Aquaculture Effluents and Water Pollution

Aquaculture facilities typically discharge into natural waters.  Their effluents are enriched with nitrogen, phosphorus, organic matter, and suspended solids because fertilizers and feeds are used to enhance production above natural productivity.  Generally, 20 to 40% of nitrogen and phosphorus applied to ponds in feed is recovered in harvested fish.  In shrimp ponds, phosphorus recovery is 10 to 15%, but nitrogen recovery is about the same as in fish ponds.  Bottom soils adsorb phosphorus, and denitrification and ammonia volatilization also occur in ponds.  Usually, less than 30% of nitrogen and 10% of phosphorus applied to ponds exits in effluent.  In raceway culture, nitrogen and phosphorus in uneaten feed and feces can be partially removed before effluents enter natural waters.  However, in cage culture, nitrogen and phosphorus not recovered in fish at harvest enters the water body.  Large aquaculture facilities or clusters of many small ones contribute considerable amounts of nitrogen, phosphorus, and certain other potentual pollutants to receiving waters.

Aquaculture facilities contribute particularly to eutrophication of natural water bodies.  Eutrophication is undesirable for other water users, but it also can be harmful to aquaculture facilities such as cage farms and shrimp farms that use the same water body as water supply and effluent recipient.  Many countries have imposed regulations on aquaculture effluents.  These may include limits on feed inputs, specifications for site selection, and effluent water quality standards.  Aquaculture “eco-label” certification programs are being established in response to consumer demand for “environmentally-friendly” products.  These programs may include effluent standards that limit discharge of nitrogen and phosphorus.  Compliance with effluent regulations and “eco-label” certification program standards usually require installation of best management practices (BMPs) to limit discharge of nitrogen and phosphorus.  Some examples of these BMPs are use of high-quality feeds that have no more nitrogen and phosphorus than required, conservative feeding practices, use of adequate mechanical aeration to oxidize waste in ponds, and discharge of effluent through a sedimentation basin.  Some large producers also are voluntarily adopting BMPs independently of regulations or participation in “eco-label” certification.  Studies of the environmental benefits of regulations, certification, and BMPs are few, but "responsible aquaculture" programs seem to be gaining popularity with seafood purchasers and consumers.