Salmon is an easy fish to love. Simple and tasty, its pink flesh is pulled from supermarket shelves and transformed into an array of increasingly exotic dishes.
Fried with wasabi, baked with sea-salt, or served Thai-style on a bed of noodles, the Atlantic salmon (Salmo salar) is an everyday stalwart of the nation's cookery shows, recipe books and domestic kitchens.
So common, in fact, that a species once an aristocratic delicacy has become the fish most commonly eaten in the UK – more popular than tuna, cod or haddock. One million people eat salmon every day.
Until now, though, little attention has been paid to its provenance. People know about the threat to cod posed by overfishing, or how tuna trawlers also scoop up dolphins. But salmon? Is it farmed or wild; kind or cruel; sustainable or environmentally damaging?
Salmon is actually one of the hidden problems in the meat and fish business, according to environmentalists and animal welfare campaigners.
Salmon are naturally programmed to swim hundreds of miles, moving downstream from their birthplaces in British rivers to the open ocean, and then back – leaping upstream in the rivers – to spawn.
In fish farms, their complex life cycle is artificially managed by man. Despite their extraordinary journey in the open sea, the fish are kept in 100-ft wide pens. Some escape and infect their wild cousins with lice.
To answer the critics, some British supermarkets have begun to insist on their salmon being produced to higher standards. This summer Marks & Spencer became the first retailer to switch its entire range of farmed salmon to the Freedom Food scheme run by the RSPCA. Sainsbury's has also introduced Freedom Food salmon as part of its pitch to middle-market gourmands.
Both companies are vaunting their newly accredited salmon as evidence that they take animal welfare seriously; that their fish is virtuous. Five million fish a year will have their lives improved as a result of the conversion to RSPCA standards.
But just how much better is fish with the Freedom Food label than ordinary salmon – and, in any case, should we be eating farmed salmon at all?
To start with, we need to look at the reality of Britain's most popular fish. The overwhelming likelihood is that unless your salmon was sold as "wild", it has been farmed, perhaps in Scotland, but quite possibly in Norway or Chile, both big players in the industry.
According to the RSPCA and Compassion in World Farming, fish farmers have traditionally paid little attention to the welfare of their silvery charges.
Environmentalists have complained that salmon farming is denuding the sea of the smaller wild fish fed to the carnivorous salmon. By the time it is harvested from a Scottish loch or Norwegian fjord, a salmon will have consumed many times its final weight in sand eels or whiting.
Whatever the concerns, there is little prospect of a large-scale return to wild salmon. Fish farming is likely to be with us for the long-term, prospering from the failure of political leaders to stop over-fishing, which, according to one recent study, is likely to destroy wild populations as soon as 2048.
With the world's seven billion people demanding protein, aquaculture is filling the gap. In the past 35 years, the proportion of farmed fish has risen from 5 per cent to 40 per cent in 2005 of all fish consumed.
Aquaculture is scheduled to go on rising, eventually soaring way above the diminished annual wild harvest.
According to the Scottish Salmon Producers' Organisation, the UK salmon industry is now worth £1bn a year. Conventional fish is stocked in densities of 20kg per cubic metre of seawater. The most cramped fish can be identified by their withered dorsal fins; their flesh is flabbier, the white fat thicker. They don't taste as nice as salmon given more space and care.
So three years ago Marks & Spencer decided to look afresh at its salmon and devise a new system that would be better for the animals and kinder to the environment. It came up with the Lochmuir brand, now used in all its products from ready meals to smoked slices. The fish are farmed in the west of Scotland, where the jagged coastline shelters the sea coves and lochs.
Instead of being hauled to shore in nets, the salmon are slowly gathered by a £10m well-boat, which lowers their temperature to make them less sensible to their slaughter. As they swim gently round the large well around which the boat is constructed, John Rea, Scottish Sea Farm's production manager, says: "I'm impressed by those dorsal fins. They're not very typical on farmed fish. They're an expression of what M&S are trying to do. If you had these in an intensive farming situation you would find the fins were quite eroded. But the fin condition here is pretty good – they are pretty close to wild dorsal fins."
The creatures certainly look calm. "If there was a lot of thrashing around here, the fish would be very stressed," says Jim Gallagher, Scottish Sea Farms' managing director.
Andrew Mallison, M&S's fish buyer, specified that the wild fish used to feed the salmon should come from sustainable species and that the salmon should have 20 per cent more space. They grow 10 to 15 per cent slower than the industry standard. Fat levels are 15 per cent, rather than 22 per cent.
"Aquaculture can be done right, but there will always be horses for courses," says Mallison. "There will be people who don't want to pay a fair price for their salmon: they will always want it as cheap as possible. And you can make it cheaper than we do here. We have incurred costs by saying that we want better feed and we don't want to push so many fish into a cage, but all those things we believe are worth it because we think that's what our customer wants: they want fish grown well with a good eating quality.
"You can go to another farming operation and you will have two boys in boiler suits sitting in a Portakabin on the shore of a loch somewhere," Mallison adds, "and they will go out and look at a couple of cages and will throw a bit of feed in. And they will harvest when they are required to, but they won't really know how good that fish was. They will never hear anything more about it unless it was really bad."
Gallagher suggests the perception of the industry is now "historic" – that salmon farming has improved, with better sea-lice treatment and stronger cages. He points out this is an industry which has only been around for 30 years – 10 life cycles of a salmon.
"Remember, our livelihood is based on not having escapes. We want to grow the fish on the farm to harvest size and then we want to sell it someone who appreciates what we have done. The industry has invested heavily in the infrastructure."
So what does Compassion in World Farming think of Freedom Food salmon?
"On an ideal level, we just don't like the idea of fish being farmed, particularly salmon," says Peter Stevenson, the organisation's chief policy adviser.
"In natural conditions, they travel and swim very long distances. They roam the oceans and therefore to farm them in cages, even to an organic or better standards is something we are unhappy with.
"But if you are going to buy farmed salmon then please go for organic fish or the Freedom Food brand. The RSPCA scheme shows that it is commercially feasible to have higher standards for farmed salmon."
Dish of the day: salmon facts
* Like trout, salmon is unusual in being both a freshwater and seawater fish. Salmon swim out to the ocean and back to lay their eggs.
* There are six stages in the salmon life cycle, which takes several years to complete, from spawn to adult: eggs/spawn; alevin (babies feeding from a yolk sac); fry (small fish that adapt to freshwater); parr (quickly growing fish that turn silvery-blue); smolts (young salmon ready to migrate); adult fish.
* UK shop sales of salmon are worth £1bn a year.
* Salmon generates £400m a year for Scottish fish farms
* Every day, one million Britons eat salmon.