It's a domino effect, Jones says. Overfishing can kill off top predators. When those are gone, populations of smaller fish balloon. Normally, these fish graze on coral reefs, which normally keeps them clean and pristine, but too much nibbling can damage or even destroy reefs. These reefs not only house marine life, but also protect shorelines from storms and floods by acting as a buffer—and, of course, rake in billions of tourism dollars.