Natural river features preserved by the Environment Agency saved the lives of thousands of fish along the River Thames. On 20 July 2007, more than two months rain fell in one day along some parts of the River Thames catchment and as a result some areas of the river experienced flows of more than ten times the usual rate for July.
The annual survey of fish populations along the River Thames – due to finish on Tuesday 18 September 2007 – has shown distinct ‘pockets’ of aquatic life in locations where the Environment Agency and landowners have retained natural river features and used progressive engineering techniques rather than hard sheet piling or concrete to protect river banks.
Displaced fish from those areas where there are no refuges such as backwaters and vegetated natural banks with tree roots were swept rapidly downstream, unable to shelter from the force of the water. Much of the aquatic vegetation favoured by species such as tench and bream was also washed away, meaning that survey found lower numbers of these fish.
However, the high flows did bring some positive changes for other Thames residents such as chub and dace. The survey team found new areas of clean gravel bed – perfect spawning and feeding habitat for these species.
Fisheries technical specialist Steve Sheridan, said: “So far we have captured 19 different species of fish. This includes the first adult silver bream ever recorded in the survey. We have also found salmon at three different locations. Flows in the upper reaches are still quite strong for this time of year, so our crew has had its work cut out – netting fish in these conditions requires rapid reactions!
“We have also used the survey as an opportunity to closely monitor an ongoing problem spotted among some large carp in the River Thames. We had received a number of reports of dead fish from members of the public. Unfortunately, dead fish are of little value for detailed examination. During the survey we have been able to find a sample of carp in very poor condition, but still alive. They have been transported safely to our laboratory and we hope to have results or our tests shortly.”
The annual survey is carried out along more than 40 miles of the freshwater Thames, from Hurley to Teddington and forms part of a national monitoring programme covering 2,700 miles (4,300km) of watercourses across England and Wales.
Survey team measuring and taking scales from a Carp
Environment Agency staff use a specialised electro-fishing boat to catch the fish examined in the survey. Two arms extending from the front of the boat pass an electric current through the water, which attracts and momentarily stuns the fish, enabling them to be caught easily in a net. Length and weight are then recorded, while scale samples are taken to determine age, before the fish are returned unharmed back to the water.
The information collected in these annual surveys provides the fisheries team with a good picture of the population structure and range of fish species living in Britain’s rivers. It also helps the Environment Agency and partner organisations to understand environmental factors that influence the health of our rivers from year to year.