At a certain point, in the warmer months of the year, reports of diseased fish start to come into the Boards and Trusts. Before long, these reports are almost too numerous to mention. Dead, fungal and moribund fish are now a familiar sight in the summer. 2023 is no different, with reports emphasising fungal and red skin symptoms. One of the first reports to reach the FRT in 2023 came from the River Tyne in mid-May (see photos below).Â
Above: A fish caught in the Tyne in May 2023 – note the heavy fungal infection in the head area.
In drought conditions, fish are unable to move upriver. They become trapped in ever-shrinking pools, which get very hot and shallow. It is easier for fish to come in contact with the substrate under these conditions, which then leads to grazing and gives an opening to fungal infections and other pathogens.
A common fungal pathogen of salmonids is called Saprolegnia. This likely is what has infected the fish in the photographs. Recent research has shown that more aggressive forms of Saprolegnia now seem to be on the rise. They don’t need a graze or a hook mark but can now attack otherwise healthy fish.
Above: A fish caught in the River Tyne in May 2023 – note the redness around the pectoral fins.
What can we do to stop as many fish from becoming diseased?
To a degree, climate change, bringing with it drought conditions and hotter temperatures, is likely to increase the incidence of diseased fish in rivers; however, there are things that anglers and rivers Trusts can do to reduce this likelihood.
Forth Rivers Trust downs tools when the water temperature reaches 18°C and above. We don’t do our fish surveys during this hot window as we don’t want to stress already-stressed fish. In the past, we have had to avoid fishing for 3 or 4 weeks in the height of summer, to prevent placing additional stress on the fish. In the future, this window will likely get even longer or occur several times in the summer.
 A large part of the FRTs work is to plant trees in riparian zones. This helps to reduce temperatures, not just in the planting area but also downstream. Riparian trees also provide shading to protect fish lying in shallow pools from the sun’s radiation.
Forth Rivers Trust will continue to work towards getting man-made barriers eased or removed to prevent fish from getting trapped below them. In 2015, a fish kill occurred below Ashfield Weir at Dunblane on the Allan Water. Over 100 adult salmon died whilst trapped in hot pools, unable to ascend the weir and reach the upper catchment (see photo below).
Above: Two salmon covered in fungus after being trapped below a weir on the Allan Water in 2015.
How can anglers help?
Anglers can also help to reduce stress on fish in the height of summer by:
- Preferably not fishing during hot weather or fishing early or late during the day,
- Not playing fish to exhaustion and reeling it in quickly and efficiently,
- Not dragging the fish over the substrate or up bankings,
- Using a landing net at all times,
- Keeping the fish in the water at all times.
In the face of climate change, the most healthy thing for migratory fish is for them to make it upriver to their spawning grounds with as little delay as possible and with as little thermal and physical stress as possible.
Fisheries Management Scotland, the national representative body for fishery boards and trusts, currently have an online app to record fish diseases such as pink skin disease or Saprolegnia. If you spot a diseased fish, please report it to us and/or Fisheries Management Scotland via the below link.