Know your Scottish Salmon

Impacts: Is salmon farming costing the earth?

The use of open net pens in salmon farming has significant environmental impacts, which can, in turn, affect human health

The impacts range from diseases which can be transferred to wildlife; discharge of chemicals used as treatments, fish faeces and excess feed into the surrounding water; mitigating actions against predators; and escapes of domesticated species. Read about each of these environmental and human health impacts in more detail below.

As an environmental charity, Fidra are particularly concerned about the impact of excess feed, waste and treatments falling on the loch or seabed below and around the cages. Of particular concern is the use of formaldehyde to treat fungal infections. Despite being phased out in marine farms, it continues to be used in freshwater sites with little evident monitoring. Scientific data demonstrates that immediate and wider ecological damage is primarily caused by a greater amount of waste than the habitat can process, being deposited onto this environment1. See here for our spotlight on the impacts of salmon farming on the benthic environment.

Environmental Impacts

Chemical treatments are used to combat diseases, parasites and biofouling of the nets.

Treatments for sea lice include preventing their attachment and development using medicines in the salmon feed, treating salmon using dissolved therapeutants in a bath treatment and biological control with cleaner fish. Chemicals used in bath treatments for sea lice include hydrogen peroxide2, synthetic pyrethroids3, and organophosphates (including azamethipos).  Lice are becoming resistant to existing medicinal treatments.  Systemic (in feed) treatments tend to be more efficient but are becoming less effective; of these, only emamectin benzoate4 is currently used in Scotland.  Excepting hydrogen peroxide, both bath and in-feed treatment chemicals can persist in the environment. Because these chemicals are designed as biocides, their persistence in the environment can create pressures on populations of non-target organisms.

There are concerns over the use of emamectin benzoate, which may take years to break down in the environment (research suggests a minimum half-life of 404 days).  Studies have shown it to slow growth, impact egg production and change lifecycle patterns in aquatic invertebrates. Due to these findings SEPA is currently reviewing the recommended levels, known as Environmental Quality Standards (EQS).

Also a cause for concern is the use of formaldehyde to treat fungal diseases, mainly in freshwater open net farms.  The EQS for formaldehyde only applies to freshwater environments and is classified as non-statutory, and so is not believed to be monitored or enforced.  The margin between concentrations needed to kill target organisms and those which may harm fish stocks can be small, with the result that it is repeatedly advised for formalin to be administered only in essential circumstances.


To read more detail about the treatments used for disease and parasite control, read our blogs on chemicals in salmon farming:

Salmon are carnivorous fish and farmed Atlantic salmon diets have been heavily reliant upon fish oil and fishmeal-based diets.  To reduce overfishing, fishmeal and fish oil is being supplemented by fish processing by-products, and non-marine products such as vegetable proteins and oils.  This can include soy and palm oil.  Other proteins that are being trialled are algae oil and insect meal.  Processed animal proteins are used elsewhere but not in the UK due to supermarket policy.  The use of non-marine products is increasing and often comprises more than 50% of fish feed.  The amount of wild fish required to produce one farmed salmon is measured by two figures; the Fish In:Fish Out ratio (FIFO), and the Forage Fish Dependency Ratio (FFDR).

  • The FIFO ratio takes the amount of fishmeal and fish oil used to produce 1 Kg of farmed fish and uses it to calculate back to the weight equivalents of wild fish.  For example, 20 tonnes of forage fish reduces to about 5 tonnes of fishmeal and 1 tonne of fish oil.
  • The FFDR is the amount (Kg) of wild caught fish used to produce the amount of fishmeal and fish oil required to produce 1 Kg of salmon.

From 1990 to 2013, the FFDR – the amount of wild caught fish used to produce the fish-based feed required to produce 1 kg of salmon – decreased from 4.4 to 0.7 in Norwegian salmon farming.  A current global figure for a FIFO ratio for salmon and trout is 0.82.  A more accurate assessment of the efficiency of salmon farming is through the use of Marine Protein Dependency Ratio (MPDR) and Marine Oil Dependency Ratio (MODR) calculations, which analyse the use of nutrients and not ingredients.  Farmed salmon in both Norway5 and Scotland6 is a net producer of marine protein, indicating along with the FFDR and FIFO ratio that less than one wild fish is needed to produce each farmed salmon.

