The most endangered species in North America are those living in fresh water, according to a Canadian study.

The study, by Anthony Ricciardi of Dalhousie University, Halifax, and Joseph Rasmussen, of McGill University, Montreal, is published in the current issue of "Conservation Biology".

Warning that the US could lose most of its freshwater species in the next century if nothing were done, Dr Ricciardi said: "A silent mass extinction is occurring in our lakes and rivers."

The study says that common freshwater species are dying out five times faster than those that live on land.

Rainforest parallel

The authors say their estimate of the loss of freshwater biodiversity "is probably conservative", because some species are likely to have become extinct without anyone ever knowing that they had once existed.

They say freshwater species are dying out as fast as those in the rainforests.

Since 1900 at least 123 species have been lost from North American waters, and the study predicts a continuing loss of about 4% of the remaining total every decade, unless the trend is arrested.

It says almost half of all freshwater mussels, a third of the crayfish, a quarter of the amphibians and a fifth of the fish could die out by 2100.

Ricciardi and Rasmussen try to judge how quickly the slide to extinction is accelerating by comparing current rates with those from the fossil record.

They calculate that the background extinction rate for freshwater fish species is about one every three million years.

Invaders

But they say the modern rate in North America is about one extinction every 2,600 years - 1000 times faster.

Threats the authors identify include the arrival in an ecosystem of non-native species, including the transfer of game fish from one lake to another.

Dams like this one at Niagara Falls can put species at risk

They say European zebra mussels are taking over from North American natives, and sea lampreys cause problems when they invade lakes and kill the native fish to which they attach themselves.

Pollution is another threat, with most US waterways affected by sediments, toxic contamination, and organic run-off from farmland.

And many species also have to contend with dams that obstruct the free flow of rivers.