Somjit Chodchoey, 59, said he and his brother Dao Promkong put out baits on Tuesday evening. On Wednesday morning he discovered a strange looking fish caught on one of the bait hooks.
Mr Somjit said it looks like a cross between an alligator and a fish, is about 1.5 metres long and weighs more than 10 kilogrammes.
His neighbours came to look at the fish after the news spread in the area.
Issara Suwittayaporn, a fishery scientist, went to view the strange catch and said he thought it was an alligator gar (atractosteus spatula). They are mostly found in the Amazon region of South America, he said.
Mr Issara said it was possible the fish was raised by a private collector who decided to release it after it grew too big.
He said the alligator gar is large, long-lived, eats small fish and birds, and can produce many offspring. The ecosystem could be affected if there were many of them in the Thai natural habitat.
Malee Promkong, wife of fisherman Dao, said the fish would not be harmed.
"it will be sold or given to fishery officials if they come to take it," she said.
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