Next time you are on a pier, look down into the sea and find schools of tiny fish.
Large fish eat smaller fish. Small fish are in constant danger of being eaten. But larger fish are in constant danger from birds so the bigger fish must remain in deeper water where they can’t be easily seen from above.
In the shallow waters the small juvenile fish are too small to be caught by swimming birds and the larger fish are in greater danger from these birds. Most juvenile marine fish depend on the shallow seas along the coast, mangroves and seagrass meadows for the first months or years of their lives. When these environments are destroyed fewer fish can breed.