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Substrate stability: safe places to hide

Stable salmon spawning area in Little Gruinard River

Stable salmon spawning area in Little Gruinard River

For juvenile salmon, the most productive areas tend to be those where the streambed is stable with plenty of voids in which fish can hide and where water remains clear and flows relatively evenly; with nearby 'spawning' habitat of loose, clean pebble-sized sediment in which adult fish can easily spawn and in which the survival of eggs to hatch is high. Such areas are found in many of the river systems below lochs, where glacial deposits have been winnowed away and gently reworked by water (and spawning fish).

At the other end of the in-stream habitat continuum for salmon are riverine areas where there is much movement of sediment and scouring. Unstable spawning areas are vulnerable to redd washout with low rates of survival of eggs. Exceptionally, summer spates may wash out fry and salmon parr, and reduce invertebrate populations.

Much of the variation in habitat stability and the amount of cover is entirely natural, the product of geology, gradient and rainfall. However, the rates at which stream banks erode are influenced by catchment management practices. Where riparian vegetation is weakened by heavy grazing pressure and riparian trees are dying back, rates of erosion and sediment input increase. Past and predicted trends in rainfall intensity are likely to exacerbate the level of habitat instability of some headwater streams.

Little Gruinard River is prime habitat for juvenile salmon         [Close]
Unstable habitat in the River Carron         [Close]
Unstable habitat in the Ullapool River headwaters         [Close]


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