Effects of human modification to freshwater flow on juvenile bull shark movement and residency in a subtropical nursery

Understanding how organisms will respond to variation in environmental conditions is critical to predicting how species and ecosystems will respond to human impacts, including restoration efforts. We used boosted regression tree analysis to investigate the factors influencing bull shark Carcharhinus leucas residency and movement within a coastal nursery (Ten Thousand Islands, Florida, USA) that experiences considerable spatiotemporal variation in freshwater inputs and other conditions due to human development of its watersheds. We used passive acoustic tracking to quantify juvenile bull shark habitat use relative to variation in abiotic and biotic factors across scales of days to seasons. Sharks exhibited considerable inter-individual variation in residency, with 4 distinct patterns observed, including long-term resident and transient tactics. Salinity was a key driver of both overall residency of sharks and their use of specific areas within the region. Within our study area, sharks used an artificially freshwater-enhanced bay far more frequently than a freshwater-deprived bay year-round. With seasonal increases in salinities, most sharks moved further upstream into a man-made canal and then moved back downstream into the freshwater-enhanced bay as salinities dropped. Based on our results, any potential increases in salinity from the completion of an ongoing restoration project upstream could shift bull shark residency, potentially increasing reliance on the man-made canal. Understanding how seasonal and daily fluctuations in salinity might influence movements of juvenile bull sharks is therefore important for predicting how the roles of these top predators might change in the face of climate change and other anthropogenic modifications, including ecosystem restoration.