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Change F s 3 '< Ever since Western Civilization started dominating the California landscape, Califor- { nia's fish fauna has been changing. Thirty-seven percent of the fish species now present " were introduced by man within the last one hundred years, and these fishes are now among the most abundant species in California's inland waters. The native fishes are quietly disappearing, especially from low-elevation regions where man's impact has f been the greatest. In Clear Lake, Lake County, and in the San Joaquin River at Friant, two California localities from which reliable collecting records exist over the years, the fish faunas have changed from being dominated by native fishes to being dominated by introduced or planted species (Tables 4, 5). Many of the native fish species now . - endangered or extinct were among the most abundant fishes in aboriginal California and served as major sources of food for the Indians and early white settlers. Among these species are the Lost River and shortnose suckers of the Klamath River, the thicktail chub, Sacramento perch, and Clear Lake splittail of the Sacramento-San Joaquin River system, and the Colorado squawfish, bonytail, and humpback sucker of the Colorado River. The causes of the decline of native fish fauna are many but they fall into three basic areas: habitat changes, iniroductions, and fishing. HABITAT CHANGES _ Most of California's major inland waterways. today bear little resemblance to the streams and lakes encountered by the fusi white explorers and settlers. The once turbulent and muddy lower Colorado River is now a giant dammed 'irrigation ditch and drain, carrying salts and other agricultural wastes to Mexico and .occasionally to the Gulf of California. The giant lakes of the San Joaquin Valley are today vast grain ~,'. farms. The Sacramento-San Joaquin Delta, once an enormous tule marsh dissected by < ~ ~ meandering river channels, has been transformed into islands of farmland protected by ~ high levees from the water that flows by in straight, dredged channels. Almost every _ stream of any size has been dammed at least once to control its flow. Thus, it is not ° s surprising that habitat modification is the major cause of the changes in California's fish fauna. Different fish species are affected by different types of habitat change, however, so it is worthwhile to consider separately the effects of (1) stream-channel alterations, (2) dams and reservoirs, (3) dewatering of streams and lakes, (4) pollution, _ and (5) watershed changes. Man has probably been altering the cha-noels of California's streams ever since the first Spaniard stepped off a boat, shovel in band. The first really drastic alterations, however, were those of the gold miners, wlm in their frantic search for tiny pieces of