over fishing

Plant-eating or herbivorous reef fish, such as parrotfish, graze on the macroalgae and thereby help maintain the balance between slow-growing corals and fast-growing algae in the reef ecosystem. Different types of herbivores play different roles in balancing a reef ecosystem—from detritivores that clean detritus from algae, to browsers that eat larger fleshy seaweeds, and excavators that scrape algae from the reef. Overfishing can throw off this balance between reef fish and their coral reef habitat causing algae to smother corals and corals to die.