) concentrations, while aspartate aminotransferase (AST) activity depended on both factors. Changes in leukograms were noted in all the groups, inter alia, significant increases in the share of lymphocytes, and after changing diets, decreases in the percentage of neutrophils, myeloperoxidase activity (AMPO) in phagocytes and cidal ability (CA). After day 28, AMPO and CA values increased and were significantly higher in groups BF and UB.

In 2022-2023 specimens of A. fallax are recorded for the first time in approximately 17 years in Szczecin Lagoon. We collected 11 specimens of juveniles, 9 premature and mature females, and 9 premature and mature males (20.3-40.7 cm of total length, aged from 1+ to 4+). The 2+ aged fish was slightly dominated (34.5% of the whole sample). Females in the Szczecin Lagoon were ready to spawn in the age of 3+ and 4+ years, while males partly in 2+, and in the age of 3+ and 4+. Fulton's condition factor of this fish was 0.93±0.14 and Clark's 0.78±0.14. Analysis of correlations between the total length and individual weight of fish revealed that the growth of A. fallax was allometric (b < 3.0). The range of gonadosomatic index (GSI) was 0.12-26.59%, while the mean of the absolute and relative fecundity was 58,756 eggs per female and 139,650 eggs per kilogram of body weight. The obtained results indicate the occurrence of an anadromous population of Alosa fallax in the studied water areas. However, there is still a need to obtain detailed information on the population status and the biology of A. fallax in these areas in order to designate special conservation areas (SACs) for their protection, especially taking into account spawning habitats. [ABSTRACT FROM AUTHOR]

Simple Summary: The aim of this study was to provide a detailed characterization of the reproductive strategy, embryonic development, and larval development of three fish species from the genus Cichlasoma: the green terror (Andinoacara rivulatus), the red discus (Symphysodon discus), and the jaguar cichlid (Parachromis managuensis). Eggs for the study were obtained from five pairs of each species (300 eggs from each female) and incubated at 26 °C. Developing eggs were observed from fertilization to larval hatching, up to the complete absorption of the yolk sac. The results of the study will contribute to the development of reproductive biotechnology for the studied fish, enabling effective and easy breeding of these fish under controlled conditions. This will not only meet the demand for these species in the aquarium trade but also protect them in their natural habitat by discontinuing fishing activities. The purpose of this study was to characterize in detail the reproductive strategy, course of embryogenesis, and development of larvae in three species of fishes of the genus Cichlasoma: the green terror (Andinoacara rivulatus), the red discus (Symphysodon discus), and the jaguar cichlid (Parachromis managuensis). Eggs for the study were obtained from five pairs of each species (300 eggs from each female) and incubated at 26 °C. The developing eggs were observed under a microscope (Carl Zeiss Stereo Discovery. V12 and Nikon 2000SE software (NIS-Elements F 4.30.01 64-bit) from fertilization to larval hatching until complete yolk-sac resorption. The largest average number of eggs per female was found in the jaguar cichlid ( x ¯ = 2991 eggs), a smaller average number of eggs was shown in the green terror ( x ¯ = 922 eggs), and the red discus showed the smallest average number of eggs ( x ¯ = 300 eggs). There were significant differences in the sizes of the eggs of the studied species: jaguar cichlid eggs were the smallest (1.060 ± 0.05 mm3), red discus eggs were larger (1.070 ± 0.07 mm3), and green terror eggs were the largest (1.365 ± 0.16 mm3). The embryogenesis time in the red discus was 2132 °H (82 Hpf), in the green terror it was 2158 °H (83 Hpf), and the longest in the jaguar cichlid was 2470 °H (87 Hpf). At the end of embryogenesis, the average size of the larvae after hatching was measured (red discus x ¯ = 4.346 mm, green terror x ¯ = 5.203 mm, and jaguar cichlid x ¯ = 5.301 mm) and the time of yolk-sac resorption from the moment of hatching to the transition from endogenous to exogenous feeding was determined (jaguar cichlid 5 days, green terror 6 days, and red discus 3 days). The results of this study may contribute to the development of reproductive biotechnology for the studied fishes that could be used in aquaculture and, thus, help protect them in their natural habitats. [ABSTRACT FROM AUTHOR]

