With its brain-boosting omega-3s and high protein, tinned mackerel has long been the smart choice for those on a health kick.

But many canned varieties of the oily fish no longer contain any mackerel at all – and nutrition conscious shoppers will only see half the benefit.

Tinned fish giant Princes – which sells its products in supermarkets including Sainsbury’s, Lidl and Tesco – has switched the contents of its branded mackerel range to Chilean jack mackerel, which despite its similar name is an entirely different fish.

However, retailers are continuing to sell 'jack mackerel' products alongside traditional mackerel under signs that conflate the two very different products, according to the Financial Times. 

Jack mackerel belongs to the Carangidae family, while true mackerel belongs to the Scombridae family. 

Prince's jack mackerel also contains just half the amount of omega-3 as regular mackerel. 

Sue Davies, head of food policy at Which? said, 'Tinned fish is an affordable and convenient way for households to add important nutrients to their diets and eat the recommended two portions of fish a week, one of which should be oily.

'It's a real concern when manufacturers quietly downgrade everyday staples like these as shoppers can end up paying the same price for a product that delivers significantly less nutritional value.'

Omega-3 fatty acids are essential for everything from heart health – helping to remove harmful fats from the blood while increasing levels of good cholesterol – to dementia prevention. 

One study found that people who regularly eat oily fish have healthier blood vessels and lower blood pressure, which may help protect against dementia.

Another found that those who have higher levels of omega-3 in their blood perform better on thinking tests. 

Mackerel provides one of the richest sources of omega-3 per 100g and is ahead of fish such as salmon and sardines.

Demand for tinned fish has soared over the last two years, with sales up nearly a fifth – as money-savvy consumers look for cheap sources of healthy, high-protein foods. 

However, some shoppers say they have noticed changes in taste and texture since switching to the canned variety, reporting that the fish is less oily than traditional mackerel.

'It smells rather metallic... like cheap rubbish tuna,' canned seafood reviewer Marcus Ansell said in a recent video posted to his 650,000 Instagram followers. 

The move away from true mackerel has largely been driven by sustainability concerns. 

Waitrose halted sales of fresh, chilled and frozen North Atlantic mackerel in April, due to concerns about overfishing. 

Its canned products are also being phased out as stocks last. 

Lidl, meanwhile, has been transitioning to sustainably sourced Chilean jack mackerel – which it sells under supermarket signs that simply read 'mackerel in olive oil'. 

And despite 'jack mackerel' being roughly a third cheaper than true mackerel, supermarkets including Tesco continue to sell the substitute fish at similar prices, with a discrepancy of just 20p. 

However, the switch is not universal. Supermarket own-brand tinned mackerel still  contains true mackerel.

The mackerel catch in the North East Atlantic has remained 39 per cent higher than recommended limits for more than 15 years. 

As a result any mackerel caught in the area no longer holds a Marine Stewardship Council certification – a sustainability badge required by many major retailers. 

Chilean jack mackerel, however, is certified. 

Until last year, the species could not be labelled simply as 'jack mackerel' in the UK.

However, weeks after Princes launched its new tins, the Department for Environment, Food and Rural Affairs amended its rules to permit the new labelling. 

But the move has sparked outrage among critics, who say the new rules blur the distinction between true mackerel and jack mackerel, potentially catching out consumers 

'Consumers will be suffering because pure mackerel is a much better species to eat than jack mackerel,' Stewart Harper, former managing director of North Atlantic Fishing Company, told the Financial Times.