The report estimates that more than 38 million tonnes of fish netted each year is essentially 'bycatch' - which means it's either untargetted, unwanted, unused or simply unregulated.
WWF's Bycatch Initiative leader, and co-author of the paper, Amanda Nickson says: A huge quantity of fish and marine life is being thrown back to sea dead or dying. Even if this bycatch is retained, there's no way of telling whether it was sustainable to remove it from the sea in the first place. This is disastrous for the health of our oceans."
Previous estimates of bycatch focused on fish that are thrown away - which may happen if there are too many fish caught, or they're too young, or the wrong species. That total varied from 7 to 27 million tonnes per year.
The new WWF paper expands the definition of bycatch to add unregulated catches of any species, whether intentional or not, and whether the catch is used or not.
Bycatch is a major killer of marine wildlife, putting several species at risk of extinction and drastically altering the sensitive balance of ecosystems.
It also costs the fishing industry time and money, contributing to overfishing, jeopardising future revenue, livelihoods, and long-term food security.
The worst case of wasteful fishing is probably sharks targeted for their fins, where 92% of what's caught is discarded back into the ocean.
Bycatch can also include turtles, marine mammals and seabirds.
WWF believes every form of fishing should be managed for sustainability, and everything removed from the ocean should be included.
WWF's Fisheries Policy officer Giles Bartlett says: "Simple, proven methods to reduce bycatch are already being implemented by many fisheries in Europe. These include more selective fishing gear and the use of onboard observers to document total catch.
"But such practices need to become much more widespread, and we urge ministers to ensure the upcoming reform of the Common Fisheries Policy addresses this issue as a matter of priority."