Shark attacks and whale migration
in Western Australia
In 1981, the author of this website interviewed Australia's last whaleboat skipper, Ches Stubbs, for a story about the Albany whaling industry that was judged winner of that year's Provincial Press Association award for feature writing in Western Australia.
The Albany industry had closed in 1978 and, during a series of interviews, the former skipper of the Cheynes III was adamant that as whale numbers recovered in future years, so too would the number of shark attacks - white pointers in particular.
Decades of hunting had taught him that where there are whales, there are sharks - particularly great whites.
Humpbacks usually begin their 13,000 kilometre migratory swim from the Antarctic in May/June. They mate and give birth from June to November in WA's tropical waters between latitudes 12 and 25˚S.
During their southern journey, usually beginning around August, humpbacks swim closer to shore and target bays or similar resting areas so they can better protect their young.
In 1935, before humpbacks were targeted by modern whaling techniques, this whale species' numbers were estimated as high as 17,000 off the WA coast.
By 1949 the population had shrunk to around 10,000, down about 40% even though humpback whaling was suspended in 1939. Research showed about 85% of these 10,000 were likely to be adults.
Humpback whaling resumed after the war from three WA stations - two in the north and one at Albany, although sperm whales were the main target from the latter.
From 1949 to 1963, a total of 12,312 humpbacks were caught from the WA land stations. A further 5,868 were believed to have been taken by other countries from the Antarctic feeding grounds.
Humpbacks were officially protected in the southern hemisphere from 1963, by which time less than 600 adults were believed to have survived in WA waters - a reduction of more than 95% in just 14 years. However, Soviet factory ships are known to have continued hunting them illegally until 1972.
During this period hardly any humpbacks were sighted off the WA coast and shark attacks were a distant memory for the people of WA.
Signs of a humpback population recovery began in the early 1970s with sightings in Shark Bay and east of the Abrolhos Islands.
Shark Bay whaling company records show the average sighting rate in 1963 was five per day. Aerial sureys showed that by 1988 it had increased to 16, with an annual increase around 9%.
By 1991, an average 28 humpbacks were seen per day and the Centre for Whale Research (desktop site) estimated a 91/92 WA population of 3,878.
Former WA Museum director-general John Bannister said in 1997 that, based on a 1994 estimate and subsequent population growth, about 5,000 humpbacks migrated off the WA coast during 1996.
Mr Bannister said in 1997 that if between 12,000 and 17,000 humpbacks migrated off WA prior to whale hunting, it would be about 10 years before the natural population had recovered.
In 2008, the humpback population was estimated at 21,750. WA's Department of Environment and Conservation stated in 2011: "Their numbers are recovering at a remarkable 10% each year since they became protected in 1963 with numbers on the west coast currently estimated to exceed 20,000." (PDF download)
In 2012, WA's Environment Minister said there there were about 28,800 migrating humpbacks off the WA coast.
The International Whaling Commission (desktop site) estimates the humpback population recovery around 11% per year and the species is nowadays regularly spotted just north of Rottnest Island, which is believed to be a resting area as they migrate south (see Report of the Scientific Committee, Panama City, 2012 - PDF download).
There have been estimates that about 30,000 humpbacks migrated along WA's coast each year prior to whaling. Albeit fanciful, one historic account claims that once “it was possible to walk across to Rottnest on the backs of whales”.
Research published in 2015 suggests WA's humpback whale population has recovered to between 50% and 90% of it's pre-whaling population size.
WA's humpback population is now the largest in the world and with a growth rate above 10% pa, it is difficult to predict how large the population will be in the coming decade or what will be the future equilibrium.
Humpbacks are not the only protected whales. Pilot, southern right and sperm numbers are recovering at a slower rate, although these species prefer more southerly waters and may add to the migrating humpback bait that favours shark attacks in WA's southern half. This is also where most West Australians live and swim.
Southern right whales
Right whales follow a cycle of migration similar to that of the humpback, breeding during winter in WA's warm coastal waters and feeding during summer in the cold waters of the deep south.
However, their migratory path covers much less distance than the humpback in either direction. These baleen whales live on food reserves stored as blubber fat, eating little for as long as eight months during their breeding season.
Southern right whales were always a favourite target of hunters, being a bulky and slow-moving animal which won't sink when dead and which produces huge quantities of oil and bone. They grow to 17.5 metres and a weight of 80 tonnes, living more than 50 years.
Right whales congregate very close inshore along the southern coast of Australia during winter and spring. They're occasionally spotted up the coast to Perth and there have been recorded sightings over the past few decades as far north as Shark Bay and off the North West Cape.
The natural and unhunted number of right whales at the beginning of the 1800s is unknown. However, in 1804 they were reported to be such a danger to navigation in Tasmania's Derwent River that small boats had to keep close to the shore between May and November.
The right whale fishery reached a peak in the 1830s and then rapidly declined. By the turn of the century, this species was considered virtually extinct in Australian waters.
The first scientifically recorded right whale sighting this century was off the Albany coast in 1955, when a cow and a calf were reported. An increasing number were sighted in WA waters during the early 1970s.
Since 1983, the right whale population off WA's south coast is estimated to have increased by 7-10% each year.
In 1993, a total of 128 southern right whales were sighted in WA waters between Cape Leeuwin and Twilight Cove. In 1996, the greatest number sighted in this area was 153, including 37 calves.
Southern Right Whales Aerial Survey and Photoidentification, Southern Australia, 2006 (PDF download), by respected whale researcher John Bannister from the WA Museum, estimated a right whale population around 2,100 in the surveyed southern coastal area from Cape Leeuwin to Ceduna.
In 1997, Mr Bannister estimated that with a 7% annual growth rate, it would take up to 15 years before the right whale species returned to its presumed natural population between 2,000 and 3,000. His aerial survey logs reflect the estimated 7.5% annual growth rate in WA's southern right whale population:
Surviving Albany whalers in the 1980s spoke of their childhood days when, in King George Sound, sperm whales could be seen from "horizon to horizon".
Between 1954 and 1978, 14,000 sperm whales were hauled back to Albany's Cheynes Beach whaling station at Frenchman's Bay. The record catch was 22 in a single day.
Sperm whale numbers plunged during the final years of Albany's whaling industry. It was estimated that, in 1979, the number of exploitable females (aged 13 and over) had fallen to 90% of the level in 1947. However, just 25% of the exploitable males (aged 20 and over) had survived.
Scientific evidence indicated the overall sperm whale population halved between 1968 and 1977. Exploitable female numbers fell close to zero by 1989, needing another 35 to 40 years to again reach exploitable levels.
WA whale numbers will soon recover to pre-hunt levels with a question over sustainable growth in the absence of natural predators due to fishing exploitation. What is the new equilibrium?
The natural balance involves predators that keep whale numbers at sustainable levels, mostly by targetting the young. Primary predators / scavengers are killer whales and sharks - particularly great whites that are more attracted to the whale's migratory habitat than during the 20th century, suggesting a possible cause for increased shark numbers.Back To Top