Whale strandings are no longer quite such a mystery – on the beaches of south-east Australia at least.
An analysis of data collected over more than 80 years has revealed that a disproportionate number of cetaceans beach themselves in the region every 10 to 12 years. The stranding events appear to be triggered by a climate phenomenon called the zonal westerly winds.
“Strandings were thought to be pretty random events – or suspected of being linked to climate events like El Niño – but nothing had been demonstrated. We have shown a clear pattern,” says Mark Hindell of the Antarctic Wildlife Research Unit at the University of Tasmania in Hobart.
Tasmania is one of the world’s hotspots for cetacean strandings. Common dolphins, sperm whales and long-finned pilot whales are three of the species that most commonly beach themselves.
Hindell’s team analysed data on all cetacean strandings in Tasmania from 1920 to 2002. In peak stranding years, there were about 10 times as many stranding events as in trough years. During the last peak, in 1992, there were 29 stranding events. When the team looked at stranding data from the state of Victoria, in southern mainland Australia, they found the same pattern.
Then they looked for a possible environmental explanation. They noticed that the occurrence of stranding events correlated with variations in summer sea-surface temperature and sea-level air pressure off Tasmania, which are indicators of shifts in the movement of zonal westerly winds across the Australian continent.
Roughly every 10 years, these winds cause severe storms and a rise in the amount of cold sub-Antarctic water that moves north to the Tasmanian coast, Hindell says.
Many whale species stick to this colder water, as it is richer in nutrients. So as the colder water gets closer to the coast, so too do the whales and dolphins, he says.
Although this does not explain precisely what causes the whales to strand, knowing about these regular peaks in beachings should help whale rescue groups prepare by stepping up community training in how to deal with a stranded whale, or even by patrolling beaches during maximum danger periods.
“Information that can help with better preparing volunteers would be very useful,” says Ron Ling, president of the Organisation for the Rescue and Research of Cetaceans in Australia, based in Sydney.