Diversity of Fish Species Is Decreasing, Study Finds

Across America, from lake to lake and stream to stream, more of the fish are alike.

The great diversity is disappearing as species popular with fishermen and cooks are introduced where they do not occur naturally, fish ecologist Frank J. Rahel reports in Friday’s issue of the journal Science.

It’s a process Rahel calls “biotic homogenization,” the reduction in regional differences in plants and animals. Small populations of native species become extinct while popular food and game fish are spread by humans. The result is that fish populations across the country become more alike.

“On average, pairs of states have 15.4 more species in common now than before European settlement of North America,” according to his study.

And, like European settlement, the main movement has been East to West, Rahel found, with only one species--rainbow trout--swimming upstream.

In recent years government-sanctioned introductions of game fish have declined but illegal and inadvertent introductions continue.

Rahel, a fish ecologist in the zoology department at the University of Wyoming in Laramie, studied freshwater fish in the 48 contiguous states, comparing the number of species each state had in common with each other state now and in the past. He used historical records to determine past populations and data from state agencies for current species.

He discovered that 89 pairs of states that formerly had no species in common now share an average of more than 25 species.

“For example,” he said, “Arizona and Montana historically had no fish species in common but they now share 33 species.”

While much has been written about the loss of native species--and that does occur--Rahel found that the bigger culprit in the growing similarity is the arrival and establishment of species from elsewhere.

Introductions for food and sportfishing were the major factors, he said, but he added that release of aquarium fish and arrival of foreign fish in ballast water play a part.

The Eurasian goldfish, for example, is not native to any of the 48 states he studied, but now is found in the lakes and streams of 42 of them thanks to owners who no longer wanted the pets.

Even more widespread is the common carp, now present in all 48 states. A popular food in Europe, the fish were introduced in the 1800s for people to grow in farm ponds as a source of extra protein.

“They quickly escaped from the farm ponds and are everywhere, but they never evolved as a desired food source for the North American palate,” Rahel said in a telephone interview.

Anglers have had the biggest impact on the movement of game fish from the East to the West. “Of the 17 most widely introduced species, 12 fit this pattern,” he said.

Those include the black crappie, yellow perch, walleye, largemouth bass, striped bass, bluegill and brook trout.

“By contrast only one Western species (rainbow trout) has been widely introduced into Eastern states,” he said.

Of 85 fish species found in Nevada, 44 have been introduced and 24 of those are game fish. More than half of freshwater fish species found in Nevada, Utah and Arizona were introduced from elsewhere, he found.

And 25% to 50% of species in Washington, Oregon, California, Idaho, Montana, Wyoming, Colorado, New Mexico, Maine, New Hampshire, Massachusetts, Connecticut and Rhode Island were introduced.

Introduced species represented less than 25% of the species in the remainder of the contiguous 48 states.