For centuries, countries have held the strict rule that fishermen could only keep fish larger than a set adult size. The idea behind this is to allow the young fish the chance to grow old enough to reproduce so the fish population stays strong.
As we have over-fished the oceans and specifically scooped up the largest of fish, nearly all the fish with large genes have been exterminated.
This is evident in the fact that adult fish commonly found on our plates have shrunken nearly 40% only since the 70’s.
If this keeps up, the next generation’s Filet-o-Fish will most likely be smaller and more expensive.