Improving aquaculture profits while reducing water pollution

students with troutNutrients in aquaculture effluent waters have come under increasing regulatory scrutiny because of their role in stimulating unwanted aquatic plant growth in public waterways. As a result, a number of states, including Idaho, developed regulations that restrict the amount of phosphorus, nitrogen, and solids that are allowed in the discharge water from freshwater fish culture facilities. Since phosphorus is the nutrient with the greatest impact on algae and plant growth in freshwater, Idaho’s Department of Environmental Quality required a 40 percent reduction in phosphorus discharged from aquaculture facilities.

To decrease the amount of phosphorus discharged while maintaining the economic viability of the trout industry, University of Idaho Extension collaborated with fish nutritionists, fish feed manufacturers and the Western Regional Aquaculture Center. The collaboration resulted in two major research and extension efforts that focused on determining ways to reduce phosphorus levels in hatchery discharge water.  The focus of the projects was to reduce the amount of phosphorus in fish feed while maintaining optimum rates of trout growth and feed conversion.

Results from the projects showed that a low-phosphorus feed provided a 33% reduction in the amount of phosphorus discharged compared to a regular trout feed, with no effects on fish quality or growth. It was also verified that the higher the feed digestibility, the better the feed utilization efficiency and the lower the cost of production. From an environmental perspective, higher feed utilization efficiency results in less waste excretion and lower pollution potential.

Farm-scale testing validated the effects of feed changes on production efficiency and the economics of production. Phosphorus levels in feeds can be reduced for rainbow trout without affecting weight gain or product quality, and higher feed utilization efficiency results in lower costs for producers, decreased waste excretion, and improved water quality.

This collaborative research and Extension effort:

  • Prevented an estimated 20% loss in trout production.
  • Allowed higher production levels worth more than $6 million per year.
  • Permitted efficient low environmental-impact trout production by reducing phosphorus levels in discharge waters by over 40% without affecting growth of fish or product quality.

For more information contact Gary Fornshell, Aquaculture Extension Educator, Twin Falls, ID ( or 734-9590).