An RSPCA inspector said SLOTXO it was "heart-breaking" to find a young swan dead in discarded fishing line.
A member of the public reported one cygnet struggling to free itself, close to the body of a second at Eastfield Park in Northampton.
The trapped bird managed to free itself by the time an inspector arrived.
The RSPCA said it received 1,510 calls last year about animals caught in discarded fishing materials - some 40% of the total of litter-related cases.
The animal charity, which works across England and Wales, urged anglers to be "extra cautious" to make sure they leave nothing behind, especially fishing line, which can be almost invisible to wildlife.
Inspector Michelle Hare said: "It's just heart-breaking to think this poor bird died as a result of someone not disposing of a fishing line correctly.
"Fortunately the second bird, who I understand was caught by the same line, had managed to get free."