Waste from uneaten food and the fish themselves contribute high levels of nutrients into the aquatic environment surrounding open net pens, especially under the farm structures in the Allowable Zone of Effect (AZE). The AZE is the area or volume of sea-bed or receiving water body designated by the relevant regulatory body, such as SEPA in Scotland, in which some exceedence of relevant Environmental Quality Standards (EQSs) or some damage to the environment is allowed.  The deposition of this waste can lead to eutrophication (excessive nutrients) which in turn can result in excessive algal growth to produce toxic ‘blooms’ that can have harmful effects on people, fish, shellfish, marine mammals and birds.  Eutrophication occurs particularly in shallow water, therefore deeper waters are now favoured for farm sites.

Interactions between salmon farms and predators such as seals, otters and seabirds are common in areas where their distributions overlap, and the high density of fish can be very attractive.  Seals in particular can damage nets, and injure and eat large numbers of fish.  This incurs costs due to revenues lost from fish, uptake of methods to protect the farms from seals and repair of any damage caused.   Until 2021 farms could use lethal methods of control under licence for predators if non-lethal methods had been tried and were not effective. However due to the amendment of the Animals and Wildlife (Penalties, Protections and Powers)(Scotland) Act 2020, from 1 February 2021 licences are no longer granted to shoot seals in order protect fish farms


To find out more about the use of a non-lethal predator control option, Acoustic Deterrent Devices (ADDs), read our blog: ‘Acoustic Deterrent Devices: An environmentally sound option?’ Following a review of Acoustic Deterrent Device use on salmon farms and subsequent Code of Practice developed by Marine Scotland7.

Each year thousands of farmed salmon escape into the natural environment, through human error, severe weather and structural issues.  In 2016, there were five incidents involving the loss of over 300,000 fish from seawater Atlantic salmon sites8.  Three additional incidents reported no loss of fish.  Escaped farmed salmon may impact wild salmon stocks by genetic interaction and transfer of diseases.  Domesticated salmon are genetically distinct from wild Scottish salmon, having been bred by Scottish farms from Norwegian stocks.

Infectious diseases represent a major problem in fish farming despite successful development and application of vaccines against a range of pathogens. Sea lice and viral diseases currently represent the largest disease problems.

Salmon sea lice (Lepeophtheirus salmonis) are parasitic crustaceans which originate from wild salmon and can breed rapidly in the concentrated populations of salmon in open net pens. Lice had have a huge impact on the salmon farming industry9. Once attached to the skin of a fish, they feed on its flesh, causing lesions (any damage or abnormal change in the tissue of an organism)10.   The lice can reduce swimming ability, create disturbances in water/salt balance and increase stress levels, leading to poor health, greater susceptibility to disease, low blood count, poor growth and eventually death.  Sea lice can also be transferred to wild populations of salmon and sea trout if they follow migratory routes which pass farms.

Viral disease outbreaks can be devastating to individual fish farms and may be inadvertently spread by live fish transport or contact between fish farms. Diseases such as Infectious Salmon Anaemia (ISA) and Viral Haemorrhagic Septicaemica (VHS) are not currently seen in any farms in Scotland, with the last ISA outbreak in 1998. Legislation to try to control spread of disease has been in place since 1937, to protect wild salmon stocks. Import restrictions apply, and farms are checked by veterinarians at least once every two years.

What makes salmon pink?

