A total of 1,400 fish have died after being starved of oxygen because of the hot weather in a lake popular with families.
Horrified children and neighbours of the lake in Cannon Hill Common in Wimbledon Chase have seen hundreds of fish of all sizes gasping for breath on the bank or floating dead in the water this week.
The Environment Agency (EA) and Merton Council have been at the scene to aerate the lake using pumps but 1,400 fish are known to have perished already.
The EA said a drop in the oxygen levels had been caused by algal bloom dieback and high temperatures, resulting in the death of a ‘significant population of the fish in the lake.’
Andy Henderson, who lives in Cherrywood Lane, walks his dog on the common.
He said: “It is extremely sad this has happened.
“It’s no-one’s fault it’s just so sad.
“It is obviously a lack of oxygen and it is not the first time – last year it happened.”
The council and EA confirmed several devices to pump oxygen back into the water had been running in the lake since Monday despite neighbours’ concerns they had not been working.
A neighbour of the common said: “Kids go fishing there and it looks horrific.
“There are dead fish floating on the top, it’s very distressing for the kids.
“It stinks as well.
“The kids are asking ‘what’s wrong with them,’ it’s horrible.”
A woman who lives nearby said: “There were 15 to 20 dead Koi carp.
“It’s disgraceful and appalling to see dead fish.”
An EA spokesman said: “Our officers inspected the lake on Monday, due to similar reports from members of the public.
“No distressed fish were seen but evidence suggested that the lake had suffered some level of algal bloom dieback, which had caused a small fish mortality.”
The spokesman said the officers returned to the lake yesterday where they found the problem to have persisted.
The spokesman added: “Aeration units and hydrogen peroxide are being used to oxygenate the lake and improve water quality.”
A council spokesperson said: “We’re already working with the EA and the local fishing club who take care of the lake to clear the dead fish and to add oxygen to the water.
“There’s been an aerator in the lake since Monday and the EA have added more aerators to further increase the oxygen levels.”