Bull shark feeding ecology in the coastal Everglades
I’m Phil Matich and I’m Mike Heithaus. This is a summary of our study published in the Journal of Animal Ecology. Understanding how environmental changes, both natural and man-made affect food webs today is important for predicting how climate change and other human impacts might alter ecosystems in the future. [MUSIC] During the dry season in the Florida Everglades, large portions of the marsh dry out. This leads to annual migrations of species in the marsh into deeper channels into mangrove-lined estuaries. [MUSIC] But the timing and intensity of marshes drying varies from year to year due to changes in rainfall and water management. Over longer time scales, they will be modified by ecosystem restoration and climate change. Juvenile bull sharks are predators within Everglades estuaries during their first few years. [MUSIC] Bull sharks are highly efficient osmo-regulators and can easily travel between marine, brackish, and freshwater habitats and therefore may take advantage of this resource pulse into the estuary. [MUSIC] We captured sharks on long lines, baited with mullet, and sat for one hour. [MUSIC] When we caught a shark, we brought it on board, where we could tag and measure it. [MUSIC] We used passive acoustic telemetry to measure seasonal changes in habitat use to see if bull sharks increased the time they spent in areas where marsh prey are concentrated. and we modeled how stable isotope values in different bull shark tissues should change relative to one another if sharks were switching their diets to make use of this resource pulse. [MUSIC] Our tracking data revealed that most sharks increased their use of upstream portions of the estuary where marsh prey are found when the prey pulse entered the ecosystem. And stable isotope data are consistent with models of numerous individual sharks switching from feeding in marine estuary food webs to freshwater species. That means that seasonal pulses in prey are likely important to the bull shark nursery and ecosystem changes that reduce the magnitude or duration of marsh drying could impact shark populations. Continued research in the shark estuary will provide insight into how annual variability in the timing and magnitude of the resource pulse affects bull shark foraging behaviour and increase our understanding of how ecological roles of predators can vary both along individuals and in response to environmental changes.