Apparently the size of the fish you fish matters if you're fishing on a large scale. You may not have thought about this, but it makes sense given broad overfishing concerns plus our knowledge of selective breeding. As MinuteEarth explains, there are laws that establish a minimum size for catching fish so young fish have a better chance of growing up and reproducing.
That sounds like it makes sense. But an unintended consequence of these laws has been fisherpeople catching the largest fish they can, thus creating a situation where small fish reproduce disproportionately with other small fish. And the issue there is that small fish don't produce as many eggs and don't equip their eggs with as much built-in food for the development process. Add that to genetic predisposition to be small and you've got small fish producing less total offspring but more . . . small fish.
You know what, it's a whole big fish small fish thing, just watch the video.