The pink colour of wild salmon comes from naturally occurring pigments called carotenoids. There are over 600 naturally occurring carotenoids in plants and animals. Those found in fish belong to a group known as xanthophylls and include astaxanthin. Astaxanthin is the major carotenoid found in wild salmon and crustaceans (shrimp, lobsters) and is responsible for their pink-red pigmentation. Nature-identical synthesized pigments are used in salmon feeds.  Astaxanthin is approved for addition to the diet of farmed salmon and trout in all relevant markets11.  Astaxanthin has been declared safe for the human consumer by the standards of the European Panel on Additives and Products or Substances used in Animal Feed (FEEDAP).  Pigment source has changed in Scotland in recent years to natural products such as extract of the red yeast Phaffia rhodozyma6.

Omega 3

Oily fish such as salmon are a source of the long-chain omega-3 fatty acids, eicosapentaenoic (EPA) and docosahxaenoic (DHA) acids, which may help prevent heart disease and have additional health benefits.  However, replacing the traditional marine ingredients of fishmeal and fish oil in salmon feed with sustainable alternatives can lead to a decrease in omega-3 levels12. Recent research indicates that farmed Scottish salmon still delivers more omega-3 acids than most other fish species and all terrestrial livestock12.  Present UK Government recommendations are to have 1 portion of oily fish such as salmon a week13. Omega-3s are also found in herring, mackerel and mussels. Eicosapentaenoic acid (EPA)14, which helps to lower the risk of heart disease, and Docosahexaenoic acid (DHA)15, which is important for brain function, can also be made from alpha-linolenic acid (ALA) found in vegetable oils (rapeseed and linseed), nuts (walnuts, pecans and hazelnuts), soya products and green leafy vegetables, but it is a slow process and only small amounts are formed.  Trials have been undertaken to genetically modify the plant Camelina sativa to express algal genes and produce oil containing  long-chain omega-3 fatty acids, to replace fish oil in salmon feeds16.

Antibiotics and other medicinal treatments

The development of vaccines has led to a significant decrease in the use of antibiotics, with some farms claiming to have ceased using them since 2012. Additional topical and in-feed chemotherapeutants are still used, and the Scotland Aquaculture website17 provides information on the use of medicinal treatments and pesticides at individual farms. It is unclear how often salmon is tested by industry, the Fish Health Inspectorate and third party auditors for residues of medicines or pesticides, as information is not all publicly available.


Read more about how changes in fish diets impact human health via our blog written by Dr Becky Gait: ‘Krilling off Omega-3s?’

Spotlight: The Benthic Environment

What is a benthic environment?

The benthic habitat, or benthos, is the lowest ecological area in a body of water; so in this context the loch or seabed.  As highlighted in the image above, and described in the ‘Waste’ section, excess food and faeces fall to the floor of the loch or seabed directly underneath the salmon farm pens, disseminating chemicals and nutrients in high concentrations.

How is the benthic environment monitored?

Scottish salmon farms are required to conduct benthic surveys as part of their Controlled Activity Regulations (CAR) licence issued by SEPA as a way to monitor their waste and chemicals discharges into the environment. From looking at benthic survey data, Fidra are particularly concerned about the negative impact that Scottish salmon farming has on the benthic environment. Read our report to find out more and understand our asks of Government, regulators, industry bodies and retailers.

Fidra’s focus on the Benthos

Available benthic survey data in Scotland, highlights regular examples of poor benthic performance18. A high number of farms regularly receive an ‘unsatisfactory’ result in their benthic surveys, indicating that there are excessive and unhealthy levels of discharges from the farm, creating conditions that do not allow wild species of the benthic environment to live sustainably.

Benthic Environment, courtesy of NatureScot

The concluding sentiments in the Scottish Environment Protection Agency (SEPA) Fish farm survey report (2018)19 states:

“Statistical analysis showed that [emamectin benzoate] had the biggest negative effect on the crustacean abundance and richness. This effect was detectable below the current [Environmental Quality Standards] (EQS),  this adds to the weight of evidence that the current EQS may not be protective of benthic ecology beyond the 100m from the cages.” (pg. 19)

Fidra is concerned that while SEPA recognise the current standards do not fully protect the benthic environment, which so much of Scotland’s marine species depend on for food, reproduction and as a habitat, there appears to be very limited action to rectify this. SEPA has a duty of care as the regulator of salmon farming in Scotland to recommend and enforce that the industry does not expand any further until the necessary EQSs are met.


