It's not poisson, it's poison!

Fish, once thought the healthiest of foods, is now bad for us. Pollution is everywhere, and there is

If you were to designate a miracle food for the 21st century - a contemporary version of ambrosia, say, or manna - it might well be fish. Oily fish - tuna, salmon, mackerel and so on - are the antidotes to the diseases and chronic conditions we fear most. They help prevent heart disease, Alzheimer's, cancer, arthritis. They are a prophylactic for depression in adults, hyperactivity in children. By speeding up our neural networks, they may make us brainier, too. They also make us less violent and aggressive.

Our new appreciation of the value of fish, however - and in particular the balance of the omega-3 and omega-6 fatty acids that they contain - has come at a bad time in history. Not only have stocks of fish such as tuna, cod, swordfish and marlin declined by 90 per cent during the past century, more than 70 per cent of the world's marine fisheries are fished up to or beyond their sustainable limit. So we have exhausted our fisheries at just the moment we have discovered we need them. But the final irony is possibly the sorriest, and it is this: oily fish is no longer safe to eat.

This is a conclusion to which the official guardians of Britain's dietary safety - the Food Standards Agency, the Department of Health - would object, in public at least, but it is one that many nutritionists are being forced to confront. Watching this happen - and in particular watching state bodies trying to calibrate the point at which a "healthy" food undergoes a metamorphosis into an "unhealthy" one - is to see a peculiarly vivid illustration of what it means to be caught between a rock and a hard place. If the implications were not so grim, it might almost be funny.

While the amount of omega-3 and omega-6 that oily fish contains is good for our brains, the mercury it also contains is not. Mercury is a potent neurotoxin that was once used in making felt hats and gave hatters their reputation for madness. It can also cause cancers and reproductive abnormalities, its effects ranging from headaches, irritability, fatigue, depression and inability to concentrate, to pregnant women exposed to sufficiently high levels giving birth to deformed babies - as happened in Minamata, Japan, half a century ago. Thanks to the burning of fossil fuels and the incineration of waste, it is now widespread in the oceans where it is deposited in rain, at concentrations that become greater the higher up the food chain you go. Bigger fish that eat others - tuna, swordfish, sharks - are full of it, as are dolphins and whales.

More than 40 states in the US have issued health "advisories" about fish and in June California sued three tuna-canning companies, including Del Monte Foods, for failing to give "clear and reasonable" warning before exposing people to "known carcinogens or reproductive toxins". In the UK, the Food Standards Agency had told us how much fish we should eat. Between one and four portions of oily fish a week for men and boys, says Sir John Krebs, the agency's chairman. However, females of child-bearing age should eat no more than two portions a week. They should not eat marlin, shark or swordfish, and nor should children. Men, and women who don't want more babies, can eat these fish, but only a portion a week. A portion is 140 grams, but you also need to distinguish between fresh and tinned tuna, because there are recommended limits on eating fresh although you can eat as much tinned as you like - unless you are a pregnant or potentially pregnant woman. But as tinned tuna is classified as white fish, not oily, because the oil has been squeezed out, you might as well not eat it in the first place.

There is a vein of lunacy in this that echoes the Mad Hatter's tea party. Perhaps Sir John has swallowed too much mercury; but, in truth, he has merely, like Lewis Carroll, pursued a skewed logic to its unhinged conclusion. In a world where poison is ubiquitous, the quest for purity or safety is destined to end if not in failure, then in delusion and absurdity. What price the next brand of green tuna - not just "dolphin-friendly" but "mercury-lite"?

The extent of the dilemma can be gauged by looking at the alternatives. If wild fish are riddled with mercury and other toxins such as polychlorinated biphenyls and flame-retardant chemicals, you could create a "clean" environment and farm them. In theory. In practice, fish farming looks as though it doesn't work - too many antibiotics, additives and pesticides, too limited a diet, not enough space to swim in, so poorer muscle development and therefore less taste and tone. This January, in a study published in Science, US researchers confirmed what some people have long thought about farmed salmon: it contains many more toxic chemicals than wild salmon. The researchers told us to eat no more than half a pound a month.

