Seamounts: Undersea mountains 'litter' ocean floor

2 March 2011

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The seafloor is littered with underwater mountains, a new global survey has revealed.

Almost 5% of the ocean is covered by seamounts, peaks rising more than 1000m above the surrounding sea floor, with 16% covered by smaller knolls.

Seamounts and knolls are important and little studied habitats for marine life, say the survey's scientists.

Seamounts alone make up a habitat equivalent in size to Russia or all the world's tropical dry forest.

Details of the seamount survey are published in the journal Deep-Sea Research Part 1.

Seamounts are "undersea mountains", normally conical in shape with the summits rising more than 1000m above the surrounding sea bed. Knolls are similarly shaped, but smaller structures.

"Both are important habitats of marine diversity," says Dr Chris Yesson of the Zoological Society of London, UK, who led the research team that conducted the latest survey.

Past estimates of seamounts and knolls have suggested there could be anything from a few thousand to a few hundred thousand.

These vary because of the different ways seamounts have been measured and the quality and amount of data used.

To produce a definitive estimate, Dr Yesson and colleagues based their work on a recent US Geological Survey map of the ocean floor.

That project mapped the ocean floor to within an accuracy of approximately one kilometre, identifying seamount-shapes from the depth of the ocean floor.

Dr Yesson's team took this map and examined it in detail, searching for specific peaks and checking the peaks' shapes to see if they were the right size and shape to be seamounts or knolls.

To check for accuracy, they then cross referenced the results against areas of seafloor that have been intensively surveyed.

"Previous attempts to quantify seamount numbers have been based on depth maps at much lower accuracy, or have been regionally focused, so this study represents a significantly better estimate of global seamount numbers," Dr Yesson told the BBC.

The team found that there are approximately 33,000 seamounts and 138,000 knolls covering 4.7% and 16.3% of the ocean respectively.

They cover an area equivalent to Russia. They also represent an unheralded, distinct habitat, equivalent in size to the world's tropical dry forests.

"The most surprising element of this study is the way that it highlights how much more of the oceans we have yet to study."

For example, only about 250 seamounts have been biologically surveyed to any degree, and most of those have had few samples of marine life taken from them.

That represents less than 1% of the total number now known to exist.

Seamounts can be areas of high biological diversity, containing habitat forming organisms such as corals and sponges, which in turn are habitats for fish and other animals, including commercially important species, says Dr Yesson.

"But seamounts are by their nature fragmented habitats, vulnerable to exploitation.

"The vast majority of seamounts have no formal protection, and these areas can be targets for trawl fisheries, and once impacted, they may take decades, centuries or even longer to recover," he says.

"There is a wealth of biological diversity on the tens of thousands of unstudied seamounts that may be lost before we have the opportunity to learn about it."

Dr Yesson's team hopes this global survey will be useful as a conservation resource, allowing scientists to find, research and help manage seamounts and knolls and limit the exploitation of deep-sea ecosystems.

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