Alien invades Black Sea
Sickness in the ocean
Big bird goes extinct
Undersea robots

Desert Pupfish

Photo by John Rinne.
Image courtesy Desert Fishes Council.

Salmon heads upstream
Image courtesy US Fish and Wildlife.

Rainbow trout and Brook trout courtesy US Fish and Wildlife.

Government fish and game departments have long grown desirable gamefish in hatcheries, then dumped them into waterways for a public that was originally hungry for protein-rich food, and lately obsessed with sport fishing. The practice exploded in the late 1800s when specially cooled railroad cars hauled fish from East to West along the transcontinental railroad.

Nowadays, scientists concerned about declining biodiversity blame these same fish distributions for a nationwide homogenization of fish populations. Homogenization -- the growing similarity of ecosystems due to human activities -- is also occurring on land, but it may be further advanced in freshwater lakes and rivers in the 48 contiguous U.S. states.

Writing in the current issue of Science, University of Wyoming zoology professor Frank Rahel examined the decline in fish diversity by pairing up states, then counting how many species were found in both states. An increase in the number of overlaps, he figured, indicated that fish populations were growing more similar.

After comparing historic and new species lists, Rahel found that 89 pairs of states that once had no fish in common now share an average of 25.2 species. The average pair of states gained 15.4 shared species.

A place for every fish, every fish in its place
Before people started lugging fish around the continent, essentially all fish in a body of water were those that had evolved from earlier resident populations. Nowadays, homogenization is increasing due to stocking and other factors:

Extirpation (local extinction) reduce the number of species. Some fish, like the desert pupfish in the American Southwest, have disappeared due to environmental change, competition from introduced species, and other factors.

Accidental releases. The ruffe and the round goby reached the Western Hemisphere in ballast water, and continue spreading rapidly through the Great Lakes.

Liberated pets. Goldfish, discarded by former owners, now live wild in 42 states, Rahel says. They are native to none.

Taking stock
After crunching the numbers, Rahel found that stocking was by far the largest single cause of the growing similarity among species.

The government role in stocking exotic species declined with the rise of environmentalism in the 1970s, Rahel says. "As a rule, for the past 20 or 30 years, fisheries managers have been very aware of the negative effects of introducing new species."

Many stocking programs now focus on restocking native fishes -- or infertile hybrids like the saugeye, a cross between the walleye and the sauger, which will not become a permanent part of the ecosystem.

However, some individual anglers continue loosing exotic fish, which explains the continuing expansion of northern pike and walleye throughout the western United States.

Salmon, which split their time between rivers and the ocean, are facing a new form of exotic introduction -- plans to sell eggs of fast-growing, genetically engineered salmon to fish-farmers. These farmers grow their crop in ocean pens, which, the New York Times wrote on May 1, "are renowned for being torn by waves or hungry wild animals."

If -- or when -- the new salmon escape, they could cross with native salmon, a valuable species that's already under enormous threat. (The Why Files covered genetically engineered food.)

So what?
Fish. You usually don't see them. You may not even eat them. Why should you care about a reduction in fish diversity? Among other reasons, Rahel says growing similarity makes fish more vulnerable to disease and environmental change. The situation reminds him of agriculture's increasing reliance on a few grains. "The same could be said about fish," he says. "As you get more homogenization, you are more susceptible to large-scale fish disease outbreaks. We are losing our genetic resources."

Homogenization is worst in the West: More than half the fish species in Arizona, Utah and Nevada are non-natives. After rainbow and brook trouts were introduced to Rocky Mountain streams, the rainbows bred with the native cutthroat trout, creating exotic hybrids. Meanwhile, the brooks outcompeted the cutthroats, which have vanished from more than 90 percent of their original habitat, Rahel says. Finally, whirling disease is slaughtering rainbows and cutthroats alike.

Rahel's study helps explain why freshwater fish are among the most endangered creatures in the United States. According to the Nature Conservancy, 25 percent of freshwater fish species are presumed extinct, critically imperiled, or imperiled. Tellingly, the only organisms in worse shape -- freshwater mussels, crayfish and amphibians -- are also aquatic.

-- David Tenenbaum

Bibliography
Homogenization of Fish Faunas Across the United States, Frank Rahel, Science, 5 May 2000, pp. 854-6.

Altered Salmon Leading Way to Dinner Plates, but Rules Lag, Carol Kaesok Yoon, The New York Times, May 1, 2000, p. A1.