Seasonal Distribution and Habitat Associations of Bull Sharks in the Indian River Lagoon, Florida: A 30-Year Synthesis

Abstract

Many coastal shark species use shallow estuarine regions as nursery habitat, but there are considerable gaps in our understanding of the seasonal distribution and habitat use patterns of sharks within these systems. We compiled all available sampling data from the Indian River Lagoon (IRL) along Florida's central Atlantic coast to examine the distribution of bull sharks Carcharhinus leucas. The data synthesized in this study spanned the 30-year period 1975–2005 and included information on the seasonal distribution, size structure, and habitat associations of 449 bull sharks. For comparison, data from an additional 106 bull sharks captured in shelf waters adjacent to the IRL were also examined. The IRL is dominated by young-of-the-year (age-0) and juvenile bull sharks, which were most abundant during spring, summer, and autumn. Shark captures were most often associated with shallow freshwater creeks, power plant outfalls, ocean inlets, and seagrass habitats with temperatures greater than 20°C, salinities of 10–30‰, and dissolved oxygen concentrations between 4 and 7 mg/L. Juvenile bull sharks were found in waters with higher mean salinities than were age-0 sharks. Although the IRL is one of the most important bull shark nursery areas on the U.S. Atlantic coast, catch-per-unit-effort data indicate that bull shark abundance decreases with increasing latitude within and north of the IRL, suggesting that the IRL is the northern limit of functional nursery habitat for this species in the northwest Atlantic Ocean.

Received August 12, 2010; accepted March 3, 2011

ACKNOWLEDGMENTS

We express our gratitude to the individuals and organizations that shared bull shark records for this study including F. Snelson, F. Murru, M. Stolen, L. Ehrhart, R. Paperno, S. Kubis, D. Bagley, J. Provancha, S. Tyson, and Cape Canaveral Scientific, Inc. For assistance with field work we acknowledge T. Vigliotti, T. Ford, E. Reyier, B. Delius, and numerous other volunteers. F. Snelson, D. Parkyn, M. Heupel, and E. Phlips provided helpful guidance over the course of this study. A. Morgan and L. Hale provided bull shark data from the BLL observer program. For logistical support and permitting we additionally thank the Florida Fish and Wildlife Conservation Commission (permit 02R-718), Merritt Island National Wildlife Refuge (permit SUP 35 Burgess), and Canaveral National Seashore (Permit No. CANA-2002-SCI-0007). This research was supported by a grant from the National Marine Fisheries Service (NMFS) Highly Migratory Species Division to the National Shark Research Consortium, and tagging supplies were provided by C. McCandless and the NMFS Apex Predators Program, Cooperative Atlantic States Shark Pupping and Nursery Survey (COASTSPAN). Comments provided by three anonymous reviewers greatly improved the manuscript.