Overfishing: The coral reef's inhabitants are its first line of defense. Fish eat some of the creatures that eat coral, while crabs and shrimp use their pincers to protect their coral homes. Fish also graze on seaweed and algae so that they don't grow to cover the reef. <br /><br />Indiscriminate overfishing can disrupt these ecosystems, leaving the coral more vulnerable to predators. Fishing boat anchors can also scar and break the corals below.