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IV. DESCRIPTION OF THE ENVIRONMENT A. Aquatic Biological Resources Forty-two nonnative fish species and subspecies have been introduced into the upper Colorado River Basin (Tyus et al. 1982). However, not all of these were intentionally stocked for sport fishing purposes. A brief description of the major species addressed by the stocking procedure alternatives is presented below. Much of this information was extracted from a report entitled "Options for Selective Control of Nonnative Fishes in the Upper Colorado River Basin" by Lentsch et-al. (1995). Additional information on the species discussed below and on other species can be obtained from Lentsch et al. (1995). 1. Colorado squawfish: This species is endemic (occurs here and nowhere else) to the Colorado River Basin. It is the largest member of the minnow family in North America. Once very common throughout the Colorado River Basin, its present range is restricted to rivers in the Upper Basin. Populations on the middle Green and Yampa rivers are doing fairly well. Populations elsewhere are characterized by low numbers, but with some recruitment. 2. Razorback sucker: This species is also endemic to the Colorado River Basin. It was probably the most abundant and widespread of all the endangered fishes of the Basin. Remnant populations remain only in the Green River in the Upper Basin and Lake Mohave in the Lower Basin. Fish spawning occurs in both locations, but recruitment is limited due to predation from other fishes. These fish are currently being stocked in the Green and Gunnison rivers to augment low numbers. 3. Humpback chub: This fish species is also endemic to the Colorado River Basin. Its early distribution is not well documented because this species occurs in canyon reaches that generally are not very accessible. The largest remaining population of this species is the Upper Basin is located in the Black Rocks and Westwater Canyon areas near the Utah/Colorado border. 4. Bonytail: The bonytail is also endemic to the Colorado River Basin and is considered the rarest native fish. Although this species appeared abundant in the system in the late 1800's, their disappearance coincides with construction of dams first in the Lower Basin and then in the Upper Basin. This species was considered extirpated from the Upper Basin, but it is currently being reintroduced into the Green River. 5. Largemouth bass: The native distribution is believed to be from northeastern Mexico east to Florida and north to southern Quebec and Ontario. Largemouth bass have been introduced throughout the Colorado River Basin, primarily in lakes, reservoirs and ponds. Although, not stocked directly into the mainstem rivers, bass that escape from standing waters move into low velocity habitats (the same habitats used by young endangered fishes). Their diet includes other fishes. Largemouth bass were ranked 7th on the list of 28 nonnative fish species 20