Members of the public spotted the floating corpses in the river next to the Abbey Gardens, where it flows alongside Eastgate Street.
The Environment Agency estimates 1,500 fish of various sizes and species have perished – including perch, pike, roach, chub and dace – due to the combined effects of hot temperatures, low flow and low oxygen levels, coupled with the weekend’s torrential rain, which stirred up sediment at the bottom of the river.
The River Gipping at Needham Market has also been affected, where the agency reports 1,000 small to medium fish have died, while about 100 fish have died on the River Blackwater in Bocking, north Essex.
Speaking of the River Lark, an Environment Agency spokeswoman said: “This is the largest fish kill in recent years on the Lark at this location, although we have attended similar, smaller scale events previously. The last incident was in 2013, however this did not include any fish mortalities.”
Penny Stevens, from Stowmarket, came across the dead fish in Bury on Monday and reported it to the agency.
“It’s awful,” she said. “The kids were all coming out of school and saw it.”
The agency spokeswoman said over the weekend its team dosed the river with hydrogen peroxide to increase the amount of dissolved oxygen in the water after levels “dropped quite severely”.
She said they went out again on Monday and the oxygen levels in the river had stabilised, but they would be keeping a close eye on the situation.
The landowner, St Edmundsbury Borough Council, said they had been advised by the agency to leave the dead fish in the river “because the process of pulling them out disturbs the substrate and makes matters worse”.
However, after inspecting the area yesterday morning the council will be discussing this with them further.
An Environment Agency spokesman said dead and distressed fish on the River Gipping, in Needham Market, were reported to them on Sunday afternoon.
“We identified the likely cause to be low levels of oxygen in the water, a natural occurrence usually brought on by hot weather. Shortly after we put two aerators into the river to re-oxygenate the water. The aerators will remain in place until oxygen levels return to safe levels for the fish,” he said.
The deceased fish in the River Blackwater were spotted as they drifted downstream to Braintree. About 50-60 fish also died on the River Chelmer, in Chelmsford. Both of these rivers recovered naturally, the agency said.