The Effects of Afforestation on the Trout of a Small Stream in Southern Scotland

One result of an expansion of tourism is an ever increasing demand for game fishing. In Scotland, many important salmonid producing areas are being afforested with plantations of coniferous trees. The effects of such plantations on the fauna of the streams flowing through them have received little attention. It was thought that useful information on the implications of widespread afforestation could be obtained from a detailed survey of a single stream. The stream chosen for the study was Kirk Burn, a small tributary of the famous salmon river, the Tweed. For most of its course, the stream flows through a Forestry Commission plantation of larch, Norway and Sitka spruce. There are, however, open areas of land above and below the forest which could, to some extent, be considered as controls. The presence of a deciduous spinney near the confluence made it possible to compare the effects of riparian coniferous trees with hardwoods. Invertebrates and fish were considerably less abundant where the stream was densely shaded by coniferous trees. This paucity of the fauna is largely attributable to the indirect effects of reduced illumination of the stream surface and banks. Disturbance of the streamside environment during silvicultural activities and the nature of the allochthonous input are also thought to be important contributory factors. The possible implications of increased afforestation on fish populations are discussed. Changes in forestry practices, to minimize deleterious effects, are suggested.