From juicy tiger prawns to succulent salmon, tasty smoked haddock to meaty crab, these days you can buy almost every type of food the sea has to offer at your local supermarket.

As a nation, we consume almost 900,000 tons of seafood a year, with manufacturers going to ever-greater lengths to keep up with demand. But, experts say, the huge selection may not be everything it seems.

From pumping fish full of dyes and preservatives to make it look better and last longer, to overlooking infestations by lice and other parasites, and mislabelling the country of origin, producers have been accused of all sorts of fishy misdemeanours over the years.

Paul Wild, an Essex-based fishmonger and founder of Fresh and Wild Fish, has gone viral on social media for exposing the industry’s secrets, with his shocking videos racking up more than a million views.

Some fish, he claims, are so ‘polluted and diseased’ he wouldn’t eat them or feed them to his children; while others scarcely resemble what is described on the packet.

With his insider knowledge, we expose the dark secrets behind your supermarket seafood…

An 18,000-mile journey over 12 months 

Much of our supermarket fish – wild salmon, cod, tuna, hake and some shellfish – comes from the Pacific, meaning it faces a long journey before reaching our shops.

A 2018 investigation found some supermarket products travel 18,000 miles, during which time they’re frozen and packed on ice to preserve them. 

Many come via a processing plant in Qingdao, China, where they are defrosted, filleted and portioned up before being speed-frozen. Tesco is transparent about its links to China, with many of its fish products packed there.

They’re then shipped around Asia and the Middle East, through the Mediterranean and around southern Europe to reach Britain by the lorry-load, where it’s all defrosted once again before being displayed in the fish aisle.

This mammoth journey can take up to a year, especially when it comes to wild salmon – which can only be caught between May and September, and is stockpiled frozen so it can be sold year-round.

So the fish you buy ‘fresh’ could, in fact, be up to 12 months old.

You won’t find any of this on the packaging, however, which tends to describe how the fish was caught and where it was processed but not how long it’s been sitting on ice on a container ship.

Dyes to make it look good 

Ever wonder what makes that smoked haddock so yellow, that tuna fillet so pink or your smoked salmon such a lovely ruby-red?

The answer is dye – designed to simulate traditional smoking techniques – some of it natural and some comprising harsher chemicals with unproven effects on our health. M&S, for example, uses paprika to dye its Scottish smoked haddock fillets, while Iceland uses curcumin (a natural yellow-orange pigment derived from turmeric) in its frozen smoked haddock.

Some suppliers, such as Ocado, also add annatto norbixin, a natural orangey pigment, to their smoked haddock and basa ranges.

‘Supermarkets use so much dye that it comes off on your hands when you touch the fish. You’d never get that with produce from a fishmonger,’ says Paul. And other dyes are less harmless. Farmed salmon are commonly fed two chemicals in their food pellets – astaxanthin and canthaxanthin – to give their flesh its characteristic colour.

This is designed to simulate the hue the fish would acquire from eating shrimp and algae rich in colourful carotenoids in the wild.

However, synthetic astaxanthin, a compound produced from petrochemicals, has been associated with human health risks, such as gastrointestinal issues, low blood pressure and skin irritation, while high doses of canthaxanthin have been linked to eye damage.

Meanwhile, tuna used in supermarket sushi is sometimes treated with carbon monoxide or filtered smoke, which keeps its flesh bright red and appetising.

Though not dangerous in itself, it is controversial, as it can make spoiled fish appear fresh.

Don’t confuse basa with bass

Basa is one of the most common supermarket fish but most shoppers would struggle to identify it.

A type of mud-dwelling catfish native to South-East Asia, the vast majority is imported from the Mekong River Delta in Vietnam.

It was originally sold there as ‘river cobbler’, then ‘pangasius’, before being rebranded around 2010 as the more palatable-sounding ‘basa’ – now a popular alternative to cod or other white fish.

‘It’s a play on bass,’ says Paul. ‘If you don’t look at the packaging carefully, you could easily confuse the two. That’s a deliberate marketing ploy to make you buy it.’

But, he warns, basa is very different to sea bass, not only in origin and price (basa is two-thirds cheaper) but in how it is produced – and the risk of parasites and disease.

Such are the environmental and health concerns over open cage farming of the species that basa is banned in some US states, though tests by Asda and Tesco in the UK have found no trace of toxic contaminants in basa.

‘Fresh’ fish – stuffed with preservatives

Look closely at the labels on your supermarket fish – especially the fresh stuff – and you’ll find a long list of chemicals and preservatives in the ingredients list to extend their shelf life.

