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Environmental Impacts

The fish farming industry in Scotland is dominated by salmon production, much of which takes place in the sheltered sea lochs of the west coast and islands.

Members of the Fish Health Inspectorate

Salmon farming can affect the marine environment, through the discharge of nutrients, solid waste, medicines and antifoulants.

Marine Scotland Science (MSS) has a strong programme of research investigating these effects and provides both the Scottish Government and the Scottish Environment Protection Agency (SEPA) with advice on applications for the leases and discharge consents necessary for fish farming development.

MSS has developed the scientific basis on which the Locational Guidelines for the authorisation of marine fish farms in Scottish waters are based.

Predicted nutrient nitrogen discharge from a fish farmNutrients

The process of fish farming releases nutrients such as nitrogen and phosphorus from fish feed into the marine environment in a soluble form. These nutrients can enhance the growth of marine plants and algae. MSS has developed mathematical models to predict the levels of nutrient enhancement in sea lochs arising from fish farming. The results of these models are used to provide advice on the amount of fish that can be farmed on a particular site. Recently MSS has conducted field surveys to measure the levels of nutrients in sea lochs and record any effects on phytoplankton or seaweed on the shoreline.

Effects on the Seabed

Waste feed and faeces from fish farms can collect on the seabed under fish cages. This increase in organic matter has an impact on this benthic environment, affecting the nature and chemistry of sediments, and can reduce the diversity of animals living there. Scientists at MSS use mathematical models to predict the area of seabed under fish farms that is likely to be affected. These results are used to advise the Scottish Government and SEPA on fish farming developments.

Predicted carbon deposition below fish cages

Medicines and Sea Lice

Predicted surface plume of cypermethrin following lice treatmentA number of medicines are used on fish farms to maintain fish health. The application of antibiotics to treat bacterial diseases has declined in recent years due to effective vaccination programmes.

Farmed salmon are susceptible to infestations of parasitic sea lice that cause considerable stress to fish and economic losses to the industry. Sea lice on farmed fish could potentially be transferred to wild salmon and sea trout. MSS has an on-going project in Loch Torridon to record the prevalence of larval stages of sea lice in the loch and to improve understanding of the factors, such as hydrographic conditions and the presence of fish farms, that might control their abundance.

The fish farming industry control sea lice using chemicals that can be toxic to marine invertebrates. The quantities of chemicals used is carefully regulated by SEPA to ensure that levels in the marine environment are safe and below environmental quality standards (EQS). Some sea lice treatments, for example Excis® (cypermethrin) and Salmosan® (azamethiphos) are applied as a bath and then discharged into the surrounding water. MSS uses mathematical models to predict the dispersal of these chemicals and their concentrations in the marine environment. MSS was involved in a five year project investigating the ecological effects of sea lice treatment agents.


Antifoulants used on fish farm cage nets can affect the marine environment. Tributyltin (TBT) was commonly used as an antifoulant until 1987, when its use for this purpose was prohibited. MSS still monitors the marine environment for the effects of TBT in selected areas. Copper based antifoulants are now used in place of TBT and MSS is now involved in researching the effects of these antifoulants on the local environment.

Farming New Species

New species of fish are being targeted by the industry for culture. Cod and halibut are now being farmed at several sites in Scotland. These species produce different amounts of waste to salmon and will therefore have a different level of environmental impact. Work is underway at FRS to estimate these differences and advise how the results should be used in regulating the industry.

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