Shark attacks and whale migration
in Western Australia
Discussion
Blaming whales for increased shark attacks in WA might not be popular or have much marketing appeal, but is a likely explanation.
The humpbacks' migration north from the Antarctic begins in late April and the journey back south from the Kimberley is from August to December at the latest.
If you're planning to go to the beach and if future WA shark attacks maintain their winter/spring seasonal trend, the evidence suggests higher odds that you'll be bitten at this time of year - although there's far more chance you'll die or be injured in a car accident on the way there.
The good news is that the safest swimming season is probably when the whales are in the Antarctic and the great whites aren't attracted to WA shores. That happens to be summer and autumn when most of us want to be at the beach - although there are no guarantees.
An explanation
No matter how many thousands or tens of thousands of great whites are in the Indian Ocean, there will be a lure toward the WA coast if 1.4 million tonnes of whale blubber has appeared over the past 40 years to create a more plentiful zone for hunting and scavenging.
Man has wiped out a lot of ocean fish stocks so there may be less reason to hunt elsewhere.
Great white shark recovery numbers since 1999, combined with humpback whale recovery numbers since 1963, provide an explanation for:
The human fatality trend is in synch. In future, recovering killer whales and great white sharks may begin to control, or cull, whale numbers until a natural balance is reached.
A counter-argument might be put that if booming whale numbers are providing a plentiful food source for great whites, there is less reason for hungry sharks to target humans.
This is probably true at times but ignores the scavenger attraction of whales which has increased the number of great whites swimming and hunting near the WA coast, not all being so fortunate to come across a whale feast and few being satiated.
It also incorrectly assumes that great white sharks might be attracted to or hunt humans when they are hungry.
A price to pay
The WA community should accept that although we love whales, there may be a steep price to pay every time we swim at what have become the world's deadliest beaches for shark attacks.
Surfers are more at risk because the winter surf is up when humpback whales are migrating. If the trends highlighted on this website are maintained, surfers might be well advised to consider electronic shark repellents built into their boards.
The Perth metropolitan coastline has a disproportionate number of shark attacks, probably due to the larger population of human swimmers, although Rottnest Island is a resting area for humpbacks and may add to the appeal of nearby Perth beaches for sharks that are following the whale pods.
It is feasible that known humpback resting bays all along the WA coast carry a commensurate added shark attack risk.
Location and Estimate Period of Humpback Whale Activity in WA (PDF download) provides a map of the humpback migration path and WA coastal resting areas.
However, the Federal Department of Environment's humpback whale database (mobile site) clarifies where the animals rest:
Along parts of the migratory route there are narrow corridors and bottlenecks resulting from physical and other barriers where the majority of the population passes close to shore (within 30 km of the coastline). These habitat areas are important during the time of migration and include:
Resting areas are used by cow-calf pairs and attendant males during the southern migration. These whales appear to use sheltered bays to opportunistically rest during migration to the feeding grounds and include:
The Department of Environment's Humpback whale recovery plan 2005-2010 (PDF download) also maps estimated times of migratory location for WA humpback whales.
It might be noted that since 1997, WA shark attack fatalities have occurred at Geraldton, Cottesloe, Margaret River, Abrolhos Island, Port Kennedy, Cowaramup Bay, Bunker Bay, Rottnest Island, Mandurah, Mindarie, Stratham Beach and Wedge Point, all within or near the resting areas of humpback whales.
Geographical and Temporal Movements of Humpback Whales in Western Australian Waters (PDF download), published in 2001 by the Centre for Whale Research in Fremantle, has a detailed analysis of particular migratory locations at different times of the year.
Both whales and great whites have enjoyed unrestrained growth in an environment changed by man which has reduced their natural predators (except recovering sharks and killer whales vs other whales) and prey (fish mostly).
These are not natural feeding or breeding conditions and it seems likely that as a scavenger of whales, great white sharks will follow them in increasing numbers during their migration along the WA coast.
Whale recovery consequences
This website might be criticised because it raises questions about the consequences of whale recovery.
A resumption of whale hunting is not being advocated.
Federal agencies and the State Fisheries Department are researching shark numbers in light of WA's spate of fatalities, a fruitless exercise unless they also investigate the whale scavenging habitat of whatever number of great whites happen to be out there, keeping in mind the global depletion of fish stocks as an alternative food source.
The West Australian press believes the government should introduce a shark cull and a commercial fishery to save WA's beach lifestyle. Politicians should instead reconsider their pursuit of a regulated cull of great whites and commission or examine research into the shark lure of Western Australia's booming whale populations.
The evidence suggests it's not a question of how many sharks are in the Indian Ocean, but of where, when and why.
Does this mean the government's 2013/14 regulated shark cull off WA beaches was illogical due to the absence of whales and thus great white sharks during summer months?
Probably, but a decision on these programs would be assisted by proper research and may depend upon recognition that the further depletion of a protected shark species is pointless if the great whites are following whale pods up and down the coast, particularly as West Australians have far more chance of being killed falling off a ladder or being bitten by bees.
Increasing whale numbers might not be the major or even a contributing cause of increased great white shark attacks, and this website might be spinning a whale of a tale.
However, Cheynes III whaling skipper Ches Stubbs may have been right in 1981 when he predicted that the recovery of whales would see an increasing number of great white shark attacks, a prophesy supported by evidence since the 1990s.
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