You always buy free-range eggs, and meat has to be from a traceable farm. Your veggies are in season and certified organic and you’ve even checked that the palm oil in your biscuits is sustainable. Yet who shops for fish with the same forensic care?

Fish and seafood are no doubt a broadly healthy choice, being low in calories and nutrient dense, with many varieties providing omega-3 essential fatty acids.

But now that more of us than ever are cutting back on meat and picking the salmon salad pots over the chicken sandwich at lunchtime, it’s time to take a closer look. Is wild salmon really superior to farmed? How worried should we be about mercury and microplastics in our sea bass? Here the experts offer their guide to getting fish right.

The trouble with supermarket salmon

With a huge amount of salmon fished coming from farms, concern is growing over the environmental and health implications of intensively farmed salmon, for example the use of antibiotics to treat sea-lice infestations and disease.

Salmon caught in the wild is considered to be nutritionally superior, because it feeds on smaller fish that have eaten omega-3-rich algae, while farmed salmon are increasingly fed vegetable oil. One 2014 study of salmon sold in the UK found farmed salmon had twice the amount of fat as wild salmon, a lower proportion of omega-3s and significantly more omega-6 fatty acids (which have been linked to diseases such as heart disease) – although the authors stressed that farmed salmon is still one of the best sources of omega-3s.

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However, Prof Douglas Tocher of the Institute of Aquaculture, University of Stirling, challenges this and says his research shows farmed salmon actually has higher levels of omega-3s than wild, because by the time a wild salmon is caught, much of its fats have been burnt up on its journey to spawn.

‘With farmed salmon,’ he adds, ‘the feed is completely controlled so we can guarantee absolutely no issues with mercury or microplastics. We can’t give this guarantee for wild salmon. I would definitely recommend eating farmed salmon over wild.’

There has been a recent growth in inland farming which implement technology where only clean water is used, guaranteeing no contamination from microplastics, fertilisers and other unwanted chemicals or antibiotics.

Lower the mercury

An Indian study published recently found that the more fish we eat, the more mercury we ingest – and fish eaters have a ‘significantly higher mean value of mercury compared with vegetarians’.

When mercury accumulates in the body in high doses it is damaging to the kidneys and other organs, but many experts say that the benefits of eating fish outweigh such risks.

Remember that not all fish are contaminated to the same extent – the highest levels come from larger predator varieties, such as tuna, shark, halibut and swordfish, which live longest and build up the strongest concentrations. In one study, a kilogram of cod contained 0.09mg of mercury, compared with 0.89mg in a kilo of swordfish (squid and mussels did even better, at 0.03g and 0.04g respectively).

Pregnant women are advised to avoid higher-mercury fish such as swordfish completely, and limit oily fish to two portions a week, says Rebecca McManamon, a consultant dietitian and spokesperson for the British Dietetic Association. Mercury can cross the placenta and impact the development of the nervous system of the unborn child.

Widen the net

Salmon carries a ‘health halo’, but it isn’t the only way to get your quota of omega-3s. Silver cod, haddock, sardines, mackerel and herring all count as oily fish (tuna, whether fresh or canned, does not count as it does not contain enough omega-3 oil).

‘Smaller oily fish where you can eat the bones, such as anchovies, are particularly good,’ says nutritionist Kim Pearson. ‘As well as providing as much omega-3 as salmon, the bones also offer calcium, which is important for maintaining healthy bones – though beware some tinned varieties, which are high in salt.’

Shellfish are also overlooked as a good source: mussels, oysters and squid contain omega-3 fats, though less than oily fish. For all-round health and sustainable smugness, mussels are an excellent and overlooked health food, says Pearson.

‘They are particularly nutrient dense – and an excellent source of iron – and are among the least intensively farmed animal food,’ she says.

‘They are typically farmed on ropes in the sea. No chemicals are needed because the mussels take all the nutrients they need from the sea.’

Gus Caslake of Seafish, an organisation working to improve efficiency and raise standards in the seafood industry, says we could all eat more crab.

‘It is low in calories, high in protein and contains plenty of omega-3 and almost as much protein per 100g as meat, without anywhere near the same levels of saturated fat.

‘Crabmeat is particularly rich in selenium [a mineral important for immunity], containing three times the amount that cod offers and 12 times that of beef.’

Brown crabmeat is more nutritious than white, he says – and is richer in omega-3s than prawns or even sardines.

Can I trust the water?

We’re hearing more and more about the sorry state of our seas and rivers – recently, scientists at the University of York reported that the world’s rivers are now contaminated with dangerous levels of antibiotics.

Belgian scientists have reported that the presence of marine microplastics in seafood could pose a threat to food safety, and a study by the University of Plymouth found around one-third of fish caught in the English Channel contain microplastics.

Prof Richard Thompson, who led the latter study, says current levels of microplastic in fish and seafood are currently too low to cause concern, though the findings are a clear indication that we need to change our ways in order to reduce the release of plastic to the environment.

In terms of purchasing fish, there’s little you can do to avoid the microscopic levels of pollutants present.

Fresh, canned, frozen or smoked?

Fish counters are decorated to give the impression the produce has just been plucked from the waves that morning, but "fresh" fish can vary in degrees of freshness, says McManamon.

‘Try to buy from reputable sources or those with a good food hygiene rating and get it straight home as it can spoil quickly out of the fridge.’

Don’t scoff at fish in the frozen aisles: as with frozen vegetables, freezing helps retain the highest levels of nutrients, she says.

Canned fish has roughly the same nutritional value as fresh, and is more likely to be wild than farmed. Canned fish such as sardines may even provide more calcium than fresh because the processing softens the bones, making them easier to eat.

Tuna, whether canned or fresh, contains DHA, believed to reduce inflammation, protecting the heart and supporting brain function and eye health. But choose varieties canned in water or tomato sauce, says McManamon. ‘Brine will add additional salt, and oil adds calories.’

What about smoked salmon? McManamon says it’s better as an occasional treat.

‘Smoked fish has less omega-3 than fresh or frozen varieties, as well as being very high in salt.’

The Daily Telegraph