Of course man has destroyed more of the fishstock than seals! Bottom draggers harvesting fish is like digging carrots with a bulldozer. It destroys most of the crop and ruins the garden for future use, but it sure gets those carrots out of the ground in a hurry.
As the fishstock declined man refined his searching using sonar in order to locate and harvest more and more fish from a shrinking resource, until there was an insufficient breedstock to maintain the species. Only then did we stop this fishery, and we are now trying to allow it to recover. Seals always ate fish without threatening the species but that is no longer the case. Today the seals are eating the few fish left that are desperately needed to rebuild the stock. Unfortunately for the seal at this point he must pay to fix the problem even though it was not one of his creation. There seems to be no other solution now other than to reduce the seal population to try to put nature back in balance.
By the way there are still many draggers from Europe crossing the Atlantic to pillage the remaining stock of fish on the fringes of Canadian territory. Perhaps Sir Paul would do better to protest in Spain or Portugal.