You can also find these miracle omega-3 fatty acids in "vegetarian" alternatives such as flaxseed oil, but there is evidence that the flaxseed does not work as well as fish oil; the impact on body chemistry is different. Some nutritionists recommend dispensing with fish altogether and just taking the oil as a supplement with the mercury stripped out. Yet it is by no means demonstrable that supplementation, although it may compensate for dietary deficiencies, can rival "natural" food - that is to say, wild-caught, whole or unprocessed - in the range and complexity of its micronutrients or in the way we metabolise it. That is to say, if the natural food had not already been rendered unnatural through contamination by mercury or PCBs.

What the fish-oil saga demonstrates is that there is no escape from pollution, whether you are a whale-eating Inuit or a western office worker who likes a lunchtime tuna and mayo sandwich. However, the problem is far greater than this, and shows how far into the bloodstream of the planet the toxins have penetrated, how insidious the damage may be, and how precarious, and possibly illusory, are our systems of control. Like just about every other environmental toxin of which we have knowledge, mercury is subject to a public health regime based on the notion of "safe" levels of consumption - though such a level for mercury has never been established and it is debatable whether the idea is not fundamentally flawed. Suppose there were no such thing as a safe level - and that balancing omega-3 intake against mercury to arrive at a "safe" intake of fish was merely accepting the lesser of two evils in order to keep the wheels of industry, however toxic, turning, and the public reasonably docile. The resulting crisis of confidence would make the BSE affair look tame.

We should not doubt the scale of the problem. This year, the US Environmental Protection Agency concluded that about one in every six (630,000) - twice the previous estimate - of children born annually in the US was at risk of movement, learning, memory and vision disorders because of high levels of mercury in the bloodstream. For millions of us, it seems, it's heads you lose, tails you don't win. Eat fish - and you dumb down. Don't eat fish - and you dumb down, too.

Yet there is a deeper narrative involved and it concerns the erosion and supplanting of the natural. What exactly do we mean by "nature"? The scientist James Lovelock argues that pollution and people are all part of Gaia, which will survive us whatever happens. In his much-discussed jeremiad The End of Nature, the author Bill McKibben said it wasn't as simple as that. Nature's existence, he suggested, was about its independence from and of humans, and this was a quality at once objective and subjective. Walking in the woods and meditating on a waterfall, McKibben concluded that it was not so much a waterfall as a reservoir overflow, given that its existence was dictated by climate, which was in turn dictated by man-made pollution. Rain had become a "subset of human activity". "Now there's nothing," wrote McKibben, "that isn't us and so there is no escaping other people."

He was talking specifically about global warming, which in the late 1980s, at the time he was writing, was declaring itself to the wider public. He conceded, however, that the "beginning of the end of nature" might have started earlier with the arrival of nuclear weapons, which gave human beings the capacity to overmaster nature, leaving "an indelible imprint everywhere all at once".

The end of nature, in short, may be a journey of several stages, each with its own marker post and distinctive characteristics, each making itself known, probably belatedly, through the pokings about in the biosphere that we call scientific research.

The past months have been especially rich in these. The journal Public Health has just reported that deaths from brain diseases - including Alzheimer's, Parkinson's and motor neurone disease - have trebled across the western world in less than 20 years. It suggests that this could be linked to pollutants. In July, the UK's Environment Agency reported that a third of the male fish in British rivers were changing sex because of the female hormones in discharged sewage. Research by the environment group WWF suggests that 300-plus man-made chemicals are found in human body tissue, and that all are present in the foetus. In May, Science reported that more than 500 billion carrier bags and other plastic waste don't just miraculously vanish every year - most end up as a kind of ground-up microscopic tilth in the oceans and find their way into the food chain via lugworms and barnacles.

In March, the American Chemical Society heard that buckyballs - the mind-bogglingly small bits of manufactured carbon that will soon be released into the environment in mind-bogglingly large quantities with the onward march of nanotechnology - get inside the brains of fish, causing damage similar to that thought to be responsible for Alzheimer's.

Defeated in the battle for the atmosphere, nature has lost the rivers and oceans, too. Worse, our interior nature - the integrity of our brains and our bodies - has been colonised and usurped: we are poisoned from without and within.