You can sustain a population of a few hundred people by hunting, but it doesn't scale to a million people. That's why we had to have agriculture before we could infest the planet. Except, when it comes to fishing, we're still hunter gatherers. Not surprisingly, the fish are going away.
It's unlikely this will continue for more than fifty years of course. Either 8.9 billion people will die or we'll get our house in order. I was wondering, however, how evolution would respond in the impossible event that we stayed around, much as we are now, for a few thousand years. There'd be a large vacant ecological niche in the ocean -- what could possibly fill it?
Poisonous fish, of course. Something we couldn't eat. The ultimate defense against the ultimate predator. In time the ocean would be full of vast varieties of poisonous fish, all inedible.
It won't happen of course, but still, something to contemplate ...