Reason for whale strandings not known

New Zealand has recorded over 5000 whale strandings -- more than most other parts of the world -- and most commonly, they are long-finned pilot whales such as the 123 which beached at the base of Farewell Spit this week.

But scientists and other researchers have been unable to say exactly why they strand.

Experts have said there about 15 main theories, ranging from pods following a sick or disoriented leader, or the pod trying to help a young whale stranded by accident.

Another possibility is that when the seafloor is gently sloping, the sonar signals the whales use to navigate may give a false indication of deeper water if the bottom is sand or mud.

And some mass strandings can be linked to an animal with a heavy load of parasites, while others have apparently involved pregnant females going into the shallows while having difficulty giving birth.

The long-finned pilot whales -- so called because they like to travel in "pods" of up to 200 animals with one whale in the lead acting as a "pilot" -- have bulbous heads and are known to dive as far as 914m below the sea's surface when they are in the deep ocean.

According to Te Papa marine mammal collection manager Anton van Helden a single pilot whale getting stuck in the sand might be enough to bring the whole family to its aid, stranding all of them.

"If an animal gets in trouble it will call for help and other animals will come round and support it."

He said an average of 80 to 85 pilot whales stranded alone on New Zealand coasts every year, and there was usually about one mass stranding a year.

The biggest recorded mass stranding involved 1000 pilot whales on the Chatham Islands in 1918, and the biggest in more recent years affected 450 of the same species on Great Barrier Island in 1985. Rescuers successfully refloated 324 in that case.

According to Auckland University biologist Dr Rochelle Constantine, refloating becomes more difficult as stranded animals become stressed and lose their balance.

Long-finned pilot whales occur widely in cold-temperate waters, including the North Atlantic Ocean in the Northern hemisphere. In the Southern hemisphere, they occur mainly in cold currents north of the Antarctic Convergence -- a nutrient-rich zone where the warmer, northern waters sink below cold Antarctic currents.

But the whales migrate seasonally as water temperature and prey distribution changes, and will move from the ocean into coastal waters in pursuit of prey, preferably squid.

The pods usually contain animals of various sizes, ages and both sexes: males mature at age 12, reach a length of up to 6.2m, and may live a total of 50 years, while females growing up to 4m may live 60 years.

Herd behaviour

Pilot whales are extremely social and show strong herd behaviour, swimming in large pods of up to 200, though pods of up to 1000 have stranded in New Zealand.

Some researchers have said their tight social cohesion is an important factor in pods re-stranding after being refloated by people such as the workers in Golden Bay.

Usually, they strand on gently sloping beaches, such as at the base of Farewell Spit.

From the 1840s to 1991, 148 strandings involving 6331 whales were recorded and between 1989 and 1993 there were 43 strandings of 1099 whales.

A national whale stranding database shows Whangarei Harbour, Mahia Peninsula, Golden Bay and the Chatham Islands are particularly prone to strandings.

In recent years, notable strandings have included Stewart Island's worst whale stranding in 1998 when 320 of the mammals swam ashore and had to be put down at Doughboy Bay, while 65 whales died at Maori Beach on the island in December 2000.

In 1997, 101 pilot whales stranded at Karikari Bay, at the northern tip of Doubtless Bay and 348 whales beached in Takaka, Golden Bay, in the summer of 1989, but most of those were saved.

The last big stranding of pilot whales at Farewell Spit was in 1998, when about 18 survived from a pod of 28.

This time last year, 55 pilot whales from a pod of 73 died on the Coromandel Peninsula's isolated Opoutere beach.

A Massey University marine mammal pathologist, Padraig Duignan, has previously called for the Government to put more money into investigating mass strandings.

Overseas campaigners have claimed some strandings may be due to high-intensity sound waves used in oil and gas exploration and by sonar on naval ships. Sonar noise disorientates the mammals which rely on their acute hearing to navigate.

But conservation officials have said such strandings should not be confused with natural whale traps, such as Farewell Spit and other parts of the coastline, where whales and dolphins had stranded for centuries.

- NZPA