This seems to happen a lot and not just in the ocean. Population dynamics are complex. Things often seem inexhaustible until they're not and that changes has a nasty habit of happening almost overnight. Look at the Passenger Pigeon, the Alaskan King Crab, bison, various whale species (some may never recover from the whaling industry), cod, etc.
I think this is exacerbated by it being hard to tell what exactly is in the ocean. With land animals it tends to be somewhat more obvious.
Some governmental oddities haven't helped here. Britain really screwed up in how it joined the EU (then the EEC) in the 1970s with the CFP (Common Fisheries Policy) [1], which is (now) a textbook case of the tragedy of the commons.
Norway didn't join. And they're swimming in oil wealth. Not that I think oil fields would've become a common resource (the North Sea oil fields aren't AFAIK).
And then there's the insatiable appetite of the Japanese for bluefin tuna that will probably drive the fish to extinction in a matter of decades at most.
IMHO all of these resource problems stem from the fact that there are simply too many of us and we're unable to live within our means.