What damages wild trout habitats?
In a landscape exploited as intensively as ours, the wild trout’s natural habitats are inevitably affected by man’s activities. Even our most remote landscapes are influenced and altered by farming, abstraction, afforestation and pollution. Wild trout habitats can only be properly protected and improved if the factors affecting them are clearly understood. Some common problems are:

Over-grazing
Over-grazing by sheep or cattle removes valuable bankside cover. The vegetation along a riverbank binds it together and reduces erosion, providing a haven for wildlife and the insects upon which trout feed. It also provides a vital buffer zone, reducing the influx of sediments, excessive nutrients and pollutants into a stream. Wet margin habitats are also important as cover and feeding areas for trout fry and other wildlife, but if they become heavily trampled by livestock, riverbanks may collapse, making the channel too wide, shallow and silty, and seriously reducing its ecological value.

Afforestation
When insensitively planted and managed, forestry plantations spoil valuable spawning and nursery streams. Ploughs and machinery can damage the surrounding landscape. New drainage channels can increase peak flows after storms and reduce flows during periods of low rainfall. Silt and sand can be released into stream channels reducing the potential spawning sites for trout. Trees planted to the very edge of a stream will block out light, reducing the growth of valuable grasses on the stream bank and also the in-stream vegetation that is the basis of the stream’s food chain. In areas of acid geology, coniferous forests can also increase the effects of acid rain, while compared with leaves of broadleaf trees, pine needles offer a poor food-source for the invertebrates upon which trout feed.

Over-shading
Away from forestry plantations, even the natural, unchecked growth of trees can cause a problem, shading out low bankside cover and waterweed. The results are bank erosion and reduced in-stream invertebrate populations. Of course this problem must be balanced with the equally important need to provide some cover and the important role that native trees play along rivers and in the wider landscape.

Poor water quality
Chemical and biological water quality can suffer from excessive burdens of industrial and domestic pollutants. Both point source and non-point source pollution affect many rivers and stillwaters. Although some forms of industrial pollution have reduced over recent decades, others – including some forms of agricultural pollution – are still present and are increasing in some areas. This leads to excess algae and plant growth, resulting in reduced oxygen levels and altered pH levels. Managing the problem before it happens is a priority.

Over-abstraction
Over-abstraction of ground or surface water has a significant effect on the health of the river environment, and occurs in both upland and lowland areas. Reduced flows change the ecology of the river: water warms up more quickly and pollutants are less diluted. Silt tends to build up, reducing habitat diversity and invertebrate populations. The growth of useful in-stream plants (such as Ranunculus spp.) can become suppressed while algae flourish. Sometimes, through abstraction, rivers dry up completely, obviously with disastrous consequences.

Dredging
Dredging is usually carried out as part of flood-prevention and land-drainage programmes – often for agricultural reasons. Indiscriminate dredging will damage or remove riffles, pools and cover from a healthy river channel. Simultaneous clearance of the trees and shrubs from the bank will result in a uniform ‘canalised’ river channel and a river corridor offering poor habitat.

In-stream structures
In-stream structures such as dams, weirs, sluices and culverts impact on habitat by impounding the stream, and can curtail the natural migratory patterns of trout and other fish. Some obstacles may be important historical features, but others may be redundant or no longer appropriate and consideration should be given to removing them.