It is well established that juvenile bull sharks (Carcharhinus leucas) use rivers and estuaries as nursery areas; however, there is a large degree of variation in habitat preference (with respect to salinity and distance upstream) between studies at national and international scales. To investigate habitat preference and the influence of water quality on movement of bull sharks, we monitored 36 juveniles using an array of acoustic receivers in the Logan and Albert Rivers, Australia, for 30 months. Acoustic data were used to estimate mean daily distance upstream, and generalized additive models were used to determine the influence of flow, salinity, temperature, dissolved oxygen, turbidity, and chlorophyll a on movement. A model that included flow, salinity, dissolved oxygen, and chlorophyll a explained 89% of the variance. Movement was tightly correlated to both flow and salinity with sharks moving downstream in response to increasing flow/declining salinity and upstream during low flow/increasing salinity. During periods of negligible flow and stable salinity, animals moved upstream and downstream in response to decreasing and increasing dissolved oxygen, respectively. Dissolved oxygen and temperature were strongly negatively correlated and interchangeable within the model. Despite fluctuations in environmental salinity (0–32 psu) and a strong declining gradient in salinity with increasing distance upstream, bull sharks remained within a narrow band of salinity (6–10 psu) throughout the tracking period. The results of this study indicate that habitat choice by juvenile bull sharks is a complex tradeoff between physiology, food availability, and predator avoidance, resulting in large differences between adjacent systems and more broadly across their range.
