Well… you may have just assumed that yourself. I assume the aggregate fish population was relatively stable since the last ice age. Perhaps not, there may have been other catastrophes, but I'm not aware of them. So now in the industrial/information age the fish/predator equilibrium is going to have to adjust.
I really don’t care it people want to ban together to preserve species for sport or economics. I only have a problem when people think we're going to drive them all into extinction without drastic new action.
Extinction per se isn't the issue. Depletion to the point of uselessness is, in economic terms, the same thing. We are certainly headed there if nothing is done.
Look at the whaling industry. It used to be big business, and while the whales aren't exactly extinct, the industry itself is gone. (Yeah, I know, government drove the last tiny remnant of it out of existence, but what really killed it was lack of whales.) We could use that industry back again. People need jobs, and I myself wouldn't mind tucking into a nice, big whaleburger. But to do that, it takes whales.