Individual variation in ontogenetic niche shifts in habitat use and movement patterns of a large estuarine predator (Carcharhinus leucas)
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Ontogenetic niche shifts are common among animals, yet most studies only investigate niche shifts at the population level, which may overlook considerable differences among individuals in the timing and dynamics of these shifts. Such divergent behaviors within size-/age-classes have important implications for the roles a population—and specific age-classes—play in their respective ecosystem(s). Using acoustic telemetry, we tracked the movements of juvenile bull sharks in the Shark River Estuary of Everglades National Park, Florida, and found that sharks increased their use of marine microhabitats with age to take advantage of more abundant resources, but continued to use freshwater and estuarine microhabitats as refuges from marine predators. Within this population-level ontogenetic niche shift, however, movement patterns varied among individual sharks, with 47 % of sharks exhibiting condition-dependent habitat use and 53 % appearing risk-averse regardless of body condition. Among sharks older than age 0, fifty percent made regular movements between adjacent regions of the estuary, while the other half made less predictable movements that often featured long-term residence in specific regions. Individual differences were apparently shaped by both intrinsic and extrinsic factors, including individual responses to food-risk trade-offs and body condition. These differences appear to develop early in the lives of bull sharks, and persist throughout their residencies in nursery habitats. The widespread occurrence of intraspecific variation in behavior among mobile taxa suggests it is important in shaping population dynamics of at least some species, and elucidating the contexts and timing in which it develops and persists is important for understanding its role within communities.
KeywordsAcoustic telemetry Fast fourier transform Niche-width expansion Shark nursery State-dependent behavior
Funding for this project was provided by the National Science Foundation to the Florida Coastal Everglades LTER Program (DBI0620409, DEB9910514, DRL0959026) and Florida International University’s Marine Sciences Program. We thank the many volunteers who assisted with shark fishing. Special thanks to Adam Rosenblatt for establishing the array of acoustic receivers and helping with downloading data, and providing analytical support. Thanks to Andrew Fritz for developing software for managing movement data and providing analytical support. Thanks also to Joel Trexler for providing logistical and analytical support for this project. Thanks to Philip Stoddard for providing comments on earlier versions of the manuscript. Research was approved by and conducted under the protocols of Florida International University’s Institutional Animal Care and Use Committee, and in accordance with sampling permits EVER-2011-SCI-0031, EVER-2009-SCI-0024, and EVER-2007-SCI-0025 granted by Everglades National Park.
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