Environmental Performance

In aquatic systems, as soon as feeds or wastes enter the water column, in situ mineralization occurs. The fraction of the produced wastes that is discharged depends on farm type, culture density, feed composition and water renewal rate. The effects of these factors on waste discharge are reviewed. All possible combinations of these factors result in large differences in the type and amount of waste products discharged to neighbouring surface waters from aquaculture operations. Few farms discharge directly to a sewage system or operate an on-farm water purification system to deal with the discharged nutrients. Using a fraction of the otherwise discharged waste as an input for other cultures is possible, but also rarely practiced. In farms applying recirculating aquaculture system (RAS) technology and relying on nitrification and denitrification, nearly all wastes produced on-farm are mineralized, resulting in a stabilized sludge which represents on a dry weight basis 4 to 8% of the feed input. The semi-closed nature of RAS farms also minimizes the possible introduction and dissemination of diseases and parasites and the use of disinfectants and antibiotics. A small water exchange also reduces opportunities for culture animals to escape. With the exception of some extensive production systems, pond, cage or raceway operations discharge more nutrients and use more water per kg fish produced than RAS. The challenge is to make all future aquaculture farms equally efficient as RAS in dealing with waste discharge. This can be done by making aquaculture operations either more or less intensive. Each approach has its advantages and disadvantages and is reviewed in terms of water use, nutrient utilization and discharge, and energy use.