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I I I I I I I I I I I I I I I I I I I BIOLOGICAL OPINION Status of the Species STATUS OF THE SPECIES Listed species/critical habitat: Bonytail Chub Listing History The bony tail chub was first proposed for listing under the ESA on April 24, 1978, as an endangered species. The bony tail chub was listed as an endangered species on April 23, 1980, with an effective date of the rule of May 23, 1980. In the final rule, the Service determined that at that time there were no known areas with the necessary requirements to be determined critical habitat. Critical habitat was designated in 1994 and is discussed elsewhere in this section. Species Description The bony tail chub is one of three closely related members of the genus Gila found in the Colorado River. Confusion about the proper taxonomy and the degree of hybridization between the bony tail chub, the humpback chub, (Gila cypha), and roundtail chub, (G. robusta), has complicated examinations of the status of these fish. The bony tail chub was originally described from specimens taken in Arizona (Baird and Girard 1853). The bony tail chub is a highly streamlined fish with a very thin, pencil-like, caudal peduncle and large, falcate fms (Allan and Roden 1978). A nuchal hump may be present behind the head. Maximum length is about 600 millimeters (mm) , with 300-350 mm more common (USFWS 1990). Weights are generally less than one kilogram (kg) (Vanicek and Kramer 1969). Bony tail chub are long-lived fish; some have reached at least 49 years of age (Minckley 1985). Life History Life history information for the bony tail chub was recently summarized in the recovery plan (USFWS 1990) and in the biological support document for the critical habitat designation (USFWS 1993a). It is important to note that life history information on this species is limited. The information presented in this BO is primarily taken from these sources. The bony tail chub was once abundant in the Colorado River and its major tributaries throughout the Basin, occupying 3,500 miles of river in the United States and Mexico (USFWS 1993a). With the confusion between the bony tail chub and roundtail chub arising from use of the common names "bony tail chub" and "trout" for both species, specific information on abundance may be lacking. However, the Service is reasonably certain that records from the LCR were bony tail chub and not roundtail chub. Records from the late 1800's and early 1900's indicated the species was abundant 38