According to a UN report published in July, 87 per cent of the world's marine fisheries are either fully exploited, over-exploited or depleted, meaning there are less fish in the sea for us to eat.
Organisations such as Greenpeace say that to preserve fish stocks, we should think before we buy and only buy fish and seafood that's been responsibly caught. Here's our guide.
Eat different species of fish
Tuna, salmon, cod, haddock and prawns make up an estimated 80 per cent of all fish eaten in the UK.
TV chefs such as Hugh Fearnley-Whittingstall and Jamie Oliver took to our screens last winter to encourage us to eat a wider range of fish, the idea being that this would take pressure off the 'big five'. It also gives us an excuse to try other sustainable species of fish swimming around our shoreline and in our rivers.
Try sardines instead of tuna, rainbow trout instead of salmon and white fish including coley and pollack instead of cod and haddock. Also try other varieties of seafood from our shores, like gurnard, queen scallops and mussels.
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Avoid certain species altogether
While it might seem macho to slurp on shark fin soup, it doesn't make great environmental sense.
A Canadian study in 2003 found that the numbers of many species of shark had declined by 75 per cent in just 15 years. And that was almost a decade ago; since then, demand for their fins has risen.
Sharks are especially endangered because they take longer to mature than many other species and therefore take much longer to recover from overfishing. Other fish on the Marine Conservation Society's 'avoid' list include conger eel, marlin and Bluefin tuna.
Eat less fish
The Food Standards Agency advises us to eat "at least two portions of fish a week, of which one should be oily." But according to Greenpeace, this could actually make the problem worse.
"To increase fish consumption by this amount for 49 million adults in the UK would require an extra 33 million portions of oily fish per week," they say.
"This implies an increase in present levels of total fish consumption of over 40 per cent and of oily fish by 200 per cent."
They argue that this current advice places even more pressure on our fish stocks. Greenpeace also point out that the important omega oils found in oily fish can also be found in other foods, such as walnuts and some oils.
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Choose sustainable and responsible fishing methods
While there have been concerns over fish stocks — especially over Britain's 'big five' — others say there's enough to go around, as long as it's responsibly sourced.
"There's a common misconception that you can't buy sustainable cod," James Simpson from the Marine Stewardship Council told us. "Actually you can."
In fact, MSC-certified cod, haddock, salmon, tuna and prawns can all be found in UK stores. Check labels for the MSC logo, which indicates that the fish was sourced responsibly, and avoid fish that have been caught by beam trawlers, which can damage the seabed.
Also, go for hand-dived scallops and line-caught fish, which are more targeted types of fishing and don't result in lots of bycatch.
If you visit your favourite restaurant and always see just the big five being served, ask why. Many chip shops have now started serving a wider range of fish rather than just the traditional cod, and according to the Fish Fight website the MackBap (a battered mackerel fillet in a bun) is now served in 626 chip shops around the country.
Before you buy, ask your waiter, fishmonger or the supermarket staff how the fish was caught: "If they can't tell you or if you are not completely satisfied with their answer, don't buy the fish!" Greenpeace say. So do your bit for the environment — and the fish — and use your consumer power.
Do you think there's a real problem with our fish stocks? Do you think about sustainability when you buy fish? Or do you always stick to the 'Big Five'?