Long-term presence and movement patterns of juvenile bull sharks, Carcharhinus leucas, in an estuarine river system

Abstract

Elasmobranchs living in rivers and estuaries may be highly susceptible to environmental and anthropogenic changes to coastal habitats. To address this, patterns of movement of 67 juvenile bull sharks (Carcharhinus leucas) were examined in the Caloosahatchee River between 2003 and 2006 using an array of 25 acoustic receivers. Individuals were monitored for periods of 1–460 days with most present for periods of weeks to months. Individuals utilised the entire monitored section of the river (∼27 km) and also moved upriver beyond the extent of the acoustic array. Daily activity spaces of individuals ranged from 0 to 14 km with most <5 km. Monthly mean home ranges were 0.9–5.6 km and although home-range sizes were consistent through time, location of individuals within the river changed through time. Individuals moved upstream during the day and downstream at night and this pattern persisted across years. Based on data collected from depth transmitters in 2006, individuals swam closer to the surface at night and remained in deeper water during the day, suggesting further diurnal patterns in habitat use. Long-term consistent use of this habitat by young C. leucas suggests this population is highly dependent on the Caloosahatchee River as a nursery area.