The biology of tiger sharks, Galeocerdo cuvier, in Shark Bay, Western Australia: Sex ratio, size distribution, diet, and seasonal changes in catch rates

Tiger sharks, Galeocerdo cuvier, are apex predators in a variety of nearsho re ecosystems throughout the world. This study investigates the biology of tiger sharks in the shallow seagrass ecosystem of Shark Bay, Western Austra lia. Tiger sharks (n = 252) were the most commonly caught species (94%) com pared to other large sharks. Tiger sharks ranged from 148-407 cm TL. The ov erall sex ratio was biased towards females (1.8 : 1), but the sex ratio of mature animals (> 300 cm TL) did not differ from 1 : 1. Contrary to previou s accounts, tiger sharks were caught more often in all habitats during dayl ight hours than at night. Tiger shark catch rates were highly correlated wi th water temperature and were highest when water temperatures were above 19 degreesC. The seasonal abundance of tiger sharks is correlated to both wat er temperature and the occurrence of their main prey: sea snakes and dugong s, Dugong dugon. Stomach contents analysis indicated that sea turtles and s maller elasmobranchs were also common prey. The importance of major seagras s grazers (dugongs and green sea turtles, Chelonia mydas) in the diet of ti ger sharks suggests the possibility that these sharks are keystone predator s in this ecosystem.