It's a bit of a mouthful - and one designed to reassure consumers about the ethics, safety, environmental impact and wisdom of rearing fish intensively like broiler chickens, pigs or cattle. Research carried out by Taylor Nelson Sofres for the Seafish Industry Authority reveals that consumers have 'overall negative attitudes' towards fish farming, due to a decade of bad publicity in the salmon industry. First, it was the damage done to the King of Fish by incarceration in overcrowded sea cages, then a series of revelations about dyed pink flesh, toxic chemicals, antibiotics and marine pollutants. Though the farmers have made determined efforts to clean up their act, abiding by stricter rules governing husbandry and food safety, ever more lurid scandals have raised their ugly heads. In 2000, Ardessie Salmon Ltd was thrown out of the Tartan Quality Mark scheme run by Scottish Quality Salmon for using toxic chemicals illegally. The following year, a Canadian government report revealed that four out of five salmon farms in British Columbia failed to dispose of sewage from crews' quarters correctly, and one in three discharged raw human sewage close to the surface where it could drift into cages where fish were fattened for market. By the following year, however, that figure had dropped to one in eight. Last month, in a hotel bar in Argyll, I heard about the latest mishap from a diver who works the salmon farms. 'One guy lost his entire stock,' he said, 'because he kept the fish too long. Salmon should be slaughtered before their migratory instinct kicks in, but these ones weren't; they suffered. We had to go into the sea cages and remove the lot - three and a half tonnes of dead fish - by hand.'

Such is the question mark over salmon that other farmed species have been overlooked, but how do we know the trout, sea bass, cod, halibut, tilapia and prawns we see in the supermarket are any more humanely treated - or better for our health? According to the charity Compassion in World Farming, they aren't. Trout on UK farms are stocked at even higher densities than salmon, its 2002 report In Too Deep reveals, with 60kg of fish per cubic metre of water (compared to 15 to 20kg/m <+>3 </+>for salmon in a sea cage) - the equivalent of 27 trout, 1ft long, sharing a bathtub of water. Nor is the method of dispatch more palatable, since trout are invariably left to suffocate in air - or in bins of ice, to increase shelf life - rather than being stunned by a blow to the head. Unlike salmon, most trout are the offspring of 'broodstock' treated hormonally so they give birth only to females, because these mature later and are better to eat. In addition, eggs are 'shocked' using heat or pres sure to produce only sterile fish (triploids) with three sets of chromosomes, not two. These convert feed to body weight more efficiently and can't breed with wild fish if they escape. 'Higher levels of spinal deformities have been found in triploid Rainbow trout compared to diploids,' In Too Deep maintains. 'Triploid fish can also be anaemic, showing lower blood haemoglobin levels.' In a separate section, the report reveals that farmed halibut, sea bass and sea bream can suffer from severe cataracts when reared intensively, causing blindness and corneal bleeding.

At the hatchery run by Trafalgar Fisheries near Salisbury, Wiltshire, I witness transsexual fish for the first time. In each of the 16 indoor tanks, 30,000 rainbow trout fry are swimming around happily like tadpoles. 'Their survival rate in the wild from egg would be two to three per cent,' says general manager John Williams, 'whereas we have raised it to 50 per cent.' Peering into one tank, I can't help noticing that some fry are writhing around and others look distorted or hooked in shape. 'That's just nature,' Williams says, pointing out that any random sample will contain aberrations. A few miles down the road, as we enter the farm, our Land Rover is sprayed and our boots disinfected to preserve the site's water quality (it is brought by gravity from the Avon and returned to the river pristine) and keep diseases at bay; I'm reminded that fish farming is just another aspect of agriculture, poised for the next viral epidemic to decimate the industry.

As we tour the 89 open-air ponds or 'stews', I am impressed. The rectangular earth pools look enchantingly natural, with swans and herons eyeing the livestock from outside the protective overhead netting - and you can see why. Though stocking densities are well within animal welfare guidelines, the 'fingerlings' crowding the shallows in anticipation of food are packed so densely they appear to squirm rather than swim. On a hot, steamy night or after a thunderstorm, Williams explains, water quality deteriorates and oxygen levels plummet. 'They're gasping,' he says, adding that an oxygen generator is used to restore gas balance. When trout are in such close proximity, damage is inevitable. 'You'll often get worn-down tails,' he tells me, showing me how the fin is rounded rather than triangular. 'Sometimes it splits down the middle, like this.'

