By Paul Rincon
BBC News Online science staff
The narwhal, a marine mammal known best for its single, unicorn-like tusk, may be under threat after aerial surveys showed a decline in its numbers.
Hunting, fishing and climate change may threaten the narwhal
Hunting and local climate change in the Greenland waters where it usually lives may be to blame, researcher Dr Mads Peter Heide-Joergensen believes.
The monitoring suggests there is a population fall of about 10% per year - and this may be an underestimate.
Details are outlined in a zoological journal called Marine Mammal Science.
Dr Heide-Joergensen, of the Greenland Institute of Natural Resources, conducted his investigation of Narwhal numbers in the Inglefield Bredning area of the island and its adjacent fjords.
He carried out two separate surveys in August 2001 and August 2002, using two digital cameras that continuously download images to separate laptop computers.
The 2001 survey, repeated four times, covered an area of 840 sq km and counted 360 narwhals. The 2002 survey, repeated seven times covered an area of 2,208 sq km and counted 566 narwhals.
When compared with the results of surveys conducted in 1985 and 1986, the estimates currently show a 10% decline per year.
However, Dr Heide-Joergensen said the decline could be much bigger because the 80s studies did not correct for perception bias, which is designed to take account of the fact that some whales may be present at the surface and not seen by the observers.
NARWHAL - M. MONOCEROS
Spiral horn normally found on the male and can grow to 3m
Its purpose is not entirely clear but males are known to joust
Narwhals will eat fish, squid, krill and other crustaceans
Narwhals (Monodon monoceros) are hunted intensively by the indigenous people of Greenland, the Inuit.
In 1999, a long-standing policy of regulating the hunt in Nunavut, eastern Canada, was abandoned. In Greenland, no direct measures for regulating the hunt have ever been taken.
"They take [the narwhals] for the tusk but they also hunt them for the skin and the meat. The skin is rich in vitamin C, which is a good defence against scurvy when you don't have vegetables," Dr Heide-Joergensen told BBC News Online.
However, several other factors also threaten the animals.
The developing halibut fisheries in Greenland may impact the narwhals because the animals rely on the fish when they winter in central Baffin Bay.
There are fears this growing industry could soon reach a commercial scale, severely depleting an important narwhal food resource.
Some scientists fear for the narwhals' future
Dr Heide-Joergensen said increases in sea ice in some parts of Baffin Bay caused by the local cooling of temperatures might also be placing pressure on the animals.
"They winter in heavy pack ice and we know that they occasionally succumb in the ice when it closes over completely," he explained.
He added that there had been a steady increase in ice in parts of Baffin Bay over the last 50 years.