An increase in algae in the River Stour in Sudbury is causing fish to die.

Conservationist George Millins, from Great Waldingfield, said over the past week about a dozen fish - including carp and bream - had been discovered in the river near the Granary in Quay Lane.

Mr Millins expressed concerns sewerage may be the cause of the problem, but the Environment Agency said while algal bloom could be triggered by pollution, it could also occur naturally.

The agency also said there was no evidence of pollution in the river to date.

Mr Millins said: “I’m battling to save our wildlife and I hate to see things like that. We cannot afford to have all these waterways polluted.”

Alan Simmonds, who works at Eden-Rose Coppice in Sudbury, which is near where the fish were found, said it was distressing to see the dead fish.

“It’s along a walkway. It’s not very nice to see,” he said.

An Environment Agency spokeswoman said algal bloom was depriving the river of oxygen in places and causing the fish to be in distress, adding the agency had been monitoring the situation for about two weeks.

She said: “Algal blooms can be triggered by pollution but they also occur naturally, particularly in warmer weather. On the Stour we have found no evidence of pollution to date, and timing of the event is similar to what we experienced last year.

“We are continuing to monitor and sample the river. Should we find significant numbers of fish in distress we will consider the most appropriate action to take, which could include a fish rescue.”

If members of the public see fish in distress along the river they should call the incident hotline on 0800 80 70 60.

The algae on the Stour is not considered harmful to dogs, but the agency advises owners to always be alert to the potential presence of blue green algae in rivers and lakes, especially at this time of year, which can be harmful.