Bird expert Dave Walters discovered the distressed cygnet as he walked beside Budworth Mere.
"I was horrified," said Dave, 30, who monitors birds at Witton Limebeds and Marbury Country Park.
"One of the cygnets was noticeably smaller and very sickly-looking. On close inspection it only had one complete leg. The other had been lost just above the knee joint, leaving a badly infected stump.
"The remainder of its leg was entwined with a large amount of fishing line, which also ran tightly around the bird's neck and body.
"I've heard tales of swans killed by fishing lines and have seen several birds injured or dying through hook damage. But I've never heard of anything as horrific as a cygnet ripping its own leg off to escape."
Now Dave, from Weaverham, is urging anglers to be more careful with their leftover line.
"If for any reason line has to be cut, let me know and I will personally swim out and retrieve it myself - anything to stop this incident re-occurring."
The trapped cygnet, which had to be put down, was one of five hatched on May 2 from a pair of Mute Swans.
The birds have been at the Mere since 1993.
They have been monitored and fed near Kid Brook on the north side.
Marbury Park countryside ranger John Cergnul added: "It is not the first time this has happened. If you don't catch the swans in time they just get worse. They get more and more wrapped up in the line as they try to escape.
"We've put the message across to fishermen so many times - it's out of our control really."
Secretary of Northwich Angling Association, John Clitheroe, said: "We are one of three clubs that fish at the mere. Our policy is that anyone who leaves line is banned - totally condemned and thrown out of the club straight away.
"We also have 36 rangers which patrol the waters we use regularly - to make sure nothing has been left behind.
"It is terrible when swans die. The problem is, as in any sport, there are always idiots and it's difficult trying to catch them."
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