Although some of the claims are provocative – and some have been challenged – fish farming is more common than most of us realise. It is estimated that around 50 per cent of all the fish we eat is farmed in pens that can sit up to 50 metres underwater and store around 200,000 fish. In countries where fish farming is a large source of income, such as Norway, a farm might have 10 pens – which would contain more fish than the wild Atlantic salmon population of the world.
When it comes to these farms, sustainability is a pressing issue. The UN’s Food and Agriculture Organisation estimates that 34.2 per cent of stocks worldwide are currently fished to unsustainable levels. However, these fish farms are crucial in catering to the rising demand for salmon: according to figures from Seafish, the UK seafood industry body, consumers ate an astonishing 92,680 tonnes of the fish in 2017, making it the third most popular seafood behind cod and tuna.
Our increasing consumption of fish is partly driven by the health benefits. Oily fish, such as mackerel, salmon and sardines, are a valuable source of Omega 3. Places where people consume lots of oily fish, such as the Mediterranean and Japan, have fewer rates of heart disease than in the UK, because Omega 3 fats, EPA and DHA, help to protect the heart and blood vessels from disease. There is also research to suggest that Omega 3 can boost our brain health: a 2017 study found that people with high levels of omega-3s had increased blood flow in the brain.
However, some research shows that there is less Omega 3 in farmed salmon than in wild salmon due to the difference in their diets. In the wild, salmon eat smaller fish. These are a source of EPA and DHA, as the Omega 3 has been passed down the biological chain from algae. However, in farms salmon are fed on high-protein food pellets, which are usually made from plant and animal sources, or wild fish.
One study undertaken by researchers at Stirling University in 2016 found that the amount of Omega 3 in salmon had halved in five years. Professor Douglas Tocher, who led the research, told a publication at the time: “About five years ago, a portion of Atlantic salmon of 130g was able to deliver 3.5g of Omega 3. This is actually our weekly recommended intake. Now, the level of Omega 3 has halved. Therefore, instead of eating one portion of farmed salmon, we would need to eat two portions.”
Fish farms have also been linked to pollution, antibiotic overuse and emissions: in 2017, the presenter Jeremy Paxman warned that salmon farming has done “enormous harm” to fish stocks and the environment. One particular cause for concern is microplastics – tiny pieces of plastic that are broken down by the elements over time. A study by Hull York Medical School and the University of Hull found up to 10.5 pieces of microplastic per gram of oysters, mussels and scallops, with the highest concentrations in those collected off the coast of Asia. Some scientists have raised concerns that ingesting microplastics may be harmful to people’s health, but we still don’t know how much is actually consumed when we eat fish.