Dead silver carp started dying and floating belly up on the surface of the Heyda lake in Thuringia in mid-April, TheLocal.de reported Monday.
No sign of bacteria or infection was found in tests performed on the fish showed, officials said, and only relatively mature silver carp were dying.
Poisonous algae is a common killer of fish, but officials said there no obvious signs of it although the lake water would be tested daily to monitor any changes.
Volunteers using boats have been scooping the dead fish from the lake near the town of Ilmenau while both local residents and experts remain baffled as to a possible cause, TheLocal.de said.
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