'The small trout gives the game away,' said Gordon Patterson, a conservation officer with the Forestry Authority in Edinburgh. 'This stream is sufficiently alkaline to support a wide variety of animal life.'
The Forestry Authority is teaching foresters and the public how to recognise whether a stream is acid. By studying the insect life it is possible to tell whether there is enough animal life to support fish at the top of the food chain. A stream with insufficient insect life may be improved to increase the diversity of species.
'Acidity is one of the major factors which prevents a diverse fauna, including fish, flourishing in a stream,' Mr Patterson said.
To make it easy for people to diagnose the condition of a stream Mr Patterson, together with Brian Morrison, a scientist working with the Freshwater Fisheries Laboratory in Pitlochry, has published a guidebook.
'If there are water shrimps, limpets and snails (found under stones), and certain types of mayflies in a stream then we can be sure that it is relatively alkaline and will support fish,' said Mr Morrison. 'While stoneflies (which can be distinguished from mayflies because they have two rather than three tails) tolerate acid streams.'
The plant acids from bogs do not limit the insect life in the same way as the inorganic acids which enter the stream from acid rain.
Trees may increase the amount of acid rain and moisture being precipitated on to the land. But by arranging drainage channels so that rain water has to pass through an area of grassland some of this acid may be filtered out. The Forestry Authority now has guidelines for the management of forests which are designed to make it easier for a diversity of insect life to flourish.
'We avoid planting right up to the edge of a stream so that sunlight can reach the stream and the algae in it,' Mr Patterson said. 'We also do all we can to conserve the grass on the banks because it acts as a buffer against erosion. And we avoid stirring up silt from the bottom and try to stop soil washing into the stream because it damages fry and covers the bottom, killing off the important insect life.'