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 ($\overline{\mathrm{x}}$ = 2991 eggs), a smaller average number of eggs was shown in the green terror ($\overline{\mathrm{x}}$ = 922 eggs), and the red discus showed the smallest average number of eggs ($\overline{\mathrm{x}}$ = 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 $\overline{\mathrm{x}}$ = 4.346 mm, green terror $\overline{\mathrm{x}}$ = 5.203 mm, and jaguar cichlid $\overline{\mathrm{x}}$ = 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.

Urbanization and technological advancements result in the dispersion of antropogenic electromagnetic fields (EMF) that can affect on ecosystems. Therefore, it is important to understand their impact on the environment. Aquatic ecosystems are subject to EMF as part of various electricity sources, e.g., high-voltage transmission lines (HVTL). We examined the impact of EMF generated by HVTL on the spatial arrangement and survival of pike (Esox lucius) embryos. Fertilized eggs were incubated under two HVTL configurations 110 kV and 220 kV compared with a control group devoid of anthropogenic EMF. Embryo orientation and survival were monitored until blastopore closure. The control group showed dominance in the arrangement of embryos along the N-S, NNW-SSE, and NNE-SSW axes, with a slight prevalence of northern directions. EMF originating from HVTL did not exert a significant influence on the spatial arrangement of pike embryos, although some deviations from the arrangement noticed in the control group were observed. Increased embryo mortality was observed only at 110 kV site, but probably due to factors unrelated to EMF. In conclusion, EMF generated by HVTL did not significantly change pike embryo orientation or chances of survival. However, longer exposure or higher EMF levels could provoke notable reactions, requiring ongoing evaluation as power networks continue to spread more widely.

The increasing expansion of energy infrastructure and anthropogenic transformation of the environment have introduced electromagnetic fields (EMFs) into aquatic ecosystems. While studies on the impact of EMFs on aquatic organisms are growing, their effects on fish embryonic development remain poorly understood. This study investigated the influence of electromagnetic exposure dominated by the magnetic (B) component, measured at 11.15 (± 2.24) μT and 50 Hz, on the heart rate of vimba bream (Vimba vimba) embryos, a species of conservation significance in aquaculture. The electric (E) component was not measured, and its potential contribution cannot be excluded. Fertilized V. vimba eggs were incubated under control conditions or exposed to an EMF, and embryonic heart rate was monitored in two experimental series to assess both short- and long-term EMF effects. In Experiment 1, embryos were incubated either under EMF (Variant B, n = 18) or control conditions (Variant A, n = 18) and observed without additional exposure. In Experiment 2, embryos incubated under EMF (Variant D, n = 30) or control conditions (Variant C, n = 30) were exposed to EMF during microscopic observation. Bayesian non-linear mixed models revealed significant EMF effects in both experiments. Embryos exposed to EMF during incubation (Variant B) displayed a 23.5% higher baseline heart rate (95% CI: 14.5–31.9%) compared to controls. In Experiment 2, embryos exposed to EMF during observation (Variant D) showed a rapid heart rate increase of 29.0% (95% CI: 21.5–37.6%). Embryos exposed to EMF during incubation exhibited a diminished response to subsequent EMF exposure, suggesting physiological adaptation. Additionally, EMF exposure during incubation was associated with reduced inter-individual variability in heart rate, suggesting a homogenizing effect on embryonic cardiac responses. These findings demonstrate measurable cardiac responses of fish embryos to hatchery-relevant electromagnetic exposure and highlight compensatory mechanisms regulating heart rate. While the results suggest fish embryos can adapt to such electromagnetic exposure, further studies are required to disentangle the roles of both magnetic and electric components and to evaluate long-term physiological and ecological consequences.

The Baltic sturgeon Acipenser oxyrinchus , once widespread in rivers draining to the Baltic Sea, was extirpated by overfishing and habitat degradation and is now the focus of a basin-wide restoration programme. Since 2006, more than seven million hatchery-reared juveniles have been released in Baltic rivers, yet the movements, habitat use and survival of stocked fish remain poorly quantified. HaffStör aims to close these knowledge gaps and to provide an operational basis for adaptive, cross-border management of the recovering population in the Oder River–Szczecin Lagoon system (Germany/Poland). The project combines movement ecology, conservation biology and social sciences in four interconnected work packages. First, we optimise rearing protocols, comparing juveniles produced under different degrees of environmental control (ambient vs. static recirculating aquaculture systems), coupled with behavioural experiments on habitat choice and anti-predator responses. Second, we release tagged juveniles in the Lower Oder and Szczecin Lagoon and track their movements using active and passive acoustic telemetry, complemented by habitat mapping and benthic sampling, to identify key migration routes, habitat utilisation and zones of overlap with human activities, mainly associated with fisheries and navigation. Third, we develop and test a standardised, cost-effective monitoring framework, based on passive gears, fishery-independent surveys and environmental DNA, aligned with Bern Convention guidelines and intended to be transferable to other Baltic catchments. Fourth, we establish cooperation with commercial fisheries through targeted outreach to initiate structured by-catch reporting, co-designed surveys and joint project activities. By integrating ecological and fishery data, HaffStör will deliver robust estimates of juvenile survival and mortality drivers, refine release and monitoring strategies and generate spatially-explicit assessments of by-catch risk. Beyond new scientific insights into Baltic sturgeon ecology, the project seeks to strengthen stewardship amongst local communities and practitioners and to support evidence-based, transnational governance for this flagship migratory species.