Fishy business in Loch Ness

If Nessie exists, what does she eat? Martyn Kelly reports

It contains more water than all the lakes and reservoirs in England and Wales combined, as well as a monster, real or imaginary, which is said to be worth £25m each year to Scotland's tourist industry. Yet here, remarkably, the superlatives about Loch Ness peter out. "Professional scientists," says Nicholas Witchell, the TV newsreader, "have tended to steer clear of the place for fear that their reputations would be stained by its commercialism and pseudo-scientific quackery."

Mr Witchell's interest in the loch - he wrote a book about the monster in 1970 - led to Project Urquhart, in which he was joined by bodies such as the Natural History Museum and the Freshwater Biological Association. Loch Ness is, after all, not just Britain's largest lake, it is also virtually undisturbed by commercial fisheries or pollution.

"This is worthwhile, serious research," says Mr Witchell. "It is not what has happened in Loch Ness before."

The story that is emerging has taken many scientists by surprise. It also offers a sounder scientific basis for the speculations about Nessie.

At the bottom of the food chain in lakes are microscopic algae called phytoplankton. The quantity of these can usually be predicted by measuring the amounts of nutrients, particularly phosphorus, in the lake. However, Tony Bailey-Watts of the Institute of Freshwater Ecology (IFE) and Roger Jones of the University of Lancaster found large amounts of phosphorus relative to the phytoplankton in Loch Ness. Dr Bailey-Watts says: "The loch is very deep and the water is very peaty, so only a tiny percentage of the water column receives any light at all." Wind funnelled along the Great Glen creates strong internal currents, which cause phytoplankton cells to spend a lot of time in darkness, far below the surface. Photosynthetic organisms cannot capitalise on the nutrients in the loch. Even at the height of summer, Loch Ness is about as favourable an environment for phytoplankton as Windermere is in winter.

As phytoplankton is the grass from which all aquatic flesh is made, this does not bode well for leviathans.

Dr Ian Winfield, a biologist who works with Dr Bailey-Watts at the Institute of Freshwater Ecology, has co-ordinated a study of the fish in the loch using advanced sonar equipment that records fish as acoustic "blips", which can be counted and measured. "There were very few fish in the open water, although there were signs of concentrations around river mouths. Those found were nearly all Arctic charr," says Dr Winfield.

The relationship between the amount of nutrients in a lake and the crop of phytoplankton extends further up the food chain, to the numbers of microscopic animals - zooplankton - and fish. "It is not very precise, but it usually puts you in the right ball park," says Dr Winfield. "Unfortunately, it did not work for us in Loch Ness. The model predicted more fish than were actually found." The quantities were, in fact, extremely low: about a quarter of a kilogram (half a pound or so) per hectare. In the bald language of science are a thousand dreams shattered: "the density of charr in open water is incapable of supporting a population of predators," write Dr Winfield and his IFE colleague Glen George.

They reached this conclusion from some simple ecological calculations, starting with the figure of a quarter of a kilogram of fish per hectare. By examining the size distribution of fish, Dr Winfield worked out how much of this biomass is produced in a year and how much is carried over from previous years. This gave a figure for annual production of slightly less than the standing stock: 230g per hectare of lake surface per year. Then he applied two ecological rules. The first states that a predator can remove only about 10 per cent of its prey each year before it starts eating into "capital" rather than "interest". If this happens, the breeding stock for the following year might not produce enough food to support it. This cuts the biomass of fish that can be sustainably consumed to about 23g - equivalent to about a tablespoon of sugar - per hectare. Multiply this by the area of open water in Loch Ness and you have a total available food supply of 93kg per year.

However, there is one more ecological rule: these charr need to be caught. Which means swimming. Fast. And that requires energy. Add to this the energy required to keep basic body processes going and the total fuel bill for a cold-blooded aquatic predator is three times its body weight a year. Which means that Nessie, individually or collectively, is at best 31kg (about 68lb) in weight. Not very monstrous.

However, the story does not end there. Military sonar experts working on Project Urquhart recorded some strong contacts that could not be readily explained. "These people," Mr Witchell says, "were extremely experienced and generally extremely sceptical. The last thing they wanted to come away with was unexplained results of this nature." But legends never die easily, and Nessie, all 31kg of him, her, it or them, remains as enigmatic as ever.