In the fossil record, one freshwater fish species goes extinct every 3 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 53-86 species of freshwater fish may be extinct by the year 2050. Since the first assessment of extinct North American freshwater fishes in 1989, the number of extinct fishes increased by 25 percent.
"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. "Freshwater fish are a good choice for analysis as their bones make clear fossil impressions, and their lake and river environments produce excellent stratigraphic sequences."
The study's author, Noel Burkhead, used an established method to compare the rate of extinction found in the fossil record with modern rates.
"Estimates of freshwater fish extinctions during the twentieth century are conservative, because it can take 20-50 years to confirm extinction," said Burkhead, a research fish biologist for the USGS.
Extinction is a natural process, Burkhead explained, so examining its rate over a long geological timescale provides biologists with a benchmark for comparing current extinctions to background rate. The accelerated pace of extinction observed since the beginning of the twentieth century suggests human causes.
In North America, assessments of extinctions are conducted by the American Fisheries Society's Endangered Species Committee, using categories to factor in a lag time since the last observation of the species. The study used the categories "extinct" (species not seen for 50 years or more), "possibly extinct" (not been seen for 20 years or more), and "extinct in nature." All these categories require that searches for the missing fishes must have been made by knowledgeable biologists.
"It is extremely rare that the death of the last individual is documented by biologists," said Burkhead, "although it can happen when a fish only is found in a specific spring or caldera, and it dries up. That's what happened with five species of desert pupfishes and the Alberca silverside—the last known fish to go extinct in North America."
The Alberca silverside was found only in the Alberca Caldera, Guanajuato, Mexico; it went extinct when the caldera temporarily dried up in August 2006.
Surprisingly, Burkhead reported that 90-96 percent of fish extinctions in the fossil record were not linked to the five well-known mass extinctions. 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.
The background rate of extinction is based on the fossil record, which includes information on when ancient fishes lived and how long species survived in the geological past. Burkhead used data on fish extinctions from well-known paleontologist Steven M. Stanley at the University of Hawaii.
"Another cause of extinction can be a change in a fish's food chain, which is what may have happened to the harelip sucker, a really cool fish that used to live in seven states throughout the Ohio River basin," said Burkhead. "It was a snail-eating specialist with cleft lips that used to pluck snails off river bottoms and manipulate the snail in its mouth in order to suck out the snail's soft parts, perhaps making little popping sounds. Sadly, snails are highly sensitive to excessive sedimentation and in the late nineteenth century, large amounts of topsoil were washing into rivers along with sewage and industrial effluents from cities. This likely caused snails to decline, which may have been what drove the fish to extinction."
Declines in freshwater fishes are only the "tip of the iceberg" for freshwater ecosystems, with mussels and snails experiencing declines greater than that of freshwater fishes.
The study, "Extinction Rates in North American Freshwater Fishes, 1900-2010," will be published in the September issue of the journal BioScience.
A summary of data on extinct North American freshwater fishes is available online on the Extinct North American Fishes website, which is updated by USGS and the AFS.
Putting the numbers into perspective:
- 39 — Number of North American freshwater fish species confirmed as extinct from 1898 to present
- 1213 — Number of freshwater fish species found in North America up until 2010
- 3,000,000 — Average number of years between each fish extinction documented in the fossil record
- 2006 — Year that the most recent fish extinction was confirmed
- 31,769 — Total number of valid fish species described worldwide up until 2010
- 43 — Percent of the world’s fish species that depend on freshwater habitats