Not Enough Fish in the Sea
Overfishing means catching fish faster than they can reproduce. Overfishing pushes the fish population lower and lower, until fish are so few that fishermen can't make a living any more. Many fisheries have already collapsed.
In the second half of the 20th century, ocean fishermen increased their catch 400% by doubling the number of boats and using more effective fishing gear. In 1989, the world's catch leveled off at just over 82 million metric tons of fish per year, which is all the ocean can produce.