Laserfiche WebLink
<br />,. <br /> <br />. <br /> <br />. <br /> <br />Area Manager <br />DRAFT <br /> <br />20 <br /> <br />Juvenile razorback sucker (average total length 171 mm [6.7 in]) stocked in isolated coves along <br />the Colorado River in California, suffered extensive predation by channel catfish and largemouth <br />bass (Langhorst i 989), <br /> <br />Lentsch et al. (1996) identified six species of non-native fishes in the upper Colorado River basin <br />as threats to razorback sucker: red shiner, common carp, sand shiner, fathead minnow, channel <br />catfish, and green sunfish. Smaller fish, such as adult red shiner, are known predators of larval <br />native fish (Ruppert et al. 1993), Large predators, such as walleye (Stizostedion vitreum) and <br />northern pike also pose a threat to sub adult and adult razorback sucker (Tyus and Beard 1990). <br /> <br />Status and Distribution <br /> <br />Currently, the largest concentration of razorback sucker remaining in the Colorado River Basin is <br />in Lake Mohave. Estimates of the wild stock in Lake Mohave have falJen precipitously in recent <br />years from 60,000 as late as 1991, to 25,000 in 1993 (Marsh 1993, Holden 1994), to about 9,000 <br />in 2000 (Service 2002b). Until recently, efforts to introduce young razorback sucker into Lake <br />Mohave have failed because of predation by non-native species (Minckley et al. 1991, Clarkson <br />et al. 1993, Burke] 994). While limited numbers of razorback suckers persist in other locations <br />in the Lower Colorado River, they are considered rare or incidental and may be continuing to <br />decline. <br /> <br />In the upper Colorado Basin, above Glen Canyon Dam, razorback suckers are found in limited <br />numbers in both lentic (Jake-like) and riverine environments. The largest populations of <br />razorback suckers in the upper Basin are found in the upper Green and lower Yampa Rivers <br />(Tyus 1987). Lanigan and Tyus (1989) estimated a population of 948 adults (95 percent <br />confidence interval: 758 to 1,138) in the upper Green River. Eight years later, the population <br />was estimated at 524 adults (95 percent confidence interval: 351-696) and the population was <br />characterized as stable or declining slowly with some evidence of recruitment (Modde et al. <br />1996). In the Colorado River, most razorback suckers occur in the Grand Valley area near Grand <br />Junction, Colorado; however, they are increasingly rare. Osmundson and Kaeding (1991) report <br />that the number of razorback sucker captures in the Grand Junction area has declined <br />dramatically since 1974. Between 1984 and 1990, intensive colJecting effort captured only 12 <br />individuals in the Grand Valley (Osmundson and Kaeding] 991), The wild population of <br />razorback sucker is considered extirpated from the Gunnison River (Burdick and Bonar 1997). <br /> <br />ScientificalJy documented records of wild razorback sucker adults in the San Juan River are <br />limited to two fish captured in a riverside pond near Bluff, Utah in 1988, and one fish captured in <br />the river in 1976, also near Bluff(Ryden 2000b). Large numbers were anecdotally reported from <br />a drained pond near Bluff in 1976, but no specimens were preserved to verify the species. No <br />wild razorback sucker were found during the 7-year research period (1991-1997) of the SJRIP <br />(Holden 1999). Hatchery-reared razorback sucker, especialJy fish greater than 350 mm (13.8 in), <br />introduced into the San Juan River in the 1990s have survived and reproduced, as evidenced by <br />recapture data and collection oflarval fish (Ryden 2000b). <br /> <br />Razorback suckers are in imminent danger of extirpation in the wild. The razorback sucker was <br /> <br />001412 <br />