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For example, the Tecopa pupfish, which once enjoyed the warm outflow from two California hot springs, disappeared about 20 years ago when the springs were modified for use as bathhouses. Another pupfish was able to move in and mosquitofish were introduced, resulting in competition that extirpated the species.

Noting that 19 of the 40 species analyzed have disappeared since 1964, the researchers concluded that "unfortunately, the rate of extinction of North American fishes is likely to increase." They believe that "present laws and recovery activities appear inadequate to stem the increasing tide of endangered fishes."

The greatest losses occurred in the Great Lakes, the Great Basin, the Rio Grande, the Valley of Mexico and Parras Valley in Mexico. But according to Dr. Andrew L. Sheldon, zoologist at the University of Montana in Missoula, the fish of the arid West are presently most endangered, the result of fast-expanding, water-hungry human settlements that drain what little water remains.

"In conflicts over development of increasingly valuable water resources, the fishes have few advocates," Dr. Sheldon pointed out. Yet, he added, people are likely to benefit from better water quality and other environmental improvements when steps are taken to protect endangered fish. In addition, many other threatened organisms, including mollusks and crayfish, would have a better chance for survival.

"We've spent most of our resources on fishes, but clams, snails, crayfishes and even aquatic insects are in equally bad straits," Dr. James Williams said in an interview. "These lowly invertebrates, which really support the entire system, have not received proper attention."

Though mollusks like freshwater mussels have even fewer champions than fish, they are in some ways more important to environmental quality. Mussels cleanse billions of gallons of water daily, removing microscopic plants, bacteria and suspended organic particles from the water as it courses through their gills.

But mussels depend on fish to reproduce. Gill-less mussel larvae latch onto fish gills, which breathe for them until the larvae mature. Each mussel species has its own fish host, and when the fish goes, so does the mussel. Limited Success in Arizona

Dr. Gordon, a research associate for the Tennessee Cooperative Fishery Research Unit at Tennessee Technical University in Cookeville, said that since each kind of mussel depends on a particular host fish, "we can't reintroduce mussels unless the host fish is already there." To get around the problem, at least temporarily, his laboratory is propagating mussels in captivity, raising them past the larval stage before they are released into the river.

In the arid southwest, Francisco Abarca and his colleagues at the Arizona Game and Fish Department are struggling to reintroduce native fish that are severely endangered or gone from American waterways. In some cases -- for example, the colorful Yaqui shiner and the Yaqui catfish, both of which are extinct in Arizona -- the fish are being imported from Mexico, where they are still relatively abundant, Mr. Abarca said.

The celebrated desert pupfish and the Sonoran topminnow, both listed as endangered species by the Federal Government, have been the stars of the Arizona project, the largest aquatic reintroduction program in the Southwest. But the program has met with limited success. In only 1 of 18 reintroduction sites did the pupfish survive, and fewer than 30 of more than 200 reintroduced populations of topminnows have managed to hang on for at least three years, with prospects for continued survival considered slim for many.

Dr. Abarca said future efforts to reintroduce endangered species in the range of their historic Arizona habitats will first seek to remove non-native fish that compete with them.

Still, according to Dr. Jack Williams, the situation could be worse. "While native fish recovery efforts may not seem very successful," he noted, the list of endangered aquatic species would be much longer if not for the efforts of Federal, state and private organizations. Behind Loss of Species Major factors in extinctions of 40 North American fishes in the last century; in most cases, more than one factor was at work.