The extinction rate of freshwater fish in North America could double over the next 40 years, the U.S. Geological Survey has warned.

From 1900 to 2010, freshwater fish species in North America went extinct at a rate 877 times faster than the rate found in the fossil record, and estimates indicate the rate may double by 2050. This new information comes from a U.S. Geological Survey study to be published in the September issue of the journal BioScience.

In the fossil record, one freshwater fish species goes extinct every three million years, but North America lost 39 species and 18 subspecies between 1898 and 2006. Based on current trends in threatened and endangered fish species, researchers estimate that an additional 53 to 86 species of freshwater fish may be extinct by 2050. Since the first assessment of extinct North American freshwater fishes in 1989, the number of extinct fishes increased by 25 percent.

The majority of these extinctions are small forage fish, such as varieties of minnow, sucker, stickleback and cisco (including the deepwater cisco of Lakes Huron and Michigan, and Lake Ontario’s kiyi), however healthy ecosystems depend on the presence of these prey fish. Additionally, some notable Canadian gamefish were also lost in the 20th century. Those include the Lake Ontario populations of Atlantic salmon (1898) and Arctic grayling (1935), the blue pike (1970) and the St. Lawrence Estuary striped bass (1968).

"This study illustrates the value of placing current events into the context of deep geologic time, as rocks preserve an unbiased record of natural rates of processes before human activities began to alter the landscape, the atmosphere, the rivers, and oceans," said USGS Director Marcia McNutt.

Extinction is a natural process, said the study’s author, fish biologist Noel Burkhead, so examining its rate over millions of years allows biologists to compare extinctions to background rate. The accelerated pace of extinction observed since the beginning of the 20th century suggests human causes.

Natural causes of fish extinction are linked to transitions in landforms and continental watercourses over time, but many twentieth century extinctions were caused by dams, channelization of rivers, water pollution and other human-induced factors. This manipulation of the environment often spawning grounds, reduced water quality or disrupted the food chain, leading to extinction for sensitive species.

Additionally, estimates of freshwater fish extinctions during the 20th century are conservative, said Burkhead, because it can take 20 to 50 years to confirm extinction. In the study, a species was considered "extinct" if it hadn’t been seen for 50 years or more, "possibly extinct" if not seen for 20 years, "extinct in nature," if only captive fish survive. All these categories require that searches for the missing fishes must have been made by knowledgeable biologists.

A summary of data on extinct North American freshwater fishes is available online on the Extinct North American Fishes website, including maps showing extinctions by eco-region, and a list of extinct fishes by province or state.