Riverfly Monitoring on the River Clyst

As part of an initiative to learn about the health of our river, members of Broadclyst Environment Group took a brief training course in riverfly monitoring over the summer and carried out the first riverfly survey at the beginning of October.

What are Riverfies?

“Riverflies” refers to a number of species of flying insects that spend their larval stage in freshwater. There are diverse types of larvae – some have flattened bodies for hiding under stones, others have hooks for attaching themselves to the riverbed, while others live in cases which they build out of small stones or plants. Some are good swimmers, while others prefer to walk along the bottom.

These insect larvae are an important part of the food chain, providing food for fish and other aquatic predators. Different riverfly species have different tolerances to pollution, so the presence or absence of particular species in a river can give a good indication of the water quality.

Also, sudden changes in the riverfly population are often the result of environmental changes brought on by pollution or other problems, and riverfly surveys in some places have highlighted pollution and led to the successful prosecution of polluters.

Riverfly monitoring was developed by the Riverfly Partnership, which grew out of groups of anglers concerned about the state of the country’s rivers. There is a standardised monitoring technique, which means that all surveys are carried out in the same way so that the results of surveys from different places on the same river or from different rivers can be compared.

Surveys are carried out using a net of specific size, which is placed on the riverbed. The surveyor stirs up the riverbed upstream of the net for exactly three minutes, and the flow of the river washes all the dislodged insect larvae and other organisms into the net. These have to be carefully identified and counted, then the results are used to give a score for that site on the river.

This article was written by Mark Phillips and origionally published in the Winter 2024 Broadsheet.