[1] Riera, R., Pérez, Ó., Cromey, C., Rodríguez, M., Ramos, E., Álvarez, O., Domínguez, J., Monterroso, Ó., & Tuya, F. (2017). MACAROMOD: A tool to model particulate waste dispersion and benthic impact from offshore sea-cage aquaculture in the Macaronesian region. Ecological Modelling361, 122–134.

[2] Marine Scotland. (2019). Marine Scotland – use of hydrogen peroxide in fish farming: EIR release. Available at

[3] EPA. (2022). Registration Review of Pyrethrins and Pyrethroids. Available at

[4] SEPA. (2017). Review of Environmental Quality Standard for Emamectin Benzoate. Available at

[5] Ytrestøyl, T., Aas, T. S., & Åsgård, T. (2015). Utilisation of feed resources in production of Atlantic salmon (Salmo salar) in Norway. Aquaculture, 448, 365–374.

[6] The Scottish Parliment. (2018). Review of The Environmental Impacts of Salmon Farming in Scotland. Available at

[7] Marine Scotland. (2021). Aquaculture – fish farms: containment of and prevention of escape of fish – draft code of practice – consultation. Available at

[8] Marine Scotland. (2017). Scottish Fish Farm Production Survey 2016. ISBN 971788512275. Available at

[9] Costello, M. J. (2009). The global economic cost of sea lice to the salmonid farming industry. Journal of Fish Diseases, 32(1), 115–118. Available at

[10] Cai, W., Kumar, S., Navaneethaiyer, U., Caballero-Solares, A., Carvalho, L. A., Whyte, S. K., Purcell, S. L., Gagne, N., Hori, T. S., Allen, M., Taylor, R. G., Balder, R., Parrish, C. C., Rise, M. L., & Fast, M. D. (2022). Transcriptome Analysis of Atlantic Salmon (Salmo salar) Skin in Response to Sea Lice and Infectious Salmon Anemia Virus Co-Infection Under Different Experimental Functional Diets. Frontiers in Immunology, 12. Available at

[11] Higuera-Ciapara, I., Félix-Valenzuela, L., & Goycoolea, F. M. (2006). Astaxanthin: A Review of its Chemistry and Applications. Critical Reviews in Food Science and Nutrition, 46(2), 185–196.

[12] Sprague, M., Dick, J. R., & Tocher, D. R. (2016). Impact of sustainable feeds on omega-3 long-chain fatty acid levels in farmed Atlantic salmon, 2006–2015. Scientific Reports, 6(1), 21892. Available at

[13] NHS. (2018). Fish and shellfish. Available at

[14] Stark, A. H., Crawford, M. A., & Reifen, R. (2008). Update on alpha-linolenic acid. Nutrition Reviews, 66(6), 326–332. Available at

[15] Domenichiello, A. F., Kitson, A. P., & Bazinet, R. P. (2015). Is docosahexaenoic acid synthesis from α-linolenic acid sufficient to supply the adult brain? Progress in Lipid Research, 59, 54–66. Available at

[16] Betancor, M. B., Sprague, M., Usher, S., Sayanova, O., Campbell, P. J., Napier, J. A., & Tocher, D. R. (2015). A nutritionally-enhanced oil from transgenic Camelina sativa effectively replaces fish oil as a source of eicosapentaenoic acid for fish. Scientific Reports, 5(1), 8104. Available at

[17] Natural Scotland. (2022). Scotland’s Aquaculture Home. Available at

[18] SEPA. (2022). Scotland’s Environment – Marine Fish farm Biomass. Available at

[19] SEPA. (2018). Fish Farm Survey Report: Evaluation of a New Seabead Monitoring Approach to Investigate The Impacts of Marine Cage Fish Farms.

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