SCIENCE / Never mind the Loch Ness monster: Deep in Highland waters lurks a species of fish that has survived since the last Ice Age. Colin Tudge on the Arctic char and its exploitation

IMAGINE a fish, a relative of the salmon and trout but more beautiful than either, living since the last Ice Age in splendoured isolation in 200 Scottish lochs. In that time it has evolved into a marvellous variety of forms - a creature as intriguing and instructive, genetically, as Darwin's Galapagos finches. It seems too good to be true. Yet the fish exists, and in abundance: it is scarlet-bellied, purple-backed, with pink spots and white-edged fins. Stunning to look at and succulent to eat, it varies slightly from loch to loch and even from niche to niche within the lochs. It is the Arctic char.

But unless ecologists Ron Greer and Derek Pretswell win what for them has become a crusade, the biological wonder of the char is about to be dissipated and its long-term commercial potential squandered for ever. Scottish fish farmers are already introducing cages of foreign char, from Canada, into the lochs.

'We've hardly begun to study our own char,' Greer says. 'We don't even know what's out there. But already the farmers are starting to repeat the same mistakes they made with the salmon and the trout.'

The salmon and trout family, the Salmonidae, includes about four species of char - or a dozen, depending whether the biologist naming them is a 'lumper' or a 'splitter' - but the Arctic char is a single species and is the only one - indeed the only member of the entire Salmonidae family - that lives all the way around the North Pole.

The Arctic char is an inveterate cold-water fish: it is distressed in water above 23C, and unable to raise eggs in water that does not drop below 8C. It shares the fresh water of northern North America with the brook trout and the lake trout (which are also chars), does well in Russia and Siberia, and abounds in Scandinavia. 'But its stronghold in Europe is the Scottish Highlands,' Greer says. 'We have the greatest number and the greatest variety.'

In fact, freshwater fish worldwide are often extremely varied. Sometimes there are many slightly different genetic 'races' or subspecies within the same species - as is the case with the Arctic char and the brown trout of Europe. But if the different populations have been isolated long enough, then the 'races' diverge to form suites of full species. The 'races' of Arctic char might form new species in time but have not yet diverged that far.

There are two main reasons for such variety, both of which operate within the Arctic char. First, water levels rise and fall, either through drought and flood in the short term - or with the coming and passing of Ice Ages. During Ice Ages, sea levels and lake levels fall as water is trapped in land-locked ice, but they rise again when the water is released in the thaw. Neighbouring lakes and rivers are sometimes joined as water rises, and then are isolated again. In times of isolation, the trapped populations of fish diverge genetically; partly by chance, and partly by adapting to local conditions. And then again, when a patch of water is big enough, different populations within it diverge by adapting to different niches. 'So wherever we look we find variety,' Greer says.

There are a few char populations in Britain outside the Highlands. The ones in Windermere have long been known to form two populations with different habits - one breeding in autumn and one in spring. In time they may well become different species. In one loch in the western Highlands the population is genetically almost uniform. Presumably this loch was first invaded by only a very few 'founders'. In Loch Rannoch there are three distinct populations. One is pelagic - feeding in mid-water - and the other two are benthic, feeding on pea-mussels and the aquatic larvae of midges at the bottom of the loch.

'I reckon there were two separate invasions of Loch Rannoch - by a pelagic type and a benthic type,' Greer says. 'After they invaded, the benthic type split into two.'

But Rannoch is one of the very few lochs that has been properly studied. Greer and his colleague Derek Pretswell have now initiated a survey to see how many types there are in the Highlands as a whole. At present no one even knows which lochs contain char, and which do not; still less have scientists explored the variety within each loch. 'Yet it is now technically possible to study their DNA directly and see precisely which is related to which, and when the races diverged,' Greer says. Furthermore, 'the Highland char offer one of the best opportunities in the world to carry out such studies and to watch evolution in action.

