A number of fish deaths in the River Stour has raised concerns over a repeat of last year when a “whole population” of fish were killed.
Fishermen have already noticed around 40 dead fish floating on the surface of the river caused by an algae bloom reducing oxygen levels in the water.
What is not clear however is what is causing the bloom.
There are concerns it could be due to reduced flow of the river, fertilisers running into the water from farm land or worse still, pollution from sewage plants.
“It isn’t a huge number of fish but it is never good,” said John Weddup who runs the Sudbury and Long Melford Angling Association. “It’s not normal to see dead fish in the water.
“We are more concerned with long term issues. It happened last year in Bures, it had never happened before.
“The only thing the algae grows on is nutrients, where are these nutrients coming from?”
Pollutants in the water can cause the deaths of thousands of fish.
An entire fish population died from a lack of oxygen in Stour Brook near Haverhill in August, caused by sewage entering the brook.
Mr Weddup said this was one possible theory for the recent deaths on the Stour but the Environment Agency (EA) has denied this.
An EA spokesperson said: “Algal blooms can be triggered by pollution but they also occur naturally, particularly in warmer weather.
“On the River Stour we have found no evidence of pollution to date and the timing of the event is similar to last year.
“We have been monitoring the effects of an algal bloom on the River Stour which was depriving the river of oxygen.
“The situation has been gradually improving and this has been further helped by the recent heavy rainfall.
“Since our last inspection on Monday (May 18), there has been no new sightings of dead fish. If members of the public see fish in distress along the river they can call our incident hotline on 0800 80 70 60.”
Darrell Graham, fishery officer for the Sudbury and Long Melford Angling Association believes a lack of flow is to blame.
He said: “Of course it’s worrying, especially with what happened last year.
“The river’s not flowing which creates the problem.”
Mr Weddup said although there was no immediate danger to members of the public pets may be more likely to suffer.
“If you drink that water it’s not going to be great,” he said. “Or if your animal drinks it.”
He added that he would monitor any further deaths, saying dead fish floating on the surface indicated many more on the river bed.
“The fish are at a very sensitive time,” he said, “They are spawning so they are very weak.”
The Environment Agency spokesman said it will continue to monitor the river throughout the summer.