Mark Sawyer, 53, said he was fishing for carp when he hooked the odd-looking specimen, which he initially thought was a common brown goldfish.
But on closer inspection he found it appeared to have the head of a roach, the body and tail of a brown goldfish and the rear fin of a bream.
Mr Sawyer, who works as the tackle editor for trade magazine the Angling Times, photographed it before throwing it back into Magpie Lake in Cambridge.
He said: "I have shown the picture to a number of marine boffins who say it is definitely the result of mixed parentage.
"I have caught thousands and thousands of fish but have never seen anything like it before. It is a proper oddity.

Brings to mind the jackalope. Well known in western folklore.