'In October 1992 we entertained char specialists from 15 countries - we have formed a club called the International Society of Arctic Char Fanatics - and the visiting scientists just could not believe how little we British have studied our fish. Yet our char are the most accessible in the world: we have roads within 50 yards of the loch sides. Our overseas colleagues often have to walk miles to get near them.'

Greer and Pretswell are not just academics. They have founded a company called Natural Resources Scotland, with a mission 'to achieve sustainable development of the natural potential of the Scottish Highlands'. Greer, like most biologists, is a romantic - but a romantic with a very canny commercial centre. The spoliation of pristine races of Highland char with foreign strains is not merely an act of ecological vandalism, he says. It is commercial suicide.

If Natural Resources Scotland had its way, then the char would be exploited as a local, natural resource - but repeated locally in many different places. The harvest taken would be sustainable - 'you should exploit the interest, not the capital'. Greer would make no attempt to increase the yield except by seeking to re-establish the pristine, post-Ice Age forest of the Highlands, which in the north-west Highlands, warmed by the Gulf Stream, was a temperate rainforest with ash, and in the cold Grampians was a taiga landscape with Scots pine, oak and birch. Their fallen leaves would manure the lochs, as they did before they were cleared.

The fish would be caught by net and by angling - Greer is a keen and knowledgeable trout fisherman - and would be low-volume but high-value. 'The harvest would be seasonal,' he says. 'It is a fundamental error to try to live off the same resource all year round. If we truly want to create a sustainable economy in the Highlands then we have to do different things at different times of year. What we have is wonderful. If we exploited it properly, we could create a solid economy and conserve - or improve - the ecology.'

By contrast, what is now pending, Greer says, is disastrous. 'In my opinion the salmon farmers have already screwed up. They have tried to turn a high-value, low-volume product into a high-volume product - but the cost of production is still high. They have over-produced. They have failed in marketing and they haven't managed to control disease, so they have failed in husbandry too. The system simply is not sustainable. So now they want to diversify. In fact they want to repeat the same mistakes with the Arctic char.'

Fish farmers are importing foreign strains even before the native fish have been surveyed. There have been few importations so far - but a very few could make a huge impact. The imports are farmed in cages, 'but it is not a question of whether they escape, but when. And one male can fertilise 10,000 eggs'. One errant male can undo 12,000 years of evolution - and spoil a unique resource for ever: 'It's like adulterating malt whisky with Vietnamese vodka in the knowledge that the process is irreversible.'

According to Greer, the farmers are not revealing what they are doing. 'I found out about one importation only by chance. A lady rang me to ask how to feed young char. I was surprised that she had any to feed because it was the wrong time of year. She told me that her boss had just got the eggs in from Canada. My advice was unprintable.'

Yet there is nothing illegal in such importation. It is against the law to bring in foreign species but not to bring in strains of species that already exist in Britain. 'I do not blame the legislators,' Greer says. 'The nature of the problem is only now coming to light. But we hope to bring about a change in the law, so that it will acknowledge the importance of discrete populations, and not simply of species. With freshwater fish in particular, 'species' can be an unhelpful concept.'

If nothing is done to reverse the present trend, the Arctic char will continue in a destructive process that began in the Highlands about 6,000 years ago when the post-Ice Age colonists first began to clear the native forest. Over the centuries, the inhabitants and exploiters of the Highlands wiped out the post-Ice Age fauna that would otherwise be found in such sub-Arctic parts: moose, northern lynx, beavers, the wolf, the white-tailed eagle, and the osprey - which has returned only by chance. All the others are gone to make way for 'wilderness': heather, sheep and the 'wild' deer and grouse that are cultivated for shooting.

'The Highlands,' Greer says, 'is sold to tourists and foreign businessmen as a haven of European wildlife. What rubbish] It is one of the most degraded environments on Earth - a sheer, utter disgrace. Yet we presume to tell the Brazilians how to run their rainforest.'

Colin Tudge's latest book, 'The Engineer in the Garden', was short-listed for the 1994 Rhone-Poulenc Science Book of the Year Prize.

(Photograph omitted)