Summer temperatures in the River Dee are reaching levels where they are a danger to salmon and trout.

The Trust’s latest report on water temperatures shows that last summer, temperatures were routinely high enough to harm fish.

The Trust has been monitoring water temperatures with the support of Marine Scotland Science and as part of the Scotland River Temperature Monitoring Network. In 2022, all 33 monitoring sites in the upper Dee catchment routinely exceeded 20°C (for up to quarter of the summer period), which are monitored automatically by data loggers every 15 minutes.

20°C is significant as it is the point that thermal stress impacts may not be repairable and in adult salmon the risk of mortality increases. However, even more concerning was that 20 of the sites (60%) exceeded 23°C, which is when juvenile fish cease growth, change behaviour to seek thermal refuge and become extremely vulnerable to other threats.

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.

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.

As 1st June marks the start of the Summer Fishing Season there is now some updated advice for anglers on fish handling on the River Dee website: https://riverdee.org.uk/fish-handling/

Pictured – Dr Al Reeve checking data loggers on the River Gairn, Aberdeenshire in May 2023

To find out more please contact debbie@riverdee.org