By Alex Kirby BBC News Online environment correspondent

Andrew Derocher, a polar bear researcher, has had a relatively easy job of it recently.

He has been tracking the movements of two bears in the Norwegian and Russian Arctic by satellite.

Before he could do that, though, he had to fit the radio collars on to the bears.

With an adult animal weighing up to 600 kilograms (1,300 pounds), that needed a steady hand and nerves of steel.

Some bears have a home range of 500 sq km, and some cover 300,000

Andrew Derocher

Andrew, based at the Norwegian Polar Institute in Tromso, in mainland Norway, collared the bears in Svalbard, halfway between Norway and the North Pole.

The collars can be fitted only to female bears - the males' necks are too thick. Each collar contains a radio transmitter in a hermetically-sealed canister, and weighs about 1.5 kg (3.3 lbs).

The signal goes to a satellite, and the animals' movements can be tracked on the internet.

Light sleepers

Shot from helicopters with anaesthetic darts, the bears take about five minutes to lose consciousness. These days they usually stay asleep until the collars are safely fitted.

But it was not always thus. Andrew Derocher explains: "The drug we use now is a major step forward.

Anaesthetised bears now sleep soundly

"When I began in 1984 we had a combination that was a little bit dicey. Any loud noise - a sneeze, for instance - could make the bear jump up, very alert, and overcome the drug quite easily.

"It tended to create a bit of excitement. We used to put a paper towel round the bears' eyes.

"So they'd wake up, but they couldn't see what was going on - not till they figured out we'd put a towel on them, at least."

The two bears Andrew has been tracking have been named Louise and Gro (after the former Norwegian Prime Minister, Gro Harlem Brundtland).

He and his colleagues are astonished at the distances some animals cover. Those around Svalbard and animals further east, in the Russian Arctic, are one population.

Individual bears range over 1,000 kilometres (620 mile), between Svalbard and the Russian territories of Novaya Zemlya and Franz Josef Land.

Choosy eaters

Andrew Derocher said: "Some bears have a home range of 500 sq km, and some cover 300,000. They have hugely diverse behaviour patterns.

The bears' size means a huge energy demand

"Their main prey are ringed seals, which weigh about 60 kg, but some bears prefer bearded seals, which can be over 400 kg. We think they're choosing different areas to find these two species."

There are plans to fit ten more bears with collars, which will offer better tracking opportunities.

Andrew's colleague Dr Kit Kovacs spends much of her time fitting tags to different species - bearded, ringed and harbour seals, bears, and white whales, which grow to four metres and weigh over a tonne.

Hungry future

She is off-hand about it. "It becomes just like walking the dog to people who do it every day", she says.

Dr Kovacs is worried by scientists' predictions that Arctic summers could be ice-free by mid-century. "Most polar bears will be forced onto the land if that happens", she says.

"But there's very limited food on land. It's incredibly difficult to imagine how the bears are going to meet their annual energy budgets without their period of heavy feeding out on the ice.

"If the ice retreats, it will have a profound influence on the bears' distribution patterns, and presumably on their abundance in the Arctic."

Images courtesy of US National Oceanic and Atmospheric Administration