Unlike most producers, Trafalgar uses an electric current to kill the trout instantly, rather than slow suffocation. A tonne of fish is transported from the stews into a system of concrete channels. From there, a rudimentary pump sprays them across a grader where fish of the correct market size are funnelled one way while smaller ones are returned to the ponds for 'on-growing'. As I watch, a 110-volt current is passed through part of the labyrinthine trough sealed with metal plates. As one stockman shouts 'Out!', the rest leap from the water. 'We're looking at even more humane ways,' says Williams, 'such as in-pipe electrocution using different voltages. That's being developed by Bristol University at our farm near Cirencester.'

It's distressing - but no more so than any sector of agriculture where beasts are slaughtered. If we eat fish, we can hardly expect it alive or complain that it has been killed. What we can expect is a product that is free of chemicals - but with farmed fish, that isn't always the case. First, there is the use of antibiotics and vaccines which remain in the flesh as residues. Then, because all cultivated species are fed fishmeal and oil extracted from small, bony 'trash fish' (or industrial catch), they may contain cancer-causing PCBs, dioxins and other marine pollutants. In the final quarter of last year, the UK's Veterinary Medicines Directorate (VMD) found residues of the pesticide DDT in three samples of Danish farmed trout, while PCBs were detected in one trout sample of dubious origin ('Produce of Denmark or France') and 16 packs of farmed tropical prawns. A further four samples of prawns were contaminated with other toxic chemicals. From this year, the VMD is extending its surveillance to farmed cod, haddock, turbot and tilapia. 'We're waiting for further intelligence from the border inspection posts,' says the VMD's Janet Rickard, 'because there may be other species coming in, too. Basically, we want to test anything that's farmed.'

No wonder consumers are wary of the word; no wonder retailers have avoided it so fastidiously in their labelling. But the truth is, we have been consuming farmed fish unknowingly for years, because until 28 March this year - a few days after my trip to Sainsbury's - there was no legal requirement to indicate its provenance. Now, fish sold in EU countries must be labelled 'farmed', 'caught at sea' or 'caught in inland waters' and information given about the species' name and catch area.

Though producers and some environmentalists argue that farmed fish are a necessary evil, taking pressure off overfished wild stocks, retailers with a conscience are less certain. 'I think fish farming has a future,' says Andrew Mallison, senior fish technologist at Marks & Spencer, 'but my personal view is that we should be trying to get wild, sustainable caught fish wherever possible and topping up with farmed.' Despite such reservations, the economics of aquaculture have never been healthier. Sales of farmed species have grown by 11 per cent a year since 1984, making it the world's fastest-growing food sector (valued at $54 billion). In 2001, some 29 per cent of all fish and shellfish eaten was farmed. The UN's Food and Agriculture Organisation has predicted that aquaculture will have to grow sevenfold in the next 25 years just to maintain the world's consumption of fish - and with such a bonanza imminent, corporate minds have focused afresh on the science-fiction dream of fish farming.

In Waltham, Massachusetts, the biotech company Aqua Bounty has submitted an application to develop a GM salmon - modified to grow up to six times faster than its wild counterpart. If the Food and Drugs Administration approves it, the transgenic fish could be on sale in US supermarkets by 2004. Meanwhile, Norway is building the world's largest industrial plant for the rearing of cod - the species most affected by overfishing. Cod Culture, Norway's factory on the outskirts of Bergen, will have the capacity to produce 30m juveniles a year. In Hordaland County, Mongstad Aqua has secured the right to take 15,000 cubic metres of cooling water an hour from a nearby refinery; it will be used to rear 10 million young cod. However, it is the Japanese who have taken marine cultivation to its absurd limit. Though experts dismiss the idea, the city of Hirado announced last year that it was planning to farm minke whales in a two-square-mile netted area as a novelty for tourists. It remained unclear whether they would be reared for their meat, supplying a restaurant trade worth £20m a year.

Against this Frankenfish background, even the most experimental fish farm in Britain looks relatively benign. At the Aquascot hatchery on Loch Striven in Argyll, western Scotland, revolutionary methods are being used to farm Atlantic cod. Nestling among bracken-covered hills in the shadow of a hydro-electric dam, the polytunnels, hangars and circular concrete tanks at Ardtaraig have a whiff of heavy investment about them - and part of it comes from Marks & Spencer, keen to see whether Britain's favourite fish can be reared from an egg and harvested initially from tanks and ultimately sea cages. 'I think we've proven that it can,' says Andrew Mallison, 'and what we're doing is the fine-tuning, the economics - what weight to grow the fish to, how to achieve a year-round supply. Farming offers convenience to the retailer, in as much as you can have what you want, when you want, to the size and quality you specify. It's controllable, whereas in the wild it's not. You rely on what is landed each day - and you haven't caught it until it's in your net.'

