Legislation and interest exists to protect and restore freshwater and marine ecosystems from the environmental impact of aquaculture. However, aquaculture-induced eutrophication remains a major environmental concern. Water soluble phosphorus, uneaten feed, feces, and metabolic waste from farmed fish increase phosphorus concentrations in adjacent waters. In open freshwater fish farms, in particular, the effects can be more immediate, as excess phosphorus is introduced directly into ecosystems. Several intestinal enzymes, transporters, and regulating factors have been implicated in farmed fish dietary phosphorus retention. For example, alkaline phosphatase and other transporters aid in the absorption of phosphorus in the anterior intestine, while pH, calcium, and vitamin D influence these enzymes and transporters. This process may also be influenced by intestinal morphology and the gut microbiome. To reduce phosphorus pollution from open flow fish farms, a thorough understanding of the processes that affect nutrient retention and absorption, as well as the impact of dietary factors, anti-nutritional substances, and intestinal morphology, is required. Aquaculture can be made more sustainable by reducing phosphorus release. This can be achieved by optimizing feed composition, adding functional feed ingredients, managing gut health, and treating effluent aquaculture waters with bioremediation and absorbing materials. Anti-nutritional factors can be mitigated through processing and through the use of functional feed additives. Addressing these issues will reduce aquaculture’s environmental impact, ensuring aquatic ecosystem health and global food security. In addition, treating effluent aquaculture waters with bioremediation and absorbing materials can remove phosphorus from the water, preventing it from entering the environment. This can further reduce the environmental impact of aquaculture and help to ensure the sustainability of this sector.

In the present study, we focused on the nitrogen and phosphorus levels generated by open flow freshwater fish farms in Greece and Iceland where salmonid fish are cultivated. There are several differences in the legislation and regulation of the aquaculture sector in Greece and in Iceland. Nevertheless, in both countries, environmental protection agencies need accurate information for nutrient processes in order to evaluate the environmental influence on surrounding waters by salmon aquaculture. Data were collected from surveys of 21 freshwater fish farms in Greece and 9 in Iceland. The surveyed farms practiced intensive production methods. The concentrations of nitrogen and phosphorus in farms’ discharges were used to estimate organic load generated. The collected data were compared with data from the literature for the production Salmonids for species cultivated in Iceland (Arctic charr, Atlantic salmon) and in Greece (Rainbow trout). The results of the analysis indicate that in both countries the highest concentrations of nitrogen and phosphorus discharges of fish farms exhibits a variability which can be attributed to the total biomass of produced fish, fish feed and feeding management protocols used, species cultivated and ambient temperature. During the last 30 years, there was a significant reduction in the amount of nutrients generated by fish farms, which to a great extent is a result of lower levels of protein content in fish feeds and improved feed conversion efficiency. It is estimated that over the past 30 years, the combined effect of reduced nitrogen and phosphorus content and the improvements in Feed Conversion Efficiency, resulted in about 50% reduction in the nutrients generated by every Kg of farmed fish produced. Nevertheless, this significant improvement was easily offset by and increased production of fresh water fish farms exhibited in both countries. It seems possible that legislative measures combined with improvements in technology and feed management of fish farms will continue to be good option of minimizing the environmental impact of fish farms in the future.

Fish physiology is a key field of study in fisheries science, providing a solid foundation for understanding aquatic ecosystems and driving innovation in the aquaculture sector. As aquaculture continues to be a key source of food worldwide, study in this subject has increasingly focused on the implications of climate change, which poses considerable threats to fish and other aquatic species. This review outlines current studies on fish physiological responses to several stressors related with climate change, such as changes in temperature, salinity, disease occurrence, and oxygen levels. The combination of these abiotic and biotic variables is vital, as climate change accelerates pathogen fate and dispersion and alters nutrient dynamics, influencing fish growth and survival. This study also reviews mesocosm experiments and modelling research to illustrate the significance of these interactions in developing sustainable fisheries management and enhancing aquaculture methods. Furthermore, the study identifies current research trends and recommends areas for future research to address the ongoing problems posed by climate change to aquatic resources.