Shark attacks and whale migration
in Western Australia
Authorities have argued that increased shark attacks are due to a rapidly growing human population and more people swimming, diving or surfing at WA beaches, as well as the attraction of recovering seal numbers.
However, this doesn't explain why attacks were common a hundred years ago, including several fatalities before the 1930s when whale numbers were the same or higher than today but there were far fewer people in WA.
In the 1920s and 1930s, WA's population was around 450,000. Following the three deaths in 1923 and 1925 (one tiger shark and two species unknown), the human population kept increasing and, apart from a shark attack fatality in 1948 (tiger shark) and one in 1967 (great white), WA beaches were comparatively safe till 1995.
The argument that WA has a growing population and more beachgoers doesn't make sense for about 70 years, during which time whale numbers were decimated in WA waters with little habitat incentive for great white sharks.
WA has since become known as the shark attack capital of the world. The disparity in population growth vs shark attacks is illustrated in these charts from Changing patterns of shark attacks in Australian waters (desktop site) published by the CSIRO in 2011:
Why did so many decades pass without shark fatalities and with fewer attacks, despite a growing human population, and why did fatalities resume when the first large migrating pods spawned a whale watching industry in the 1990s - less than 30 years after humpback whaling was banned and 17 years after the last whaling station closed at Albany?
These CSIRO figures suggest that whale numbers synchronise far more closely with shark attacks than do the number of human swimmers.
Changing patterns of shark attacks in Australian waters (desktop site) concludes that Australia's increase in shark attacks over the past 20 years is due to increased population and the following unsubstantiated assumption:
In the absence of definitive beach-visitation numbers, and for the purpose of the present study, a conservative figure of a 20% annual increase was assumed as a surrogate for assessing the probable increase in beach visitations and water-related activities over the past decade. This figure is derived from an average of the Australian population increase of 15% and an increase in surf life-saver beach rescues of 29% over the past decade.
The claim of a 20% annual increase in water-based activities in Australia is at odds with the figures below extracted from The Australian Sports Commission's ERAS reports for 2001 (PDF download) and 2010 (PDF download).
This data shows fewer Australians participating in swimming, surfing and similar marine-based activities over the decade - down from a total 2,782,100 in 2001 to 2,615,300 in 2010, with total participation down from 18.4% to 14.9% of the population.
The number of shark attacks in Australia has risen from an average 6.5 per year in the 10 years from 1990-2000 to 15 incidents per year over the past decade. The number of Australians swimming didn't increase by 230%.
Participation in Sport and Physical Recreation (PDF download) surveys by the Australian Bureau of Statistics show the number of respondents who went swimming/diving in the previous 12 months were:
2002/03 (18yo+) - 1,576,000
2005/06 (15yo+) - 1,448,000
2009/10 (15yo+) - 1,293,000
Australia has been ranked the world's most obese country in recent years and this lethargy has been exacerbated by media exaggeration that has caused a widespread fear of shark attacks at Australian beaches with a resultant decline in people swimming.
WA has enjoyed increased tourist numbers over the past 20 years but Asian tourists in particular aren't renowned as beach-lovers and inbound tourism fell after the 2008 Global Financial Crisis, since when there have been seven fatal shark attacks in WA waters. In WA by mid 2012, international visitors were up about 65,000 per year since 2008 and interstate visitors were down about 100,000.
With a new international reputation as the deadliest place in the world for shark attacks, WA is also unlikely to attract tourists who want to go to the beach.
Is it the seals?
Seals have been protected in WA waters for almost a hundred years and their numbers have been slowly recovering, providing a larger menu attracting sharks to hunt closer to the WA coastline.
Seals are highly sought by white pointers but they are agile and a lot more difficult to catch than 50 tonnes of floating blubber in a dead whale carcass.
Much of the WA coast south of the city of Bunbury is a resting place for humpback whales and growing seal colonies in the area have also become more attractive to great white sharks over recent decades.
Great whites may confuse a human body for a seal if they happen to find one swimming near them. Sharks have very poor eyesight but an acute sense of smell and whale odour is very attractive. There is no instinctive attraction to the smell of humans who as prey are not worth swimming several thousand kilometres.
Without the increasing density of whales, there would be fewer great whites swimming along the WA coastline and there'd be less chance that a shark would make the mistake of attacking a human.
In March 2013, Feeding requirements of white sharks may be higher than originally thought (PDF download) reveals they eat far more frequently than previously estimated due to their high metabolic requirements. This study deals only with white sharks preying on seal colonies and does not consider the implications of greater food requirements influencing the sharks' attraction to whales as a target prey.