That, I always believed, was the romance of fish (though I have to admit the cod I am looking at have a certain charm as they break the surface to stare at us from their concrete enclosures). 'They have more personality than salmon,' site manager Willie Young assures me, as the juveniles in Tank G1 sense our shadows and wait for food. In this operation, little is left to chance. To keep disease at bay, cod are vaccinated by hand using a gun, by specialist teams capable of injecting 40,000 fish in a day. 'They're netted or pumped into a bath of MS-222, a licensed anaesthetic,' regional manager Andy Reeves explains, 'then injected.' Antibiotic use is kept to a minimum, heeding lessons learnt from the salmon industry - and male and female broodstock are allowed to mate naturally and alone (rather than being encouraged by hand), in a darkened tank referred to by Reeves as 'the honeymoon suite'. The true breakthrough, though, has been the rearing of live feed - tiny artemia shrimp and rotifers - to sustain the fry before weaning on to fishmeal. In a humid shed, vast tanks are used to cultivate rampant algae growth ('It's like a bubbling witch's cauldron,' says Reeve) to serve as food for the rotifers. These are in turn eaten by wriggling artemia, which the aggressive cod hatchlings devour in the water as soon as they sense movement. 'We're growing a food chain artificially,' Reeve explains, 'which is new.'

Despite Aquascot's smart technology and its attention to welfare, all is not as squeaky clean as our yellow PVC boots suggest. 'Apart from the dye issue,' says Don Staniford, an environmental campaigner and author of the forthcoming book Cancer of the Coasts , 'all the criticisms levelled at salmon apply to cod. You still require feed which depletes the oceans, and chemicals to control parasites and disease; those are inputs. Outputs are uneaten feed, escapes, discharges and waste. Cod farming pollutes the environment and isn't sustainable.' Even organic farms are dismissed as 'a consumer con' because, though they use no chemicals, they discharge effluent into the sea. 'Organic fish are still fed wild-caught fish,' he says, 'and any caught in the North Sea will be contaminated with PCBs and dioxins.' In a world where wild stocks are being fished to extinction and organically farmed fish is unsustainable, we are caught between the devil and the deep blue sea. 'The glimmer of hope is shellfish,' Staniford believes, 'because filter feeders such as mussels, scallops and oysters are indicators of pollution, not agents of pollution like salmon and prawns. They're herbivores, and their feed is already in the water so they don't need inputs. The caveat is the word carnivorous: any fish that eats more fish will create problems.'

At M&S, I ignore Staniford's advice and shop for farmed cod; there are two packs left, with the word 'FARMED' screaming at me in capital letters. No ambiguity there, but the wording on the back is an impressive piece of copywriting. 'Our fresh cod is farmed in the clear waters of the lochs of the west coast of Scotland,' it reads - which neglects to mention that the water is pumped into a concrete tank. When I taste it fried in flour, the fish is just as it says on the pack - 'chunky, moist and succulent', though more fibrous than wild cod. 'We had initial concerns about the raw flesh colour,' Mallison admits, 'which was looking grey for some reason. But we've developed the diet and overcome that, so the raw fillet looks very attractive and cooks to a snowy white. I think lessons have been learnt from the salmon industry - and the main one is, don't rush into it. A lot of people bought into salmon, set up farms and, when the market fell due to overproduction, began to cut corners. With cod, we're taking it one step at a time.'

What's in it for you?

Who farms it? Mainly Norway, followed by Chile and the UK. Worldwide production exceeds one million tonnes a year.

How? Juveniles are produced from eggs 'stripped' from female broodstock by hand and artificially inseminated. They are reared in freshwater tanks (as parr), then 'put to sea' (as smolts) in cages housing 5,000 to 50,000 fish.

