We are motivated by the same thing that motivated Charles Darwin and Captain Cook

Jesse Ausubel, Alfred P Sloan Foundation

"Our motive is discovery," said Jesse Ausubel of the US-based Alfred P Sloan Foundation, which is helping to finance the project. "Very large areas of the oceans are unexplored.

"We have looked at the top layer and a few square metres of the sea bed, but most of the mid-layer is unknown. We are motivated by the same thing that motivated Charles Darwin and Captain Cook, but there is a practical benefit."

The worldwide census will not only provide valuable information for conservationists, it will also help provide maps of unknown regions of the oceans, he told BBC News Online.

Practical benefits

"Imagine setting out on a hike and knowing that your map only covers 5% of where you're going. That's the situation now for the seas around developed countries like the US, Canada and Australia."

Scientists have pictures of thermal vents on the sea bed but they have no idea whether there are 10 thousand or 10 million of them, Jesse Ausubel added.

The survey starts at the top of the food chain

Others who stand to benefit include those whose job it is to manage the world's fisheries.

"Even for commercially fished stocks there's a tremendous amount of uncertainty about numbers - getting the numbers right can help," he said.

Governments deciding where to create underwater National Parks will have an easier job, too. "In some cases, it's obvious - you could draw a circle round the Great Barrier Reef and say 'there!', but in others it's not. Some animals stay put, others move around," he said.

Jesse Ausubel admits that it will not be possible to count every form of life in the sea, even during the course of a 10-year programme.

Starting at the top

"We are starting with the largest critters - fish, marine mammals, squid - but we're maximalists: we want to count as much as possible."

The census will use acoustic and optical techniques to carry out its headcount, but it will not employ the other techniques which would be needed to count microbiological lifeforms, he added.

Imagine setting out on a hike and knowing that your map only covers 5% of where you're going

Jesse Ausubel Alfred P Sloan Foundation

The scientists meeting in Hobart are working on Australia's contribution to the project. They have been involved since the survey was first proposed in 1997 and launched last year. Now, they are working out how to collect and organise the vast amounts of data which the census will produce.

Csiro, Australia's official scientific research body, says that scientists will tag large animals at the top of the food chain - animals like whales, sea turtles and tuna.

The international census provides a welcome boost to Australia's own surveying efforts, Dr Nan Bray, Csiro's marine research chief, said.

"At present funding levels, it will take another 100 years just to map Australia's 11-million-square-kilometre exclusive economic zone," he said.