South East Rivers Trust (& the Wandle Trust)

The heatwave, low flows and fish

While we’re all suffering in the current heatwave, please spare a thought for the fish!


While we may be jealous of those spending all their time laying around in water 24-7, the fish can be affected at this time of year too. River water temperatures can exceed 20 degrees during the summer months, which is warm for some fish species. For example, water temperature is recorded at Knapp Mill on the Royalty Fishery (Hampshire Avon) and can be viewed here  and is approaching thresholds where some fishes may start to suffer.


Effects of elevated temperatures on fish

High temperatures can be directly lethal to fish, with salmonids (salmon & trout) being the most at risk during periods of elevated temperature, as these fish species are most adapted to the cooler (i.e. northern and/or high altitude) regions of the UK.


For example, juvenile Atlantic salmon (Salmo salar L.) show a behavioural and physiological stress response when water temperature exceeds 23 °C. Other fish are more tolerant of elevated water temperatures – for example, barbel (Barbus barbus L.) can tolerate water temperatures in excess of 30 °C. Barbel are a common species on the continent, and populated UK rivers during the last ice age, 10,000 years ago, when lower sea levels linked the Thames, and other eastern English rivers, with the Rhine in a large productive estuarine system in Doggerland, now covered by the North Sea. Having evolved in southern latitudes, barbel are therefore more tolerant of higher water temperatures.


However, directly lethal temperatures for fishes are unlikely to be reached in many rivers, and certain areas of rivers will often provide cooler spots for fish to find some relief and refuge from the heatwave. For example, deep pools can be several degrees below average water temperatures and in groundwater fed rivers, cooler springs (groundwater being generally constant 10 °C, summer and winter) will also provide some respite. This is why habitats like these are so important for resilience.


The most likely effects of elevated water temperatures are associated with oxygen levels in the water, rather than the temperature of the water itself.


Dissolved oxygen is the main issue

Dissolved oxygen (DO) is necessary to all aquatic life including fish, invertebrates, bacteria and plants. These organisms use oxygen in respiration, similar to organisms on land. Microbes, use DO to decompose organic material at the bottom of a body of water. However, the solubility of oxygen decreases as temperature increases – meaning that warmer water ‘holds’ less dissolved oxygen.


With warmer water holding less oxygen, there is less to go around between all the organisms in the river. This is exacerbated during periods of high air/water temperature, as increased temperatures boost microbial activity, thus increasing their oxygen demand. Additionally, low river flows during a heatwave reduce the natural water movement that helps dissolve oxygen in the water in the first place. So there is less going in and more going out, which can create a shortfall in extreme circumstances and lead to fish kills. The photo below shows fish gasping at the surface – a clear sign oxygen levels are low. 



Impounded stretches of river behind weirs are particularly vulnerable as they have extreme slow flows and little water movement to help dissolve oxygen, and often a large accumulation of organic sediments that can carry a high oxygen demand when being broken down by bacteria.  In these circumstances, fish can be at a greater risk.


How can I help?

You can do your bit to help by keeping an eye on your local watercourse during this heatwave, and reporting any fish in distress to the Environment Agency on 0800 80 70 60. It is important you use this number as it is open 24/7 and there is more capacity within the EA to respond, potentially coming to rescue the fish in difficult circumstances.


So remember – 0800 80 70 60!