1.6 million tonnes of blubber
A few hours gorging on a whale carcass can nourish a great white for up to two weeks and this is a greater incentive to scavenge in coastal waters than hunting small seals.
The comparative nourishment value of seals, humans and whales for great whites can be described with a tongue-in-cheek analogy.
In 2014, close to 40,000 humpbacks were likely to have made the WA coastal migration. If an average whale weighs almost 40 tonnes, that is 40,000 x 40 = about 1.6 million tonnes of a primary great white food source that wasn't swimming close to the coastline 30 years ago.
As for the swimming human data: 283,000 less swimmers in Australia (see above) x WA's national population proportion of 10% = 28,300 less WA swimmers x an average weight of 70 kilograms = 1,981,000 kilograms = 1,918 tonnes less human flesh to eat since 2002.
Compared to past decades, great white sharks are lured by nearly 1.6 million tonnes more fatty whale meat and blubber swimming through WA waters. There's about 2,000 tonnes less of the alternative human meat which is bony and less nutritious.
The analogy does not pretend to be accurate but the evidence suggests less swimming, particularly off WA beaches where the shark scare has been simmering for almost a decade.
There is no evidence that great whites have a taste for human flesh but it's well known that they adore whale meat, particularly calves and the carcasses of adults that die on the final leg of their 13,000 kilometre Antarctic migration when they hug the WA shore for calmer waters.
Is it the sheep?
Another line of argument (mobile site) receiving media coverage is that WA's live-export sheep trade is luring more sharks because dead or dying livestock are thrown overboard during the voyage north.
It is estimated that in 2011, a total of 19,212 sheep were thrown overboard. At an average ewe weight around 35 kilograms, this is about 672 tonnes of sheep meat or the equivalent of about 17 mature whale carcasses.
The live trade argument promoted by conservation groups effectively concedes that carcass scavenging can influence the migratory habitat of great white sharks.
However, it is a weak argument as a result of the comparatively low quantity of livestock food mass, its disposal hundreds of kilometres from the WA shoreline and mostly far beyond Australian waters enroute to the Middle East, as well as the lack of hereditary or instinctive shark attraction to this food source.
Sheep-ship shark probe sinks (desktop site) published in The West Australian newspaper, 15 October 2012.
Instead of dead livestock luring sharks way out to sea and away from humans, a more likely source of scavenger attraction causing shark attacks are whales which migrate close to the shoreline for protection and which don't serve a political purpose.
There has been a global increase in reported shark attacks, partly enhanced by digital reporting of all incidents, but so too has there been a global increase in whale numbers.
In the 1960s there were only an estimated 5,000 surviving humpbacks migrating up and down almost all continental shores but in 2012 there are closer to 90,000. Some studies have suggested an historic global humpback population well above a million before sailboat harpoon whaling.
One or more of the six great whale species migrate annually up and down almost all continental coastlines fronting all oceans. Most whale species have been protected for decades and it is logical that their increasing numbers will influence the hunting territory of predators or scavengers.
Large sharks follow whales. They always have and always will. They don't follow humans and don't particularly care if there are 10,000 (700 tonnes) or 20,000 (1,400 tonnes) people near the surf at any particular time during daylight hours. Blowfish have more chance of affecting their preferred hunting and scavenging habitat within the Indian Ocean.
Influence of other whales
Killer whale (orca) numbers have also been increasing over recent years and there have been reports of pods near Jervis Bay, Rottnest Island (desktop site) and numerous other locations.
Packs of killer whales can target small or weak adult humpbacks, plus calves and other cetacean species. While gorging for the next day or two, there is a strong smell permeating the surrounding ocean and plenty of leftovers for great whites waiting till the killer whales have full stomachs. It is known that when feasting on a whale carcass, there is little aggression between different species.
There have also been reports of increasing killer whale numbers along Australia's east coast, where humpback recovery is estimated at 10-14% per annum, with evidence (mobile site) they were once common in east coastal waters before other whale species were decimated by humans. Killer whales are thought to migrate where there is available prey, an instinct likely shared by great white sharks.
Great whites can take out newborn whale calves, although it's difficult if there is a pod of healthy protective adults, and many whales have great white shark bites on their skin and blubber from failed attacks. However, they are usually scavengers of the dead.
The populations of other WA whale species such as right, sperm and pilot have also been recovering, although at a slower annual rate than humpbacks. Other whale species usually don't travel as far north up the WA coast as humpbacks to reproduce, but they are also prey for great white sharks.
Most evidence suggests booming whale numbers are luring great white sharks closer to WA's coast where humans occasionally fall victim, and it is evidence worthy of discussion.