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.. , , . ." ,- ~ .. .. . \ 'i i' -, HOLDEN, ET AL.: THREATENED FISHES OF UTAH S4 Factors Influencing Decline: Since no "'before" data are available, it is questionable whether humpback chubs have actually declined in abun- dance. It appears likely that this form was a phenotype near extinction when discovered. The apparent introgressi~ hybridization with bonytails may be a sign of this extinction process. If so, alteration of the Colorado River by man may have hastened the demise of this phenotype through loss of habitat and competition with introduced species. Xyrauchen texanus-Humpback or Razorback Sucker Status: Endangered species Description: Adult humpback suckers are readily distinguished by the prominent bony keel behind a flat head (Figure 3). The keel becomes obvious in 8-lO-inch fish. Juveniles have seldom been identified. They may appear much like juvenile flannelmouth, except lateral line scales are fewer (68-87 to 90-116) (Sigler and Miller. 1963). Postlarvae are distin- guished from tlannelmouth and blueheadsuckers by large black spots on the dorsal surface (Winn and Miller, 1954). Adults exhibit orange, yellow, and violet lateral coloration during spawning season (Spring). Large breeding tubercles appear on the anal and caudal fins of adult males. Most adults are 12-18 inches and 3-5 pounds. The humpback and tlannelmouth sucker hybrid is common but easily distinguished by a poorly developed keel and intermediate scale counts (Hubbs and Miller, 1953). Documentation;' Humpback suckers originally were found in major channels and tributaries throughout the Colorado River basin (Jordan, 1891; Jordan and Evermann, 1896). They were used as food by Indians and settlers, even sold commercially at one time. They are nearly extinct in the free-flowing sections of the lower Colorado basin (Minckley and Deacon, 1968). Fairly large numbers of adults are found in Lake Mohave "x"~~ "-1-/~~"!~ >%;-1"" ~. ~j,j~;:~:f~~~~~I~ ~~ '~~~:-S;t Figure 3. Humpback sucker adult. w' ~ -..".." . .. -' . <".