close up of mayfly on finger

Riverflies vital to river ecosystems

Healthy rivers support a vast variety of fauna including the eggs and larvae of the flies that can be seen over and around flowing waters in Spring and Autumn. These invertebrates are an important source of food for certain species of fish, nesting birds and bats.

Around 15 years ago there were concerns among anglers that the number of these flies were in decline. In cooperation with entomologists from the Natural History Museum, an organisation called the Riverfly Partnership was formed, with the purpose of monitoring the riverfly populations in the nation’s rivers, using a standardised procedure.

In April 2007, six members of Canterbury and District Anglers Association joined the organisation and started collecting monthly samples of the riverfly nymphs and freshwater shrimps from the River Stour in Sturry. The findings have been recorded and shared with a national database.

More recently, volunteers from the Westgate Parks Project formed a new group and are doing regular surveys. It is hoped that more groups can be formed in order to monitor the river further upstream.

Every monitoring group has an Environment Agency contact in case there are signs of a sudden drop in numbers, as this could be an indication of a pollution incident (freshwater shrimps are very sensitive). To-date there has been no reason to believe any pollution has had a significant effect.

There has been a recorded incident of diesel spillage into the river in the centre of Canterbury, however swift action by the Environment Agency prevented any effect in Sturry.

Unfortunately, the routine sampling for November at the Sturry site was abandoned because the low water level and slow flow meant that we could not replicate the standard method and any result could be misleading.

The most encouraging finding during the 10 years, has been a steady rise in the number of True Mayflies (Ephemera Danica). This indicates that the water is free of pollutants such as insecticides and the river bed is free of silt. We are hoping that a reduced level of phosphates from the water treatment works will result in fewer weed blooms which reduce the water flow. An increased flow will result in increased dissolved oxygen which is essential for healthy fish, riverfly eggs and nymphs.