Up to three dead dolphins a day are being washed up on the beaches in the south west of England.
That's up by more than a quarter on 2002, and the Cornwall Wildlife Trust think thousands more have been killed at sea that we don't know about.
It's called pair trawling, and involves two paired fishing boats dragging a giant net the size of a football pitch between them.
They catch loads of fish - but loads of dolphins too.
Pair trawling is banned in America, and the campaigners want it banned here too.
They also want someone on every European fishing boat to keep an eye on what's going on.
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