If there really is such a thing as a wonder food, it is probably a safe bet that it has got fins, scales and lives underwater. Studies have long confirmed that fish tops the menu as the most nutritional dish around. But this week's astonishing news that giving your children lots of fish could even prevent them from becoming anti-social layabouts keen on a punch-up is further food for thought. According to research from Mauritius, children given lots of fish from the age of three were 64% less likely, at the age of 23, to have criminal records than a control group, and 35% less likely to have been involved in some criminal activity but not got caught.

The study is controversial in suggesting a link between what you eat and your tendency to commit a crime; but what is universally acknowledged is that fish is as an excellent source of what are called omega-3 fatty acids. These fats - specifically EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) - are known to improve your cardiovascular system and so reduce heart disease, boost your brain power and maybe even stave off dementia, and protect your joints from the ravages of arthritis. Worth a trip to a fish and chip shop any day.

Official advice from the government, however, is that to optimise these beneficial effects, you need to eat two portions of fish a week, with at least one being oily (ie rich in the omega-3 fats) - fish such as mackerel, pilchards and salmon. So far, so good: the message is clear that, if you want to look after your health, don't miss out the fish counter on your weekly supermarket trawl.

Some fish proponents reckon there are other benefits, too. Dermatologist Nicholas Perricone says that if you eat a diet packed with salmon, your wrinkles will smooth away, your skin will tighten, and you'll generally look - as well as feel - a whole lot better. Not surprisingly, New York has taken his message to heart, and in its week of publication, his book, The Perricone Prescription, caused such a run on salmon on the Upper East Side that, briefly, there was none to be bought there at all.

Nutritionists have cautioned against the Perricone diet - eating too much of even a very good thing can be counter-productive, they argue, because you miss out on other elements of healthy nutrition. But that isn't the only fly in the fish ointment: over the past few months, alongside study after study proclaiming this benefit and that protection from eating fish, has come a steady stream of worrying reports that fish have more than their share of contaminants, as well as more than their portion of healthy oils.

Mercury and dioxins, in particular, have been found in high concentrations in some fish. So worried have nutritionists been that the government recently took the step of issuing new guidelines to pregnant women, suggesting that they eat no more than two medium-sized (140g drained weight) tins of tuna fish a week, or one tuna steak. The high mercury levels, it was feared, could interfere with foetal development.

According to a spokeswoman at the Food Standards Agency, the fish we eat are especially vulnerable to accumulated pollutants because they tend to be high up the marine food chain. In other words, the smallest fish ingest contaminated plants, then the bigger fish ingest the contaminated smallest fish, which means the big fishes we eat have, by the time they turn up on our plates, already amassed a fair sprinkling of contaminants in their food intake.

It is not, though, all bad news: she says that a recent three-year study, published in July, found that the level of dioxins in oily fish has declined by 50% over the past three years, the result of environmental clean-ups stretching back to the 1970s which are only now having their effects felt.

But it's mercury that is currently causing the biggest worries, and that could prove harder to eradicate. Mercury occurs naturally. It seeps into the earth's oceans through its crust, and is then absorbed into fish and other marine life. There is some mercury sloshing around as a result of pollution, but not to the same extent. The question is - how dangerous is it - especially in light of the FSA advice to pregnant women? If mercury might be a danger to them, surely it can't be healthy for the rest of us, either? Adding weight to the concerns, a study last year found that high mercury levels in the fish at the very top of the food chain - shark, swordfish and marlin among them - could actually cause heart disease.

This question is currently being taken very seriously at the FSA, which has set up an expert committee to look in detail at, on the one hand, the research on the benefits of fish and, on the other, the studies on whether the pollutants and contaminants they contain could do us more harm than good. For the moment, says the FSA spokeswoman, the agency is sticking firmly to its recommendation of two servings of fish a week: but when the committee eventually presents its report, that advice could change.

So how worried should we be? Tom Sanders, professor of nutrition and dietetics at King's College in London and an expert in this field, is unequivocal: the biggest risk from eating fish, he says, is that you'll choke on the bones. He is serious about that: fishbones, along with peanuts, are apparently the foodstuffs Britons most often get stuck in their windpipes. But choking aside, he says, the general picture on contaminated fish is that, while there are some particular places in the world where levels of metals could be a problem, most of the fish we eat in the UK don't come from those areas.

"The Atlantic and the Pacific fisheries are quite clean," he says. "The problem is if you have a sea that doesn't exchange water very much - that means contaminants accumulate more in the fish.

"There are also some places in the world where there are individual problems: in the Faro Islands, for example, women have tended traditionally to eat a lot of pilot whale during pregnancy, and studies have linked problems with mental development in children to this. And the seas in parts of the former Soviet Union tend to be heavily polluted with industrial waste."

Another question, he points out, is whether these contaminants in humans actually cause harm: the level of PCBs, for example, is known to be higher in the bodies of the high-fish-eating Swedes, but research doesn't show that that makes them more vulnerable to health problems. Equally, some experts have questioned whether, given that mercury is naturally occurring, it can really cause so much harm, given that fish-eating human beings have been ingesting it for generations. Just last month a piece of research from Canada said warnings of the dangers to pregnant women and unborn babies from tuna might be unwarranted, as the mercury levels, though they seem high, could be stored in the body in a safer form.

If the jury is out on that, though, there's little doubt that - contaminants aside - fish is one of the best things you can find on a table in front of you. "Even moderate consumption reduces the risk of heart disease by a quite dramatic 30% or so," says Sanders. Dr Carmel Moore, of the medical research council human nutrition research department, says fish could also protect you from diabetes: other studies have found that you can stave off depression (even postnatal depression), slow mental ageing and reduce your risk of suicide. Not to mention lowering your risk of having a nightmare teenager, which must surely rank among its greatest accolades.

Fish: some food for thought

Oily fish, such as sardines, mackerel, trout and salmon (fresh or canned) are particularly rich in essential long-chain omega 3 fatty acids, which can help prevent heart disease. It's fish at the top of the food chain - swordfish, marlin, tuna, shark - that contain the highest levels of contaminants.

Salmon: High in protein and packed with vitamin A and carotenoids, which are thought to help prevent cancer, as well as B vitamins and vitamin D, which helps the body to absorb calcium. Studies have shown that farmed salmon is as nutritionally good for you as wild. All salmon is low in mercury.

Sardines: Both sardines and pilchards are better for omega 3s than tuna.

Mackerel: An excellent source of selenium, niacin, Vitamin B6 and Vitamin B12. However, king mackerel, swordfish and shark contain high levels of mercury.

Tuna: An excellent source of lean protein, vitamins and minerals which contains medium levels of mercury.

Cod: Like other non-oily fish such as plaice or haddock, it contains high levels of selenium, a nutrient essential for the normal functioning of the immune system and thyroid gland, as well as niacin, Vitamin B12, magnesium and potassium.
Helen Pidd