The Dannemore lake, near Östhammar in the east of Sweden, was discovered to be covered with floating fish this weekend as the ice began to melt. Among the dead fish were pike, perch, roach, and bream.
"It was not a pleasant sight, there were tonnes on fish on the lake," Timo Hakulinen, who made the discovery, told the Aftonbladet newspaper.
"It was my friend who tipped me off about it. He'd seen a bunch of sea eagles out on the ice and I went to look closer."
Hakulinen guessed that the fish had suffocated after a particularly long winter had left ice on top of the lake for longer than usual.
"It's been cold and no water has come. The lake was frozen to the bottom and the fish were trapped," he added.
In nearby Uppsala, experts agree that the sub-zero temperatures are likely the cause of the occurrence.
"I've not heard anything yet, but these things quite often are about lack of oxygen. The ice stays on top and the water doesn't get oxygenated," Lennart Nordvarg, head of the Uppsala County Board's Nature and Environment Department, told the paper.
"But you can never rule something else out."