Feed is a major component of the cost of farmed salmon production and the efficient use of feed is an important aspect of farm management – 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 salmon 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 increase. 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, salmon diets have improved to make them more digestible. This results in greater food absorption, less faecal production, and computer controlled demand feeding systems result in less food use and wastage.
Licensing and farm management can ensure facilities are located in areas where the potential environmental impact from the release of nutrients are minimal, and this includes undertaking full environmental impact assessments, including modelling impacts of release of nutrients before licenses are issued. As technology has developed salmon net-pens have been able to locate to more exposed locations with stronger currents and in deeper water, and this ensures waste materials are more widely dispersed. Net-pens are also regularly rotated between farm sites to enable fallowing. The regulatory framework has also become more sophisticated1. In Scotland for example the regulation of discharged wastes is exercised through Controlled Activities Regulations2.
Another proposed solution towards ameliorating inputs from net-pen farming is the adoption of IMTA, whereby filter feeders (shellfish), detritivores (e.g. sea cucumbers or marine worms), and macro-algae (seaweeds) are grown alongside the salmon farms to remove the various outputs from the fish. Whilst this solution has initial appeal, it has yet to demonstrate its viability in biological, environmental and financial terms, when it is applied to the dynamic open sea environment3.
Land-based RAS can mitigate nutrient pollution risks by removing solids prior to the discharge of effluents and the removal of dissolved nutrients in waste water streams by establishing areas such as reed beds.