Introduction
Under certain circumstances livestock grazing on the banks of streams can have highly detrimental effects on the habitat for fish like salmon and trout. There are several ways in which the habitat can be impaired.
Lost cover
In certain types of streams vegetation such as coarse grasses and reeds which grow along the stream margins can provide very important cover for young fish. When these grasses are eaten out or the banks trampled in, valuable cover can be lost and fish numbers can be reduced as a consequence.
Erosion
If grazing intensity is high enough, not only is the bankside vegetation lost, but the banks themselves may be exposed and become eroded. Thus the stream can widen, become overly shallow and perhaps also become very silty, which in certainly circumstances can compound the lack of cover.
On what type of stream is this a problem?
The severity of the effects of grazing depends on many factors and it should be understood that it is not always a problem in all river types.
For example if the bed of the stream is composed of coarse material, such as cobbles and boulders, there may be a surfeit of cover anyway and the margins may be relatively unimportant; for salmon anyway. But, where the bed is composed of uniform gravel, pebbles, silt or sand then the banksides may be of critical importance. Furthermore the best conditions for bankside cover to grow are on streams with good soils.
So, bankside cover tends to be most important on lower gradient lowland streams. In the Tay district this means areas like the lower Isla and lower Earn catchments and the Eden in Fife. In the really rough upland areas bankside cover may not be important at all as the beds and banks tend to be composed of rocks and boulders and vegetation hardly grows anyway. As a rough rule of thumb it could be said that if a stream will support reed canary-grass (Phalaris arundinacea) bankside cover will be important.
It is also the case that on some upland streams, those which have very coarse substrates and little gravel, erosion might actually be encouraged, as eroded areas can be important areas for salmon spawning.
How big a problem is grazing on the Tay?
Compared to many other more southerly or westerly rivers grazing damage is actually not that big a problem on the Tay.
In most of the areas where grazing would be damaging, e.g. the Isla, the predominant land use is arable. Really big problems occur in areas like Devon where the predominant land use is dairying, but there is scarcely a dairy in Tayside. Therefore most of the lowland streams in Tayside are not affected by grazing, though they may be impacted by another suite of factors.
In the uplands of Tayside stream and river banks are fequently grazed, but probably in most upland catchments the beds and banks are too coarse and soils too infertile for there to be much of a problem.
So, in summary, grazing is not a large problem overall on the Tay but there are local areas where damage has occurred.
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Bankside cover, in this instance provided by reed canary-grass which forms undercuts and a mass of emergent stems provides excellent cover for young trout and salmon
In this lowland Isla tributary prolific numbers of salmon fry were electrofished from under the draping reed canary-grass which rested on the water surface for half a metre out from the true bank. As the bed is composed of gravel which provides little cover this type of bankside vegetation is critical for young fish in lowland streams like this.
Where a soft banked river or stream is heavily grazed as here on the Isla at Coupar Angus, the bankside cover is stripped away. Here the bank has also eroded for several metres (old bank supports can actually be seen sticking out of the little promontory in the river) resulting in slower flows and siltation as well.
Similar problems on a silty banked stream in Wales.
Fortunately, the greater part of lowland Tayside supports arable or mixed farming and so most banksides in lowland areas are currently not grazed, although some localised problems do exist.
It is in the wetter west where dairy farming predominates in the lowlands (e.g. parts of Ayrshire, Dumfries and Galloway, Lancashire, Devon and Cornwall) that bankside grazing is most of a problem.
In upland Tayside most riverbanks are subject to some grazing pressure
But this is frequently light and often does little damage
But even where vegetation has been stripped away, upland streams generally have a coarse stony bed which provides alternative cover. Furthermore, the relative infertlity and power of spates mean bank vegetation can never form such dense cover as in lowland areas. Although this stream may appear denuded, such streams still seem to accommodate large numbers of juvenile salmon.
In streams like these two bankside cover is almost irrelevant owing to the nature of bed and bank materials
Occasionally there are eroded gravelly reaches in the uplands and while these may affect fish cover, in some instances such areas may provide valuable spawning habitat which can be relatively scarce in torrential mountain rivers.
In upland streams attempts at stream stabilisation might not always benefit fish if all they do is destroy spawning areas.
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