The 'fresh' fish you buy at the supermarket counter may be as much as nine days old, it's been revealed, and could keep less well than the stuff you buy frozen.
In order to keep counter-bought fish appealing and safe to eat, it's likely to have been frozen along the way, says laboratory Premier Analytical Services, as after freezing, it deteriorates more slowly.
Supermarkets aren't allowed to sell previously frozen fish as fresh, and any previously-frozen fish will be labelled as such - but the signs aren't always easy to spot.
And the results of the lab test indicate that you really might as well buy frozen fish straight from the supermarket freezer section and then defrost it yourself, as it's likely to be a lot cheaper.
In research carried out for Iceland, the lab tested sea bass, tuna, rainbow trout and Atlantic salmon from Iceland, Tesco, Asda and Sainsbury's, both immediately after buying and 22 hours later.
It compared frozen fish from Iceland with fresh fish from its competitors. And, it found, the fish from rival supermarkets went off faster than Iceland's own.
In the case of sea bass, for example, Iceland's frozen fillet and Sainsbury's fresh fish counter sample both had 'mild' odours and hadn't deteriorated much after 22 hours.
Asda's fresh fish counter sea bass, though, had 'significant' deterioration, and Tesco's had a 'pungent' smell.
Iceland and Sainsbury's tuna deteriorated the least, while Sainsbury's was top when it came to trout and Iceland and Asda's salmon kept fresh for the longest.
"We are keen to shine a light on the benefits of frozen fish - and highlight the nutritional benefits, quality and great variety available," Neil Nugent, head development chef at Iceland, tells the Mail.
"Customers who buy straight out of a freezer will enjoy fish that's been preserved soon after it was caught for maximum freshness."
It's not just fish that can be a lot less fresh than you'd imagine. Some supermarket apples, for example, have been stored for up to a year; even bags of salad can be as much as three weeks old.