The native North American bivalve species Rangia cuneata was unintentionally introduced into European waters during the first decade of the 21st century. In the Baltic Sea, it is mostly found along the southeastern coast, but in 2018 researchers also discovered the species in the Bay of Pomerania, which indicated that it could eventually inhabit the adjacent Szczecin Lagoon and Odra River. In 2021, the species was discovered for the first time in the Szczecin Lagoon during a sampling campaign, at 5 out of the 12 dispersed study sites with diverse bottom substrates. The goal of this study was to ascertain R. cuneata population density, morphometric parameters, individual growth, and the potential for further expansion in the southern Baltic Sea waters. For the study, 201 individuals of this species were collected. Compared to other sites in the southeast Baltic, the Szczecin Lagoon had a much lower average R. cuneata population density, at 13.2 ± 7.11 individuals m-2 of the bottom area. The highest population density was found at sites with more silt (4--63 µm) and less sand (>63 µm). R. cuneata shells had an average length of 30.9 ± 4.6 mm and an average weight of 6.6 ± 2.8 g. The collected specimens were greater in size than other populations of the species in the Baltic Sea and were comparable in size to populations from the nearby Bay of Pomerania. There were no specimens that were under 10 mm in length, and the population was dominated by specimens in the 25-30 mm and 30-35 mm ranges, as well as the 3+ and 4+ age groups. Given the R. cuneata's invasive potential and its fast rate of colonization of new areas, it would be prudent to monitor this population and the species migration patterns across the estuary waters of the western Baltic. [ABSTRACT FROM AUTHOR]

The paper discusses the current prognoses of aquaculture development worldwide putting an emphasis on its effect on the environment and the issue of the protection of water reservoirs in different countries. Water consumption in diversified aquaculture systems is presented herein as well as the characteristics of the mechanical and biological water treatment methods in fish farms, with particular attention paid to the recirculating water systems. New aquaculture technologies using post-production waters are presented. The paper provides a discussion on the contribution of aquaculture to the global greenhouse gas emissions and the means of limiting this emission. The effect of climate change on aquatic ecosystems is presented in the context of the changes of the aquaculture production profile. The paper includes a brief presentation of the methods of mitigating the changes with respect to contamination of aquatic ecosystems as well as climate change. Reducing the water footprint can be achieved through selective breeding, species diversification and implementation of more technologically advanced aquaculture systems such as: integrated multi-trophic aquaculture, aquaponics and recirculation systems in aquaculture. The need for certification of fish farms with water recirculation systems is justified in the paper. The issues addressed herein are summarised and the main areas for extending the research promoting preservation of aquatic ecosystems in aquaculture are presented. [ABSTRACT FROM AUTHOR]

Magnetoreception is the ability of organisms to perceive magnetic fields in the surrounding environment and changes in its properties such as field direction, intensity and gradient, where the effect on organisms can manifest as an array of reactions. As the magnetic sense is found in many taxa, both evolutionarily young and old, it can be assumed that magnetoreception came into existence as one of the first sensory systems. Many studies on the effect of magnetic fields on fishes have considered both fishes that migrate for long distances and those that are more or less sedentary. Research has focused on tracing the perception of the geomagnetic field by fishes and understanding magnetic fields that are smaller and larger than the ambient Earth's geomagnetic field. The question of the effect of magnetic fields of values higher than the Earth's is gaining importance with the increasing effect of anthropogenic magnetic and electromagnetic fields in aquatic ecosystems. This review draws together the results of studies on the effect and reception of natural and human‐generated magnetic fields on fishes at various stages of ontogeny, chronologically arranged from gametes, through embryonic development, embryonic and larval motor function, directional reactions of embryos and larvae, orientation of fishes, to the mechanisms of magnetic field reception. The present state of knowledge indicates a common nature of effect on various ontogenetic stages of fishes. However, understanding of the mechanisms of magnetic sense in fishes and its relevance for ecological outcomes highlights that further progress requires more detailed research. [ABSTRACT FROM AUTHOR]

Background. The stocks of cod, Gadus morhua Linnaeus, 1758, have been declining within recent decades and the Baltic Sea is one of the most affected areas. The latter body of water with its low salinity, pollution, and vast areas of demersal hypoxia is particularly vulnerable. Areas of natural spawning are scarce and the reproductive output of cod has been irregular and more and more limited. One of the proposed solutions for this problem might be an artificial reproduction. One of such areas has been the Gdańsk Deep, but the number of potential brood fish is relatively low, which makes the eggs and sperm poorly accessible. The aim of the presently reported study was to conduct spawning and egg incubation under controlled conditions to obtain viable hatchlings and to monitor the embryonic development of cod originating from the area of the Gdańsk Deep. Materials and methods. The parental fish specimens were obtained during commercial trawl catches. The eggs were fertilized artificially at the field laboratory using filtered brackish sea water with salinity of 15%, and then were incubated at a constant temperature of 8 ± 0.5°C. Eggs and larvae development, and larvae beginning exogenous feeding were observed under a stereo microscope, and photographed regularly to determine the sizes of the eggs and larvae and to document the results of the experiment. Results. The mean percentage of fertilization of eggs was 70%. Hatching began 236 h after fertilization, and the newly hatched larvae comprised 30% of the initial stocking density. The mean length of the larvae was 3.95 mm. The cod larvae began exogenous feeding before the yolk sac was fully absorbed. Conclusion. Artificial reproduction with gametes obtained from cod spawning in the Gdańsk Deep is possible, and the fertilization rates obtained were similar to those achieved with cod from the Bornholm and Gotland basins. Therefore, our results also show that cod spawning in the Gdańsk Deep can be suggested as a model species for further research. [ABSTRACT FROM AUTHOR]