Western Australian Coastal Shark Bites: A risk assessment

Background As Western Australia’s (WA) Government enacts shark bite mitigation, the personal risk of shark bite in WA has not been studied. Aims Model the risk of large (>3m) white shark bite (Carcharodon carcharias, LWS) in southwest WA. Method Observational study, 1/1/1974 to 31/12/2013. Analysis of prey abundance, location, water temperature and water activity participation. Shark bite risk was benchmarked against serious or fatal recreational cycling crash risk in WA. Results Total and fatal shark bites have grown exponentially over 40 years (3 to 29 total, 0 to 7 fatal per 5yrs), correlated with the 10% annual growth in WA Humpback whale (Megaptera novaeangliae) abundance (rtotal=0.96 95%CI 0.77-0.99, P<0.001; rfatal=0.96 95%CI 0.81-0.99, P<0.001) but not water activity participation rtotal= 0.25, 95%CI -0.45 0.76, P=0.48). LWS were implicated in 10 of 12 fatalities. Metropolitan Perth beach summer/autumn bathing less than 25m from shore in water less than 5m deep (risk lower than 1 in 20 years) is estimated to be at least 50x safer than cycling. Off-shore diving and surf sports off Perth, during winter/spring have a similar risk to cycling. Winter/spring off-shore diving south of Perth has between 3 and 11 times the cycling risk. Conclusion WA’s shark bite risk is likely to increase as whale abundance continues to increase off the WA coast. However, the risk to bathers less than 25m from shore in shallow water during the WA summer, is likely to remain very low, and well below the risk of other recreational activities undertaken in WA.