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 2 August, 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.

Comprehensive

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.

More than 360 scientists have spent the past decade surveying 25 regions.

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 per cent-10 per cent of 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 overfishing, 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.

Most diverse regions

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. — © Guardian Newspapers Limited, 2010

Keywords: censusmarine lifeCOML