‘Fresh fish shouldn’t last three weeks like some supermarket products claim to,’ says Paul. ‘We buy crevettes direct from France and, properly vacuum-sealed, they last a week at most.’

Common preservatives include: sodium tripolyphosphate, used to retain moisture and improve texture; potassium sorbate, used to prevent spoilage; and sulphites, used on crustaceans to prevent black spots forming on their skin.

Iceland’s frozen wild red shrimp, which hails from Argentina, contains sodium metabisulphite, a white powder that coats the shellfish and inhibits the enzyme that makes it go brown.

Frozen seafood may be 60 per cent added water

Supermarket fish often contain added water, especially frozen products, which means you’re paying more for less product.

Look for the words ‘ice glaze’ in the ingredients – some products contain up to 60 per cent added water by weight.

As well as bumping up the profit, treating supermarket seafood with water or liquid chemicals can make it look plumper and more appetising – especially with products such as prawns or scallops.

During the defrosting process, Paul explains, fish products are put in a large chamber fitted with a tank containing a solution that raises their temperature slowly while pumping them full of a water-based solution (often brine).

‘This makes them last longer and can make them balloon to twice their natural size,’ he says. ‘But they lack texture and flavour, and when you cook them you’ll get loads of water coming out.’

Diseased and infested prawns and salmon

Farmed salmon is susceptible to outbreaks of sea lice, a parasite that feeds on the fish’s skin and blood and can be fatal to the fish.

In November, Tesco was forced to suspend supply from Bakkafrost Scotland, a salmon farm in the Highlands, after an animal rights group covertly filmed fish covered in sea lice in a pen.

While the parasite doesn’t pose a risk to humans – and is removed during processing, as well as being killed during cooking – the pesticides used to treat it may be. These have been linked to risks of cancer and other health issues, especially in children and pregnant women. There is also controversy over the use of formaldehyde, in a form called ‘formalin’, that is used in Scottish salmon farming to treat parasites, fungi and bacterial diseases.

This is classed as a carcinogen and exposure can cause respiratory irritation, eye damage and skin concerns, especially to those working with it on fish farms.

‘Fish’ fingers with just 58 per cent cod

They may be a staple for young children, but do you really know what’s in those breaded fish fingers you’re serving up for dinner? 

Some supermarket own-brand products (such as M&S fish fingers) are made with haddock, while others (including Waitrose Essential fish fingers) contain cod.

And, in some cases, there’s very little fish in that coating – which is made with potato starch, flour, salt, breadcrumbs, spices and oil, as well as natural colourants (such as turmeric and paprika) to give the fish fingers that golden crumb.

Iceland’s ten breaded fish fingers, for example, contain just 58 per cent fish (the cheaper Alaskan pollock, not haddock or cod), while Morrisons’ ten cod fish fingers are made with 64 per cent pollock.

The origin of the fish is another point of controversy: in 2022, leading brands such as Birds Eye and Young’s were called out for continuing to use Russian whitefish after the Ukraine invasion.

Other fish fingers claim to contain added omega-3, the polyunsaturated fats essential for heart health – but experts say this is just a marketing con. A typical serving of four enriched fish fingers provides just 130mg of omega-3, while one portion of oily fish can contain up to 2,000mg – so the benefits are negligible.

Processed in Britain – but caught abroad

Up to 80 per cent of supermarket fish comes from abroad – some from the other side of the world – where production standards and labour contracts differ wildly to those in the UK.

Most varieties come from large international fishing operations in China, Taiwan and South Korea, while some are caught in Vietnam and the Philippines.

A significant portion of fish sold as ‘Atlantic cod’ is sourced from the North-East Arctic and Russia.

Meanwhile, much of our sea bass comes from fish farms in Turkey. Last year, an investigation linked these farms – which supply sea bass or bream to Morrisons, M&S, Sainsbury’s and Tesco via wholesalers – to a fishmeal factory in Senegal, where food insecurity and unemployment is rife.

And prawns come from all over: larger king or tiger prawns from India, Vietnam, Thailand,

Madagascar and Honduras; while smaller, cold-water prawns are sourced from the waters around Norway, Greenland and Canada.

Don’t be fooled by the British flag often printed on seafood products. All this means is they were packed or processed here. You’ll find the country of origin in the small print. If you want to buy British fish, try trout, salmon (check that it’s Scottish) or sardines.

‘Some cod may also be sourced from UK waters,’ says Paul.