Development of innovative technologies for water improvement in recirculating aquacultural systems

In intensive land based production of smolt of Atlantic salmon, and also other fish species, carbon dioxide (CO2) becomes an increasing problem due to the limitation of high quality water. Increasing size of the farms with reduced inlet water flows and ex tensive use of oxygen supplementation has resulted in high concentrations of CO2 in the water. High CO2 levels can be toxic to the fish, and have become the limiting factor in many farms. Several technological measures are in use to reduce the CO2 level i n the tanks, including air lifting and stripping by mammoth pumps, use of tray and column strippers, and use of air injectors. However, the efficiency of the systems has been limited, by typical CO2 removals in Norwegian farms in the range of 15-20%. Re circulating systems, including biofilters for conversion of total ammonia nitrogen (TAN) to nitrite (NO2-) and nitrate (NO3-), have mainly been developed for warm water species. Experience on performance and water quality data in cold water systems is lac king. When biofilters now are used more frequently in Atlantic smolt recirculating systems, several factors for optimization have to be considered, including: 1) The strong pH-dependency of nitrification, and 2) the nitrification rate in water at low temp eratures is expected to be low. The Conservation Fund's Freshwater Institute in Shepherdstown, WV, USA, has a very strong scientific group in the field of water quality and technical solutions to improve water quality in freshwater aquaculture. They hav e special competence on dissolved carbon dioxide and technological measures to reduce the concentration, and nitrification. It will be very fruitful for scientists from NIVA and the technical management of the industrial partner in the project, Flatsetsun d Engineering AS and Smøla Klekkeri og Settefisk AS, to have the opportunity to work with the scientists from The Freshwater Institute, and to initiate a common project on water treatment in aquaculture.