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 come under the categories of disease, treatments, feed, waste, predators and escapes.

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.  Once attached to the skin of a fish, they feed on its flesh, causing lesions.   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.

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 peroxide, synthetic pyrethroids, 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 benzoate 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).

Salmon are carnivorous fish and farmed Atlantic salmon diets have been heavily reliant upon fishmeal and fish oil-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.  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.  This can include soy and palm oil.  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 for fishmeal 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 Norway and Scotland 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.  Farms are required to use non-lethal methods of predator control before resorting to lethal methods, which require a license. The number of seals killed has fallen since a new licensing system was introduced in 2011. Latest figures for 2019 can be found on Marine Scotland’s web pages.

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 sites.  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.

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 naturally 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 markets. No markets have set an ADI (Acceptable Daily Intake) for astaxanthin and thus no MRL (Maximum Residue Limit) has been set.  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 rhodozyma.

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 levels. Recent research indicates that farmed Scottish salmon still delivers more omega-3 acids than most other fish species and all terrestrial livestock.  Present UK Government recommendations are to have 1 portion of oily fish such as salmon a week. Omega-3s are also found in herring, mackerel and mussels. EPA and DHA 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 feeds.

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 website 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. Fish Health Inspectorate analysis in 2012 and 2013 of farmed fish was reported to have found residues in 5 out of 3,142 samples.  Of these, 2 out of 96 samples of farmed salmon tested in 2012 had residues of emamectin benzoate, used to treat sea lice.  Tests in 2016 found emamectin benzoate residues in 3 out of 91 samples of farmed salmon tested.

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