We are all eating much more fish than we used to - but are we eating the fish we think we are?
Official figures show that global consumption of fish and seafood per person is rising steeply - but research also reveals that much of what gets sold turns out to be not as described on the packet.
Earlier this year Europe's horsemeat scandal revealed how processed meat can get mislabelled in a complicated supply chain. That appears to be an issue with fish, too.
On a large scale, cheap fish is being substituted for expensive fish without the consumer knowing. Moreover, new varieties, never before consumed, are being detected in fish dishes.
Take a British national dish, for example: fish and chips. It is often thought to be the epitome of Britishness - "as British as fish and chips", the saying goes.
But scientific testing reveals that the traditional cod or haddock and chips is often something else entirely. Research reveals that 7% of cod and haddock - the deep-fried staples of British fish and chips - actually turn out to be cheaper fish substituted to cut costs.
In the Republic of Ireland, a similar study of samples bought in Dublin restaurants, shops and supermarkets revealed that a quarter of products labelled as cod or haddock were in fact completely different species.
In the United States, a study showed that 25% of the fish served in restaurants in New York were not what they were said to be on the menu.
And in Europe, about a quarter to a third of fish products tested turned out to be not what was described on the packet or menu.
The global industry transports large amounts of frozen fish around the world in containers, with China producing much of it. This means, for example, that one of the biggest points of entry for fish into the European Union is not a port at all - no wharves or boats or even water. It is Frankfurt airport.
Samples here and elsewhere across Europe are tested at the big Eurofins laboratory in Hamburg. Its Director of Scientific Development, Dr Bert Popping, said that tests were turning up types of fish which had never been in the food chain before.
"The authorities at the airport in Frankfurt have found some new species - species which have not been caught previously; fish species which have not previously entered the food chain; which have not previously been commercialised," he said.
So researchers believe that there is large-scale deception going on when it comes to fish - cheap is being substituted for expensive, so deceiving the consumer and bumping up the profits of the deceiver.
Dr Stefano Mariani, a biologist at the University of Salford in the north of England, did one of the studies. He said: "Consumers should be able to go to a shop and know they are eating what they paid for."
His findings in Britain and Ireland were that cod was being substituted with cheaper fish like pollock and Vietnamese pangasius, which is farmed in estuaries in South-East Asia.
Nobody claimed there was a health risk - just that people were being deceived when they bought what they thought was an expensive fish. The lobbying group Oceana, which campaigns for tighter controls on fishing, said the industry was "murky and complex".
One of its scientists, Kimberly Warner, told the BBC that mislabelling of fish and seafood mattered not only because of the deception of consumers, but also because threatened fish, in overfished parts of the ocean, could be sold as unthreatened, abundant varieties.
"If you are going to pay for a wild seafood product, and you want to choose that seafood carefully for your health or for conservation concerns, you will not have that opportunity if you are just being served anything which the industry wants to serve up to you," she said.
The scientists who have studied the matter believe that mislabelling of fish is too widespread to be just an accident. They suspect fraud.
Dr Mariani noticed, for example, that the mislabelling in Britain and Ireland seemed to be concentrated in a few fish producers.
"We noted that there were some suppliers that were consistently handling fish that was proven to be mislabelled, which suggests that a lot of mislabelling occurs before the fish gets delivered to the supermarket," he said.
He wants tougher regulation and more effective labelling, so that fish can be sourced and traced.
He is joined in that by some of the reputable sellers of fish. Mark Drummond is the vice-president of the National Federation of Fish Friers, the trade association for Britain's fish-and-chip shops. He also owns a busy fish-and-chip shop in the district of Idle in Bradford, Yorkshire.
His shop bustles with customers, even though he admits it isn't the cheapest in the area. On the wall behind the chip pans and sizzling haddock, there are signs saying when the fish was caught and by what ship.
"I think it would help everyone if every fish consignment had a label saying exactly what it was. The pubs or cafe or restaurant could pass that information on to their customers," Mr Drummond said.
He says the problem with mislabelling happens more with "wet fish" - fish which is not fully deep-frozen but which is only kept cold on ice.
The fish he uses, for example, is frozen at sea and labelled immediately.
"It's produced on factory trawlers by Icelanders or Faroese or Russians. It's all frozen on the boat within an hour-and-a-half of coming out of the water, and it's labelled where it was caught, the date it was caught, what the species is, so I always know exactly what I've got.
"If you're just buying wet fish and it just comes in an unlabelled polystyrene, insulated box, that's when it becomes more difficult to be absolutely sure that you're getting what you're supposed to be getting."
Just how fresh?
He adds that fish frozen immediately at sea is fresher than so-called fresh fish. The reasoning is that fish landed on the dockside and then chilled for transportation may have spent some days in that state before reaching the consumer.
"If I didn't use frozen fish here in West Yorkshire", he said, "it would have been brought in probably from Aberdeen.
"The boats sail out for a couple of days, fish for a couple of days, sail back for a couple of days. [The fish] can be five days old when it lands in Aberdeen and another day being transported to West Yorkshire.
"A day or two in the shop and it can be seven or eight days [old] when it's used. There's nothing wrong with that when it's been packed in ice. It's not off, but our fish is frozen within an hour-and-a-half of coming out of the water and we use it on the day when we defrost it."
Either way, he wants more accurate labelling to protect the consumer, but also the reputable restaurant which does not cut its costs through deception.
This global industry is a fishy business - but better labelling, he feels, might help it become less so.