Fishing while selecting for big specimens and throwing smaller specimens back leads to the unfortunate result of us creating selective pressure for small fish. Those big fish have, when they are smaller, inefficient reproductive systems which means they have a harder time creating enough offspring in the wild. Even if what you are saying had anything to do with how things actually work (it doesn't), this selective pressure contributes to our favourite fish dying out.
I agree that taking one specific animal out of a complex system that had a long time to adjust will produce big changes. Killing the wolves allowed the deer to thrive. We wanted lots of deer, so that was good for some time. Now we don't hunt enough of them, so we need wolves again.
I agree that the oceans change - a lot - due to human influence. Some species will dwindle in numbers and disappear. It takes a long time for new species to emerge. But what I'm not so sure is that the overall change necessarily would have to be so negative in the long run.
I thought that overfishing the kinds of big fish we like to eat will just lead to a growing population of smaller fish and other marine life.