Salmon and Trout Association: Sedimentation

A key issue of concern to the S&TA is the increasing loads of fine sediments being delivered to our water systems. Healthy freshwater ecosystems require sediment inputs, but human activities are increasing this supply at a rate which is proving detrimental to fish, aquatic invertebrate and plants.

The human activities responsible for this large increase in sediment reaching our watercourses are primarily;

• Intensification of arable production. This leads to greater areas of exposed soil, which can be easily eroded into rivers during heavy rainfall events.
• Poor agricultural practises, such high stock density and allowing cattle to access rivers to drink and graze banks. This can destabilise the ground and lead to increased erosion of river banks.

Excessive fine sediment can have damaging impacts on all life stages of fish, particularly salmonids. This has been made worse for salmonids by a shift in the timings of arable cultivation in the UK, from spring to autumn sown cereals, which now coincides with their egg incubation times.

The effect of sediment on fish and invertebrates is, however, fairly complex, and will depend on several factors, including; the amount of fine sediment, the time of exposure to fine sediment, and the sediment composition.

The impacts include;

• Fish and invertebrate mortality through smothering, scour damage or suffocation
• Reduced fish reproduction and growth through the degradation of spawning habitat/redds
• Fish behavioural changes, such as impeding movement and altering feeding behaviour

Fine sediments entering freshwaters also bring, bound up within them agricultural and industrial contaminants, such as phosphorus (See: Excess Nutrients), heavy metals and organic pollutants, like sheep dip insecticides (See: Sheep Dip). These associated pollutants can poison the water system, and severely affect fish and invertebrate assemblages.

Excessive fine sediment can also reduce water clarity, thus reducing light penetrating submerged plants. This can alter the available river vegetation, and habitat.

Current Management

The current UK standard for suspended solid (SS) concentration is set by the EU Freshwater Fish Directive (FFD). The FFD stipulates that SS concentrations should not exceed a guideline annual mean of 25mg/l. This is only a ‘guideline’ standard which should be achieved where possible. No ‘imperative’ standards (standards which must be met) currently exist for SS in the UK. In August 2006, UKTAG made the decision to continue running with the guideline threshold in the FFD until this Directive is repealed in 2013.

Excess sediment is a major threat, and therefore must be managed in order to meet the Water Framework Directive (WFD) objective of ‘good ecological status’. In order for sediment management to progress in England and Wales, better informed sediment targets are urgently required for compliance testing.

Action Required

The S&TA calls for:

• Urgent action by Defra to identify and use a suitable framework for establishing revised sediment targets for catchment compliance across England and Wales, taking better account of the impacts of sediment on ecology.
• Defra to monitor and quantify the efficacy of sediment mitigation options currently being used, such as those under Catchment Sensitive Farming. If quantifiable improvements cannot be shown, further measures must be put in place, such as water protection zones (WPZ).

For information on what research the S&TA is doing to address the issue of fine sediments impacts on fisheries click here.

For further information see;

S&TA Scientific Briefing Paper: The Effects of Excess Sediment in Rivers
EA. (2004). Technical Assessment Method; Sediment delivery.
EA. (2004). River Diffuse source pressure; Sediment delivery map.