WASHINGTON It has been the biggest and most comprehensive attempt ever to answer that age-old question how many fish are there in the sea? Published on Monday, a 10-year study of the diversity, distribution and abundance of life in the world's oceans, the Census of Marine Life (COML), estimates there are more than 230,000 species in our oceans.

The survey covers “from coast to the open ocean, from the shallows to the deep, from little things like microbes to large things such as fish and whales,” said Patricia Miloslavich of Universidad Simon Bolivar, Venezuela, the co-senior scientist of the COML. It also covers crabs, plankton, birds, sponges, worms, squids, sharks and slugs.

More than 360 scientists have spent the past decade surveying 25 regions, from the Antarctic through the temperate and tropical seas to the Arctic, to count the different types of plants and animals.

The results show that around a fifth of the world's marine species are crustaceans such as crabs, lobsters, krill and barnacles. Add molluscs (squid and octopus) and fish (including sharks) and that accounts for up to half the species in the seas. The charismatic species often used in conservation campaigning - whales, sea lions, turtles and sea birds - account for less than 2 per cent.

The surveys have also highlighted areas of concern for conservationists.

“In every region they've got the same story of a major collapse of what were usually very abundant fish stocks or crabs or crustaceans that are now only 5-10pcof what they used to be,” said Mark Costello of the Leigh Marine Laboratory, University of Auckland.

“These are largely due to over-harvesting and poor management of those fisheries. That's probably the biggest and most consistent threat to marine biodiversity around the world.”

The main threats include over-fishing, degraded habitats, pollution and the arrival of invasive species. But more problems loom rising water temperatures and acidification thanks to climate change and the growth in areas of the ocean that are low in oxygen and, therefore, unable to support life.

The most diverse regions identified by the COML are around Australia and south-east Asia. “It's also a hotspot for terrestrial biodiversity and this has been known for about 100 years,” said Costello.

“It looks like that region with the coral reefs has always had a very high rate of speciation. It also has a very diverse range of habitats - from the deepest areas of the oceans to large areas of shallow seas, which can support coral reefs.”

Australian and Japanese waters contain more than 30,000 species each. Next in line are the oceans off China, the Mediterranean Sea and the Gulf of Mexico.

Apart from algae and the seabirds and mammals that travel around the sea, the COML identified the manylight viperfish (Chauliodus sloani) as the most “cosmopolitan” marine creature. Its presence was recorded in around a quarter of the world's seas.

“This inventory was urgently needed for two reasons,” said Costello. “First, dwindling expertise in taxonomy impairs society's ability to discover and describe new species. And secondly, marine species have suffered major declines - in some cases 90pc losses - because of human activities and may be heading for extinction, as happened to many species on land.”

Miloslavich said the COML data would “allow policymakers to make more informed decisions on what areas should be protected for the better management of resources and the ecosystems, in order that they keep providing good services.”

The survey is being published online at the PLoS ONE journal. More detailed results will be published in October.

For every marine species known to science, COML scientists estimate at least four have yet to be discovered. They said that around 70pc of fish species have been discovered, but for most other groups probably less than a third are known. As of February, the number of marine fish species known to science stood at 16,764, and was growing at around 100 a year.

The most fruitful potential areas for discovery include the tropics, deep seas and the southern hemisphere.

“At the end of the Census of Marine Life, most ocean organisms still remain nameless and their numbers unknown,” said Nancy Knowlton, a biologist at the Smithsonian Institution, leader of the COML's coral reef project.

“This is not an admission of failure. The ocean is simply so vast that, after 10 years of hard work, we still have only snapshots, though sometimes detailed, of what the sea contains. But it is an important and impressive start.”—Dawn/The Guardian News Service