Multi-year assessment of immature bull shark Carcharhinus leucas residency and activity spaces in an expansive estuarine nursery

Abstract

Although portions of the Indian River Lagoon (IRL), Florida (USA), serve as essential fish habitat for US Atlantic coast bull sharks Carcharhinus leucas, past studies were short-term (days to months) and encompassed only small parts of this expansive estuarine system. In this study, 29 immature bull sharks were tracked in the IRL between Port St. John and Port Salerno, Florida, and in adjacent shelf waters for up to 4 yr using passive acoustic telemetry. Dynamic Brownian bridge movement models showed small daily (50% utilization distribution [UD] = 1.00 km2, 95% UD = 4.36 km2) and monthly (50% UD = 4.88 km2, 95% UD = 24.67 km2) mean activity spaces that seasonally shifted (October-March) to include adjacent coastal waters. Tracked bull sharks were found to display residency in the IRL and in distinct subregions of the system. Analysis confirmed that bull shark nursery habitat extends south of Sebastian Inlet to Port Salerno, approximately 86 km farther south than previously described, and that adjacent shelf waters, which had not been studied, are important to immature bull sharks during cooler months. This study provides the first multi-year assessment of bull shark space use in the IRL, with improved resolution and over a greater expanse of the system than past studies. These movement data will be important to understanding how young bull sharks may be affected by anthropogenic stressors in this highly impacted lagoonal estuary.