Non-lethal human-shark interactions and their ecological consequences

Collapses of predator populations, caused mainly by unsustainable fishing, have been documented in many marine ecosystems. Predators are thought to play critical roles in marine environments where, through direct predation and fear effects, they can shape demographic processes and community structure. My thesis focusses on the effects of two non-lethal anthropogenic impacts on sharks: prey depletion and shark provisioning tourism. Using stable isotopes and a time series of shark vertebrae, I first examine the historical isotope ecology of seven shark species from the southwest Indian Ocean. Two species with generalist diets showed no change over two decades in δ15N or δ13C signatures. Large individuals of five primarily piscivorous species exhibited isotope signatures that deviate from historical baselines, suggesting long-term changes in diet and/or foraging strategy. Next, I measure the effects of tourism-related provisioning on the trophic signatures and movement patterns of Caribbean reef sharks Carcharhinus perezi in the Bahamas. Combining stable isotope analyses, acoustic telemetry and direct observations, I show that individual sharks that are provisioned more frequently have elevated δ15N signatures, but similar residency and movement patterns to un-provisioned conspecifics, suggesting that their broader ecological roles are not affected by long-term provisioning. Finally, I use the gradient of shark abundance generated by provisioning for ecotourism to reveal the wider coral reef community corollaries of reef shark presence. Benthic community structure varied across this gradient, with less macroalgae and more turf algae at sites with more sharks. Herbivorous parrotfish were abundant but fed less selectively and consumed more macroalgae at sites with more sharks, suggesting that fear effects may drive the patterns observed. Teleost fish biomass was almost twice as high near the provisioning site than further away, a pattern driven by fisher avoidance of areas of more sharks. Effective shark conservation may thus deliver broad cascading benefits to coral reef communities. While most marine predator declines are due to direct fishing mortality, my thesis evokes additional mechanisms by which anthropogenic activities may drive change in predator populations and their communities.