Habitat Ecology of the Bull Shark, Carcharhinus leucas, on Urban Coasts in Eastern Queensland, Australia

Abstract

The habitat ecology of the euryhaline bull shark (Carcharhinus leucas) is poorly understood, yet vital for their conservation and management on our rapidly urbanising coastlines. This study investigated the habitat ecology of the bull shark in relation to ontogeny using a variety of approaches, including long-term catch data, acoustic telemetry, chemical indicators for trophic and habitat preferences and conventional methods for dietary analysis. Fishery-independent data from 1,060 C. leucas caught at ten locations along the east coast of Queensland (17 to 28°S) by the Queensland Shark Control Program (QSCP) from 1996 to 2006 indicated size of sharks captured ranged from 0.6 to 4 m total length (TL). Onset of female maturity was determined at 1.8 to 2 m TL. Catch per unit effort (CPUE) was higher during summer for drumlines and gill nets in tropical north QLD. Significantly larger individuals were caught on both gears in the sub-tropical southeast of Queensland compared to the tropical north. Principal components-regression analysis indicated this difference could be attributed to coastal bathymetry, i.e. distance to the 100 m contour. The number of estuaries in a region also directly influenced the C. leucas catch. General linear models (GLMs) indicated latitudinal differences in CPUE along the eastern Queensland coast with a significant decline in sub-tropical areas. QSCP data from 1996 to 2007 were analysed to investigate the effects of climate parameters on catch of this species in the nearshore habitat. Conditional generalised linear modelling (CGLM) showed that the numbers of C. leucas caught by both techniques were significantly correlated with total rainfall within eight days prior to capture. C. leucas catch on drumlines increased with prior rainfall > 300 mm and sea surface temperature, as these conditions may encourage feeding in nearshore areas. Pregnant C. leucas caught in gill nets increased with a positive Southern Oscillation Index, suggesting higher pupping during wet periods. Catch of sharks < 2 m in TL in nets increased with prior rainfall and were negatively correlated with lunar phase (i.e. lowest catches during full moon).