Environmental Impacts of Discharge

Depending upon the aquaculture system, water may or may not be discharged into the environment. Pond aquaculturists, such as those farming catfish, striped bass, baitfish, or ornamental fishes typically do not discharge their waters. Evaporation water is replaced by precipitation or pumping from shallow wells. In these culture practices, discharge of water occurs mostly when ponds are completely drained for pond maintenance. This occurs infrequently on an as needed basis. Recirculating water production systems also discharge water regularly but only in very limited quantities. Concentrations of substances may be high, but total load may be quite low.

Raceway culture systems, such as occurs with salmonids, do routinely discharge water which may contain elevated (above influent water quality) quantities of nutrients such as phosphorus. The impact of these nutrients is site specific. In some cases, the impact of elevated nutrients is obscured by other factors such as elevated sediment deposition from non-point sources or hydromodifications such as dams. For example, managed river or hydropower schemes often result in a dramatic reduction in the frequency of large magnitude floods which are important for normal river ecosystem function. These factors alone can obscure the significance of elevated phosphorus. In some cases however, the nutrients from an aquaculture facility may cause eutrophication.

The impact of aquaculture operations must be determined within the context of a specific water body. Net pen culture, whether in a lake, reservoir or coastal marine area must also be evaluated within a site-specific context. Normal current actions may minimize the potential impact of a specific facility or group of facilities. In all cases, careful evaluation must be made based on site specific evaluations and allow for corrective action to be taken.