“By altering the food web, overfishing can trigger what ecologists call a trophic cascade, where the removal of one consumer has knock-on effects across the entire reef. Corals rely on herbivores, such as fish and sea urchins, to prevent them from being overgrown with seaweed. When grazers become scarce, the plants can smother the coral. And the removal of large predatory fish can lead to an increase in the population of coral-easting invertebrates, such as starfish.”
Don’t flush the fish
London Review of Books 3 July 2008
More on trophic cascades from wikipedia.