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Physiological Ecology of Bull Sharks Carcharhinus Leucas in the Brisbane River

Richard Pillans and Craig E Franklin

University Of Queensland, ST LUCIA, AUSTRALIA

THEME: ASFB (Poster)

The bull shark Carcharhinus leucas is an elasmobranch capable of surviving for long periods of time in both freshwater and seawater. Despite this remarkable ability, large size and worldwide distribution, there is little biological information on this species. Female bull sharks are believed to give birth to 1 - 13 live young in estuaries or river mouths. The juveniles are then thought to move into freshwater and are probably restricted to fresh/low salinity water for their early years, however, there is no data to support this theory. The physiological mechanisms that enable C. leucas to survive in both freshwater and seawater have also been investigated. Measures of plasma ionic and osmotic concentrations of C. leucas captured in freshwater, estuarine and seawater environments showed that animals in freshwater have a different osmoregulatory strategy to seawater acclimated animals. Histological and biochemical analyses of the gill and rectal gland from freshwater and seawater acclimated animals revealed significant changes in the structure and function of these two osmoregulatory organs. This study will also be investigating the movement patterns and distribution of C. leucas in the Brisbane River by tagging large numbers of sharks with plastic tags. Acoustic tags will also be used to track individuals. The length of time juvenile C. leucas spend in freshwater and their movement patterns within the Brisbane and Caboolture River will be determined. Size at age data and growth rates of sharks will be determined from vertebral ring counts and tag-recapture data. Over 250 sharks ranging in size from 56 - 156 cm total length have been tagged with 37 recaptures to date.