DESTRUCTION OF CORALS AND OTHER REEF ANIMALS BY CORAL SPAWN SLICKS ONNINGALOO REEF, WESTERN-AUSTRALIA

In March 1989, most of the corals near Coral Bay, off the north-wester n coastline of Australia, spawned several nights earlier than usual. F lood, rather than ebb, tides at the time of spawning combined with lig ht northwest winds and low swell conditions to restrict the dispersal of coral propagules and, as a result, large amounts of coral spawn wer e trapped in the bay, forming extensive slicks. Fish and other animals began to die almost immediately, and over the next few days, over 1 m illion fish, representing at least 80 species, were washed ashore. A s urvey of the benthic communities revealed extensive mortality of coral s and other reef animals over an area of about 3 km2. Live coral cover in this area decreased from 42.9% to 9.4% and several large coral col onies up to 10 m in diameter were killed. The observed mortality was p resumably the result of hypoxia (oxygen depletion) created initially b y the respiratory demand of the coral spawn and maintained by the biol ogical oxygen demand of the decomposing spawn slicks and dead animals. Anecdotal reports of corals and other reef animals dying in the vicin ity of coral spawn slicks on other reefs in Western Australia suggest that this phenomenon may be a relatively common event on shallow coral reefs where coral mass spawning occurs. These records and observation s document, for the first time, a new source of natural disturbance th at has a significant influence on the community structure of some cora l reefs.