More than 38 million tonnes of fish worldwide, and one fifth of the north east Atlantic catch, are unmanaged or unused, the wildlife group claimed.
Much of this by-catch was thrown back in to the sea dead or dying, according to the WWF, which has called for stronger action on discards. It said the practice of wasting fish cost fishermen time and money, as well as jeopardising future revenues and long-term food security.
Some European fisheries had implemented practices such as more selective fishing gear, but this needed to become more widespread, added WWF-UK fisheries policy officer Giles Bartlett.
"We urge ministers to ensure the upcoming reform of the Common Fisheries Policy addresses this issue," he said.