Gilthead Sea Bream

Feed is a major component of the cost of farmed sea bream production and the efficient use of feed is an important aspect of farm management; it is vital to ensure that as much of the feed as possible is consumed by the fish and that as little as possible is wasted. Efficient monitoring and management of feed regimes minimises nutrient discharges from sea bream net-pen farming and reduces the potential of pollution arising from uneaten food entering the marine environment. This is important as the accumulation of uneaten food and faecal matter beneath net-pens has the potential to affect aquatic life through de-oxygenation and algal blooms which can be associated with nutrient increase¹. The key nutrients likely to cause problems for receiving waters are nitrogen and phosphorus.

In addition to controlling the amount of feed dispensed to the fish, sea bream diets have improved to make them more digestible². This results in greater food absorption, less faecal production, and computer controlled demand feeding systems result in less food use and wastage. To improve performance, farms should monitor feed efficiency, effluents and water quality in the receiving water body.

Licensing and farm management can ensure facilities are located in areas where the environmental impact through release of nutrients is going to be minimal. Full environmental impact assessments are undertaken which include modelling impacts of the release of nutrients before licenses are issued. As technology has developed sea bass net-pens have been able to locate to more exposed locations with stronger currents and in deeper water which ensures waste materials are more widely dispersed.

Computer modelling is often used as a guide to determine licensed discharge quantities of organic waste (and chemicals) arising from marine fish-farm operations. It is typically a requirement of net pen operators that before they can obtain a site license they model the effects of potential discharges released from the aquaculture site, using methods such as MEROMOD³.

Modelling and monitoring methods are mandated by regulations and certification standards and records documented by farms. Farms should also document and record how they dispose of any other solid wastes such as fish mortalities. There have been attempts to mitigate the release of nutrients into the marine environment by growing shellfish or algae (seaweed) species in proximity to cages (i.e. integrated multi-trophic aquaculture or IMTA) which can utilise the dissolved nutrients and provide additional crops^{4, 5}.

Enclosed hatcheries and land-based farms (recirculating aquaculture systems or RAS) can mitigate nutrient pollution risks by removing solids prior to the discharge of effluents and remove dissolved nutrients by establishing areas of reed beds or salt marshes around discharge points.