What's in it? The colourings astaxanthin (E161j) and canthaxanthin (E161g) are used to dye flesh pink, though the permitted concentration of canthaxanthin was reduced by the EU in 2002 due to links with retina damage in humans. Fish are treated with antibiotics, some of which may remain as residues, and routinely injected with vaccines. The fungicide malachite green (a carcinogen) was banned last year, but traces have since been found in four samples of Scottish salmon and two from Norway. Because they are fed on fishmeal and oil extracted from 'trash fish' living in polluted waters, farmed salmon may contain cancer-causing PCBs, dioxins and mercury as well as pesticides. They contain more fat than wild fish.

Are the fish harmed? Though intensive farms are cleaning up their act, overstocking is still a problem. This contributes to the spread of diseases such as ISA (infectious salmon anaemia). Fish are starved before slaughter, then stunned with a blow to the head, followed by gill cutting to bleed them to death. Some are anaesthetised in CO 2 , which irritates the gills, then bled.

What about the planet? Diseased salmon can easily escape from cages and infect wild stock. Farmed fish that have lost their ability to migrate can breed with wild salmon, diminishing their urge to spawn. The chemicals cypermethrin, azamethiphos, teflubenzuron and emamectin benzoate (used to treat sea lice), together with faecal waste, pollute the oceans.

Who farms it? France, Italy, Denmark and the UK. Britain produces 16,000 tonnes a year, or 35 million fish.

How? Young female brood stock are fed or injected with testosterone, turning them into functional males; sperm from these 'males' contains only X chromosomes, so resulting progeny are female (females mature later than males, retaining better flesh quality). Equally common is triploidy, where eggs are manipulated using heat or pressure to produce sterile offspring; these grow more efficiently and cannot breed with wild stock if they escape. Raised in freshwater tanks and weaned on to fishmeal pellets, fry are transferred to earth ponds ('stews') or gravel raceways fed by rivers.

What's in it? The same E colourings are used for trout as for salmon. Antibiotics and vaccines are routinely given for diseases such as PKD (proliferative kidney disease) and ERM (enteric redmouth). Many trout contain geosmin, a chemical produced by a soil bacterium which gives the flesh a muddy taint, the result of poor water quality.

Are the fish harmed? Trout are kept at even higher stocking densities than salmon, some equivalent to 27 portion-sized fish sharing a bathtub of water. On muggy days, they gasp for breath. Fin damage and injuries are common. Further stress is caused by grading, where trout are pumped from the pond and filtered through grids to sort them by size. Slaughter is by suffocation on ice (to increase shelf life), though some favour CO <+>2 </+>baths or electrocution.

What about the planet? Trout may escape and breed with wild stock, or spread disease.

Who farms it? Norway will farm 10,000 tonnes a year by 2004. Scotland is more cautious: one of its companies, Aquascot will produce 3,000 tonnes by 2006, pending further research into juvenile production. A similar project is under way with haddock.

How? Broodstock mate with wild-caught males and lay their eggs naturally, though artificial light can be used to encourage spawning out of season. Juveniles are reared in tanks, then in some cases transferred to sea cages. Unlike salmon, cod fry have no yolk sac on which to survive. They feed on live planktonic animals, before being weaned onto fishmeal pellets. Farmers cultivate their own live feed, which is new.

What's in it? Cod are injected with vaccines. The most common is against marine vibrios (bacteria carried by all wild fish). Like salmon, the fishmeal fed to cod contains PCBs, dioxins and other pollutants.

Are the fish harmed? Cod are cannibalistic in the wild, so larger ones are removed from cages by grading (see trout) which causes stress. They are pumped or netted into an anaesthetic bath prior to vaccination. Stocking densities in cages are similar to salmon, with higher mortality rates. The oily diet fed to farmed cod early on results in swollen livers and deformities.

What about the planet? Though much has been learnt from salmon, such as siting cages in clean, fast-flowing waters, uneaten food and faeces are still discharged into the sea. Farmed cod can escape and breed with wild stock. Environmentalists say that, for every tonne of cod farmed, five tonnes of 'trash fish' will be used as fishmeal.

Who farms it? Production is in its infancy, led by Norway followed by Iceland and Scotland.

How? Juveniles are reared in hatch-eries then tranferred to sea cages. They have a diet of live plankton, followed by fishmeal pellets.

What's in it? The same drugs, pollutants and fat as all farmed fish.

Are the fish harmed? Stocking densities, grading and slaughter are the main issues; the eyes of halibut are easily damaged by overcrowding and percussive stunning. Suffocation on ice is common, as is bleeding after CO <+>2 </+>anaesthesia. Halibut are attract-ive because they are disease-resistant and tolerate stress well.

