by John Rinne.
Rainbow trout and Brook trout courtesy US Fish and Wildlife.
4 MAY 2000 It
seemed so helpful -- the practice of dumping new fish into local lakes
and streams. What pioneer wouldn't want to catch and cook a submarine-sized
common carp in the local fishing hole? What angler wouldn't want to hook
a toothsome brook trout -- without having to visit the trout's native
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.
place for every fish, every fish in its place
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.)
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
Altered Salmon Leading Way to Dinner Plates, but Rules Lag, Carol Kaesok Yoon, The New York Times, May 1, 2000, p. A1.
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