Nutrient Pollution — Seafish

Rainbow Trout

Oncorhynchus mykiss

Nutrient Pollution

Feed is a major component of the cost of farmed rainbow trout 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 reduces the potential for pollution of the freshwater or marine environment arising from solid (including uneaten feed) and dissolved nutrient discharges from rainbow trout farms. This is important as the accumulation of uneaten food and faecal matter beneath net-pens and/or around discharge points and in both the marine and freshwater environment, 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 improvements in controlling the amount of feed dispensed to farmed fish, rainbow trout diets have improved to make them more digestible. This results in greater food absorption, less faecal production and lower nutrient discharge.

Licensing and farm management can ensure facilities are located in areas where the potential environmental impact from the release of nutrients are minimal, and, where applicable, this includes undertaking full environmental impact assessments, including modelling impacts of release of nutrients before licenses are issued.

Farms should monitor nutrient utilization efficiency, the quality of the pond effluents, and water quality in the receiving water body. The monitoring methods are documented within certification standards. Farms should also document and record how they dispose of sludge and any solid wastes such as fish mortalities.

In land-based freshwater trout production in the UK, where flow-through systems are usually in operation, sludge is collected in sedimentation ponds and many farms operate to a Code of Good Practice1, 2 with regards to the disposal of mortalities and other solid wastes.

The use of sludge and/or waste water to fertilize agricultural land surrounding trout farms is sometimes practiced and was studied as part of a European AquaEtreat project3. Sedimentation basins or ponds to capture dissolved particulates are effective but alternative methods are required to remove dissolved nutrients. One area of growing global interest is the use of clean-up technologies such as constructed wetlands in which harvestable aquatic plants are cultured in the waste-stream and help trap fine suspended particles as well as absorb dissolved waste nutrients.  Methods available for the clean-up of aquaculture discharge waters have been reviewed for Scotland’s freshwater aquaculture4.

Recirculation aquaculture is often cited as a sustainable means to reduce environmental impacts from discharges (as well as from escapees and disease). This has been encouraged and has taken place for rainbow trout farms in Denmark5. However, investment costs are significant and trout farmers in other locations are unlikely to consider this unless they receive a significant investment subsidy or increased price for their fish.

In marine net-pen trout aquaculture, as technology has developed, some trout net-pens have been able to locate to more exposed sites 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 sophisticated6. In Scotland for example the regulation of discharged wastes is exercised through Controlled Activities Regulations7.

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 environment8.


  1. Code of Good Practice for Scottish Finfish Aquaculture
  2. Quality Trout UK
  3. AquaEtreat project
  4. SARF
  5. CEFAS
  6. Little, D. and Shepherd, C.J., 2014. Aquaculture: are the criticisms justified? II – Aquaculture’s environmental impact and use of resources, with special reference to farming Atlantic salmon
  7. Marine Scotland
  8. Institute of Marine Research, Norway