Farmed salmon is becoming less nutritious, a major study has found.

The amount of the key nutrient omega-3 it contains has halved in the past decade.

The fatty acid is recommended by doctors to combat a variety of conditions such as heart disease, diabetes and arthritis. It also helps brain development in children and is almost exclusively found in oily fish.

A major study into the nutritional benefits of farmed salmon found that in 2006 a 130g portion contained 3.5g of omega-3 – the recommended weekly intake.

The same portion analysed in 2015 contained just half the amount – 1.75g.

The amount of the key nutrient omega-3 salmon contains has halved in the past decade

Douglas Tocher, a professor of nutrition at Stirling University and co-author of the report, stressed that farmed salmon was still one of the richest sources of omega-3 – only mackerel has more.

He said levels had fallen due to a change in the feed provided at farms. A decade ago there were complaints that the salmon were being fed too many smaller fish, such as anchovies, and fish meal.

The huge demand for the smaller fish to produce the fish meal was criticised by environmentalists as unsustainable and led to over-fishing.

But Mr Tocher said it was these marine sources that provided the omega-3.

‘Ten years ago people were complaining about the level of marine ingredients in salmon feed,’ he said.

‘Now, the majority of materials in feed are plant-based – it’s the complete opposite. The consequence of this is the level of omega-3 will go down.’

One million smoked salmon meals are eaten in the UK every week. A Government report published earlier this year into Britain’s changing food and drink habits found that salmon purchases had increased by 550 per cent.

And in 2013, salmon overtook shrimp as the most popular fish worldwide – accounting for 17 per cent of the total traded value of seafood.

The soaring demand is part of the reason why farmed salmon are being fed less of the oily fish as demand outstrips supply. In 2006, 80 per cent of the average salmon’s diet was made up of oily fish. Now it is just 20 per cent.

A fishing boat at a salmon farm in Lofoten, Norway; the way the fish are fed has affected their nutritional value

Dr Tocher and colleagues, who analysed the omega-3 content of 3,000 farmed Scottish salmon harvested between 2006 and 2015, published their analysis in the journal Scientific Reports.

The professor added: ‘I don’t want this to sound negative. Despite everything, farmed salmon is still the best source of long chain omega-3s. Even with the fall, they still have more than wild salmon.’

He said farmed fish have more of the fatty acid than wild because by the time a salmon is caught, much of its fat and oil has been burnt up on its long journey to spawn. In comparison, farmed fish retain more fat and oil, and more omega-3 as a result.

The plant-based ingredients fed to farmed salmon are mainly oil seeds. To overcome the fall in omega-3, attempts are under way to cultivate microalgae, which are part of the marine food chain and contain the nutrient.

Rothamsted Research in the UK is also attempting to genetically engineer camelina, a relative of rapeseed that accumulates high levels of omega-3 oils in their seeds, as an alternative to marine sources.

Professor Jonathan Napier at Rothamsted said: ‘The production of fish oils is no longer limited by what is available from wild capture from out of the oceans and is also going to be helped by agriculture.