Temperatures in the River Dee are reaching levels dangerous to salmon and trout

Temperatures in the River Dee are reaching levels dangerous to salmon and trout, it was warned.

The River Dee Trust has been monitoring water temperatures but its latest report shows that last summer temperatures were routinely high enough to harm fish.

In 2022, all 33 monitoring sites in the upper Dee catchment routinely exceeded 20°C, the temperature at which impacts of stress may not be repairable and the risk of mortality increases in adult salmon.

Even more concerning was that 20 of the sites exceeded 23°C, which is when juvenile fish stop growing, change behaviour to seek thermal refuge and become extremely vulnerable to other threats.

The sites are monitored automatically by data loggers every 15 minutes.

The impact of juvenile salmon not feeding through parts of the summer will mean they have reduced growth and may be of smaller size when they undertake their migration to sea in the following year.

The smaller these juvenile salmon – ‘smolts’ – are when they migrate into the North Atlantic, the lower their chance of survival and returning to the river to spawn.

River director, Dr Lorraine Hawkins, said: ‘Atlantic salmon are in crisis and one of the major threats is climate change impacting our rivers – the Dee has had severe floods in recent years and summers are getting hotter and drier.

‘Last summer, juvenile salmon in the Dee would have suffered stress due to high temperatures and would have struggled to feed.

‘This is concerning as we know that poor growth can impact on their survival when they migrate to sea.

‘We recorded high temperatures in some of our upland tributaries last week, giving us more concern for what’s to come later this summer.

‘It is essential that our rivers have suitable shade and shelter for salmon to seek refuge from floods, droughts and high temperatures.’

The Dee District Salmon Fishery Board and River Dee Trust are undertaking riverbank tree planting to provide crucial shading over the rivers and streams that salmon use as this is a proven method for reducing water temperatures – once the trees are large enough to cast shade.

Their conservation work also includes the creation of Large Wood Structures in the rivers and streams, which replicates fallen trees from the riverbanks and creates habitat and deeper waters to support fish in increasingly drier, warmer summers.

High water temperatures – above 18°C – are increasingly stressful for adult salmon and is linked to decreased immunity and increased risk of fungal infections.

The stress can also have a direct effect on their survival and the production and health of the subsequent generation.

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