What about the planet? Halibut are happiest in sluggish waters, so the dispersal of effluent is a problem. Like other farmed fish, they consume fishmeal which depletes wild stock.

Sea Bass and Sea Bream

Who farms it? Greece (more than 60,000 tonnes a year, or 60 per cent of European production), along with Turkey, Italy, Spain, France, Portugal, Cyprus, Malta and Israel.

How? Fry are reared in hatcheries and fed on live crustaceans and plankton, then released into sea cages at high stocking densities. Water temperatures of 20 C are required, hence the proliferation of bugs that love warmer waters. However, one Belgian firm has successfully farmed sea bass in the cooling water from a nuclear reactor.

What's in it? Both receive vaccines against vibriosis (see cod) and the infections pasteurellosis, myxobacteriosis and viral nervous necrosis (VNN). Environmentalists claim that farmed sea bass is 17 times more fatty than its wild counterpart.

Are the fish harmed? Captivity doesn't suit these species and the stress of handling makes them prone to sickness and high mortality.

What about the planet? The same problems as other marine species.

Tilapia

Who farms it? Though native to Africa, this freshwater fish is cultivated in China (54 per cent of the market), Thailand, Asia, South America, the Pacific, North America and the Caribbean. Production totals more than 1m tonnes.

How? The male is preferred because it grows to market weight quicker than the female. Incubated males are sexed by hand, using dyes to highlight their genitalia, or fry are fed a sex-reversal hormone to produce an all-male population. Females are sold for fishmeal. Tilapia, which are herbivores, are reared in unmanaged ponds, in net enclosures on lakes, in crowded cages or tanks. Some eat plants, or are given protein-rich feed.

What's in it? The dyes malachite green (banned in Europe), halcyon blue and ink are used for sexing. Tilapia are what they eat, and in unmanaged systems, animal manure and even human faeces contribute to the mix. Pond-reared fish can have a muddy taint caused by geosmin. Ponds and lakes are often laced with pesticides.

Are the fish harmed? Even in natural ponds, tilapia are overstocked. Stocking densities are as high as 150kg/m3 in high-tech oxygenated tank systems.

What about the planet? Fine, if you like it queasily organic.

Tuna

Who farms it? Mainly Spain, Croatia, Italy, and Malta. In 2001, just 12 Mediterranean farms produced 11,000 tonnes of endangered bluefin, half the global total.

How? The technique known as 'tuna penning' isn't strictly aquaculture, since no juveniles are bred and the farming isn't sustainable. Instead, tuna are taken from the wild, enclosed in nets and dragged to shore where they are corralled in pens and fattened on an oil-rich diet. The aim is to supply the insatiable Japanese market, which prefers oily tuna for sushi. Because penning is covered neither by legislation governing aquaculture, nor fisheries regulations, the industry is open to exploitation.

What's in it? Fat - loads of it - plus the PCBs, mercury and dioxins associated with game fish living in polluted waters.

Are the fish harmed? They don't take well to captivity.

What about the planet? Bluefin tuna is classed as 'critically endangered'. Effluent from pens pollutes the oceans.

Turbot

Who farms it? France and, to a lesser extent, Spain, Ireland, and Scotland. One pioneer is Aquascot, a producer of farmed cod. Production of the flat fish totals 6,000 tonnes. A Greek firm farms turbot in Wales.

How? The breeding cycle and behaviour of turbot are similar to halibut, and farmers use similar technology. Fry require live feed.

What's in it? See halibut, above.

Are the fish harmed? Yes, in the same way as halibut, though they are more susceptible to infection. The appearance of viral haemorrhagic septicaemia (VHS) led to the closure of a land-based Scottish farm on Gigha in 1994. This is the island where Aquascot conducted its cod trials.

What about the planet? The same problems as other marine species.

Shrimps and prawns

Who farms it? Thailand, Indonesia, the Philippines, China and Latin America (notably Nicaragua, Honduras and Ecuador). Production exceeds 1m tonnes.

How? Shrimp fry are harvested from the wild, then reared intensively in coastal ponds where they are fed a carnivorous diet of fish protein.

What's in it? Tropical crustacea have a poor record for disease, with outbreaks of deadly white spot syndrome virus (WSSV), yellow head virus (YHV) and Taura syndrome virus (TSV), spread due to overcrowding, and treated with large doses of antibiotics, pesticides and chlorine.

Are the fish harmed? Welfare isn't an issue.