An increased threat
Sedimentation is a growing threat to salmon in Britain. This problem seems to have escalated rapidly over the last 30 years or so.
The problem seems to have largely stemmed from the intensification of agriculture and the causes include increased stocking densities of cattle and sheep leading to increased erosion of river banks. However, the main contributor has probably been the increased cereal production and especially the widespread practice of autumn sowing which became prevalent from the late 1970s with the advent of new strains of winter barley and other crops like oilseed rape which were introduced in the 1980s.
Sediment can also be introduced as a result of forestry operations, for example at planting or harvesting, general land drainage activities and construction activities.
How does sediment get into rivers?
Soil is eroded from fields during rainstorms during periods of the year when the soil is bare. Autumn sowing exacerbates this problem because the soil is exposed for a longer period and during the wettest part of the year. On sloping fields water flows into little rills and picks up silt and clay particles which eventually get washed out of the field into a watercourse.
Where livestock cause a problem, sediment is introduced where banks are poached by their hooves or where overstocking occurs, by rainwater off bare soil.
What harm does fine sediment do?
Fine sediment can have serious effects on river ecology.
- Silt, clay and fine sand can clog
the pores and crevices within a salmon
redd. This prevents the through flow of water
and the eggs or alevins can suffocate. They can also
bind up gravel making redds harder to dig in the first
place.
- Sediment, especially sand, can fill up crevices and cracks among the stones on the riverbed where juvenile salmon would otherwise take refuge.
- Fine sediment carpeting a streambed will also alter the habitat for insects and will reduce the abundance of many of the species which are the preferred food of salmon.
- If present in high enough concentrations fine sediment may directly abrade fish's gills and with adult salmon may abrade the protective mucus and make them more prone to fungal infections.
Where is this likely to be a problem
In Tayside, fortunately, sediment is only likely to be a localised problem. Much of the Tay catchment is upland and underlain by hard crystalline rocks and does not have the soil or land use types which lead to this type of problem.
However, in some of the lowland parts of the district it could be a problem. This might include lowland tributaries of the Isla, for example the Dean Water, and even parts of the Isla itself. Some lower tributaries of the Earn could also be at risk, but the greatest risk is likely to be on the River Eden which carries a significant silt loading.
To date little research has been done on this problem
locally to assess likely impacts, but in southern England
it is now widely believed that sedimentation has been
a major factor in the big decline in chalkstream salmon
populations.
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Probably the main source of increased sediment has been the autumn sowing of cereals or oilseed rape. Bare soil is easily eroded by autumnal or winter rains and enters streams.
Concentration of livestock can also lead to significant sediment inputs. This can also be a problem in summer when streams are low, a time when erosion from arable fields is likely to be low.
Construction activities, in this instance a road cutting, can potentially be sediment sources.
Land drainage works in rivers and streams can release a lot of damaging sediment, usually at times of low flow when the effects are worst.
Fine sediment does more than just discolour the water. It can be very damaging to the ecology. Were it not for human activity stream life would not have to cope so frequently with this type of discharge.
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