Environmental correlates of relative abundance of potentially dangerous sharks in nearshore areas, southeastern Australia

Human-shark encounters garner a disproportionate amount of public attention. Long-term datasets from shark mitigation programs can help determine the environmental conditions that influence abundance of potentially dangerous sharks. We used 25 yr (1992-2016) of shark catches from the New South Wales (NSW) Shark Meshing Program (SMP) to model the abundance of all potentially dangerous shark species (tiger Galeocerdo cuvier, white Carcharodon carcharias and whaler sharks [genus Carcharhinus]) and individual species/genus to determine: (1) the temporal/spatial variability in catches and (2) the oceanographic and physical variables that could influence abundance. Too few tiger sharks were caught to individually model their abundance. Generalised additive mixed models revealed seasonal and inter-annual abundance trends that differ between white and whaler sharks. Overall, sea surface temperatures (SSTs), years with SSTs colder or warmer than the long-term average, El Nino events, moon illumination, and beach length influenced the abundance of shark groups tested. White shark abundance was highest during water temperatures of similar to 17-18 degrees C and declined when SST increased above 19 degrees C. Whaler abundance increased with higher SSTs. Shark abundance was higher during El Nino events than during La Nina, although the number of whalers caught was highest during neutral phases. All groups showed a decrease in the number of catches with increasing moon illumination and higher abundance on longer beaches. These results may aid public safety methods aimed at reducing human-shark encounters by highlighting when higher numbers of sharks may occur.