Seasonal occurrence, distribution and movements of the whale shark, Rhincodon typus, at Ningaloo Reef, Western Australia

Abstract

Aerial surveys between 1989 and 1992 demonstrated that large numbers of whale sharks appear on Ningaloo Reef in north-western Australia during autumn, shortly after the coral has undergone mass spawning. This movement into the reef waters would allow whale sharks to capitalize on the increased production of zooplankton brought about as a result of this mass spawning of corals and other marine organisms. Sharks occupied mainly the relatively turbid waters on the reef front, where a northerly current prevailed, rather than the offshore, warmer waters of the southerly flowing Leeuwin Current. The sharks moved in to the reef front from offshore but, once inshore, the majority swam parallel to the reef. The maximum density in any sector of the reef at any one time was four sharks per km, recorded in May 1992. The longer the time since sharks first appeared on the reef, the greater was their tendency to aggregate in a particular region of the reef. Evidence is presented that indicates that whale shark numbers at the northern end of Ningaloo Reef declined during the latter half of the 1980s; this decline may be related to the massive destruction of coral by the gastropod mollusc Drupella cornus during this period.