Not many fish in the sea, scientists say

By AAP with AG staff 9 February 2012
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Despite the vast ocean volumes, most species of fish are found in freshwater lakes and rivers, new research shows.

CONTRARY TO POPULAR BELIEF, there aren’t a lot of fish in the sea, researchers say.

Despite covering 70 per cent of the Earth’s surface, marine environments contain only 20 per cent of all its species.

In the new study, scientists made the surprising discovery that freshwater rivers and lakes contain more fish species than salty oceans. Yet freshwater environments occupy only two per cent of the Earth’s surface. It could be evidence of life re-inventing itself after mass extinctions long ago, the experts believe.

Evolution after mass extinction of fish

The findings suggest that most marine fish alive today are descended from freshwater ancestors, even though life is thought to have first evolved in the oceans.

A marine apocalypse may have been followed by a re-emergence of life in watery environments on land.

A team led by Dr John Wiens, from Stony Brook University in New York, USA, studied the biodiversity of ray-finned fish, which make up 96 per cent of all fish species. The scientists created evolutionary trees based on molecular data and fossils, and a large database of fish habitats.

More fish in lakes, rivers than in the sea

“There are more fish species in freshwater than in saltwater habitats, despite the much greater area and volume of the oceans,” says John. “More remarkably, our results suggest that most marine fish alive today are descended from freshwater ancestors.”

“Our results suggest that ancient extinctions in the marine environment may have wiped out the earliest ray-finned fishes living in the oceans, that the oceans were then recolonised from freshwater habitats, and that most marine fish species living today are descended from that recolonisation,” he says. “This pattern of ancient extinction and more recent recolonisation may help explain why the oceans are now so species-poor, even for fish.”

The findings are published online in the journal Proceedings of the Royal Society B.