The creature was spotted early yesterday and late on Tuesday at the Swan Pool at Sandwell Valley Country Park, West Bromwich, according to Sandwell Borough Council.
A council spokesman saidthat six reported sightings of an animal "matching the description of a crocodile or alligator" had been received. The spokesman said: "People have claimed to have seen it devouring pike and some fish have been spotted with lumps bitten out of them."
Officers from the authority's education and community service department cordoned off the pool, which is used extensively for fishing, sailing and other watersports. All activities at the lake have been cancelled while experts from the RSPCA and a Birmingham-based reptile rescue charity investigate the sightings.
Geoff Lewis, Sandwell Borough Council's leisure service committee chairman, said: "It is hard to imagine the mentality of people who would put something like this into a public pool.
"We are doing everything within our power to protect public safety while we check out these reported sightings.
"Our main concern is that youngsters are on holiday and with forecasts of good weather for the weekend we can't afford to do anything other than treat this with the utmost seriousness," he said.Reuse content