Reservoir water physicochemical characteristics provide important information about water suitability for fish production. Accordingly, the study aimed to characterize the physicochemical characteristics of Dembi reservoir water for sustainable fish production. The study was conducted in Dembi reservoir during the dry season. Water samples were collected in triplicate from selected 10 sampling sites of the reservoir water using manually prepared water sampler made from polyvinyl chloride (PVC) tube. The depth integrated sampling technique was employed to take water samples for all physicochemical characteristics analysis. From the selected 14 physicochemical characteristics, four (temperature, electrical conductivity, pH, and dissolved oxygen) were tested onsite using a multisystem HQ4d electronic meter (probe), whereas the rest 10 water quality characteristics were tested in the laboratory. The result showed that the current average depth of the dam was 5.6 ± 1.61 m. The overall mean values of the water quality characteristics at different sites of the reservoir were as follows: turbidity (26.4 ± 0.44 FTU), total hardness (22.2 ± 0.51 mgL−1), NO3 (5.4 ± 0.48 mgL−1), NO2 (0.3 ± 0.11 mgL−1), NH4 (2.1 ± 0.06 mgL−1), PO4−3 (1.7 ± 0.27 mgL−1), total alkalinity (52.5 ± 0.91 mgL−1), and BOD5 (2.7 ± 0.24 mgL−1). There was a significant difference ( $p<0.05$ ) in all physicochemical characteristics among 10 sampling sites of the reservoir water. The recorded values of all physicochemical characteristics, except NO2, NH4, and PO4−3, were found within the recommended standard limit for fish production. The change in reservoir water depth and increase in nutrients shows the presence of sediment siltation and nutrient enrichment. Therefore, proper watershed management practices and waste management should be carried out for sustainable water quality maintenance and fish production.

Pond management requires that a specific fish culture is conducted while taking into account both production possibilities and profitability, as well as the impact it may have on the natural environment. This study aimed to evaluate the effect of three water management systems used in rainbow trout culture on water quality in fish ponds. It was conducted at six trout farms and differing in water management strategy. After water had flown through the fishing ponds, its quality was significantly less impaired at farms operating in the flow and cascade systems. In turn, waters discharged from farms using the recirculation system were characterized by the poorest quality and lowest values on the Water Quality Index (WQI). It was found that the flow and cascade systems can be used to maintain the water quality and give less fish mortality for trout. It has been shown that the use of a water recirculation system in rainbow trout cultures significantly affects the quality of water in fishponds and can potentially lead to suppression of fish resistance and in extreme cases, to fish death. This study will help fish farmers in choosing the optimal variant of water management, taking into account both the best fish health with the least negative impact of fish farms on the environment.