Water temperature has a fundamental influence on the health of Atlantic salmon populations. We are facing a crisis from the changing climate with nine of the ten warmest years occurring since 2002, and seven of the ten wettest since 1998. Climate change could see summers in Scotland up to 4.8 degrees warmer and 40% drier.

The Scotland River Temperature Monitoring Network (SRTMN) has been established by Marine Scotland Science to provide predictions of river temperature and sensitivity to climate change. This will provide information to rivers and fisheries managers on where the maximum river temperatures occur, and which rivers are most sensitive to climate change.

Changing temperature patterns
The optimum temperature range for Atlantic salmon is thought to span 6-20°C, within which maximal growth occurs at 16-17°C. Atlantic salmon exhibit thermal stress at approximately 23°C with mortality at approximately 33°C. Temperatures recorded during the summer of 2018 show that 69% of Scottish rivers experienced temperatures that cause thermal stress for salmon on one or more days.

River temperature is an important control on the health of fish populations. Under climate change it is expected that river temperatures will rise with potential consequences for fish and invertebrate populations.

Loss of shading
Loss of tree cover and other vegetation in the riparian zone can contribute to increases in water temperature, through a lack of shading. Bankside trees can reduce river temperatures, but this effect varies depending on the characteristics of the river (e.g. width, channel orientation, speed) and their surrounding landscapes (such as tree density, landscape shading). Loss of tree cover is often associated with land-use, particular in upper catchments of rivers. Fisheries managers are now actively planting native trees in the riparian zone in order to provide dappled shade to reduce river temperatures.

Whilst dappled shade may help to reduce the impacts of extreme water temperature, over-shading can also have an impact on salmon production. Such shading can give rise to significant reductions of the numbers of juvenile salmon a stream might otherwise be able to support.

Thermal Discharge
Discharge of thermal effluents (e.g. from distilleries) is a controlled activity that is regulated by SEPA through the Controlled Activity Regulations (CAR). A discharge should not increase the ambient temperature by more than 2°C in waters of high ecological status or 3°C in waters of good ecological status. In addition, a maximum 10°C during spawning is designed to protect spawning of salmonids.

Water held by impoundments is often much colder and lower in dissolved oxygen than a natural water body, due to the additional depth of water held upstream. When water is released by a hydro-dam or reservoir, downstream temperature can be changed. This can have a detrimental effect on native species downstream, in particular invertebrate populations where water temperature may be critical for emergence timing and overall survival.

What are we doing?
Fisheries Management Scotland are actively working with our members to mitigate the impact of high temperatures on our rivers. One of the most effective interventions is the planting of native trees in the riparian zone in order to provide dappled shade and reduce river temperatures. Over one million trees have now been planted in Scotland for this purpose, but given the scale of the issue, many more trees are required. Fisheries Management Scotland are advocating for changes to the Forestry Grant Scheme, and other funding mechanisms to help facilitate the planting of native trees beside those rivers which Marine Scotland Science has identified as vulnerable to damaging temperature rises.

A number of NGO’s have been carrying out riparian tree planting in recent years.  In Scotland this has largely been led by the rivers and fisheries trust with support from the likes of the Woodland Trust.  More recently SWT have championed Riverwoods under the £1 billion conservation finance challenge, bringing together a wide range of bodies with goal of encouraging more riparian woods. This initiative has received funding from Fishmongers’ Company via the Missing Salmon Alliance for the partnership to deliver the project. Delivery in Scotland will be overseen by a working group consisting of Tweed Forum, Fisheries Management Scotland, Woodland Trust, Scottish Wildlife Trust, Scottish Land and Estates and Scotland the Big Picture.