It’s been hot, and we all know it. Temperature records beaten, fields scorched yellow, and very low river levels. None of us can say for certain, but it seems these extreme weather patterns are likely to continue. But what does a hot spell like this mean for a river?
We can see that levels are very low. The Environment Agency has river level gauges at Chudleigh and Clifford Bridge, and unsurprisingly these are showing very low levels. As of the 20th July, the level at Clifford Bridge (between Dunsford and Fingle) is just 0.18m. When water levels drop, the concentration of pollution increases and temperatures rise. Water scarcity and high temperature also decrease the concentration of oxygen in the water, which again spells disaster for flora and fauna especially salmon and trout. In addition to all of this, low water levels also prevent certain species of fish from migrating, which can completely disrupt important stages of their lifecycle.
One of the ways we are learning about how this heatwave has affected the Teign is through the use of water temperature loggers. There are 8 of these throughout the catchment, taking regular readings which tell us about maximum temps, variation of temps across the catchment, and how quickly the river heats up and cools down. Our recent spot checks on temperature recorded 18C at Chagford, and 17.5C further downstream. Above these temperatures salmon and trout (including sea trout) suffer in several ways such as reduced migration; slower parr feeding and growth; increased stress which can lead to increased mortality.
Fish have little control over their body temperature. If the temperature of their environment is uncomfortable or unsuitable, they may migrate or seek thermal refuges locally. So those deep, cool pools of water which look so inviting to dogs and some humans are also where the fish can be desperately trying to survive.
Another factor that can affect temperature is the water quality. Rivers that contain high levels of sediment and colour, and are slow flowing, absorb more heat and therefore reach higher temperatures. As the Teign rises on Dartmoor, it has a relatively steep decline to the sea, helping to keep water flowing. It is also, compared to many other rivers, quite clear and clean.
The low flows and high temperatures have affected nearly all rivers of the UK. The Wye and Usk Foundation has had some very serious concerns about migratory fish in the river, with river temperatures at risk of becoming fatally high. In mid July, the Foundation came to an agreement with Natural Resources Wales and Welsh Water to use the water within the Elan valley reservoirs to increase the flow in the river. Due to the length of the river, the water required a few days for it to reach the bottom, when they expected the situation to be at its most serious. There was also a risk of released water increasing the water temperatures as it flowed over scorching rocks. The regulatory bodies agreed to increase released water by 400 million litres per day or 4.6m3/s for 3 days. In addition, the farmers of Hereford provided support by reducing or pausing irrigation temporarily at this critical time. This level of effort and co-operation really highlights the seriousness of the situation.
Back to the Teign, and we are fortunate our fish are not in such desperate times. But if the future holds more of these extreme weather events, we need to be prepared and do all we can to ensure the survival of our fish.
If you would to know a bit more specifically about the effects of temperature on fish, the Environment Agency have produced an informative science note on the thermal biology of brown trout and salmon.