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<br />stomachs (n =4) in.a bay below Headgate. Rock Dam, Colorado River to be filled with silt "rich <br />in microscopic organisms" and filamentous algae. <br /> <br />By examining otoliths from Lake Mohave specimens, ages of razorback suckers were found to <br />range from 24 to 44 years (McCarthy and Minckley 1987), longevity typical of big river <br />endangered fish. <br /> <br />Spawning <br /> <br />Unlike the humpback chub that successfully recruits young, but actual spawning has not been <br />observed, the recruitment of razorback sucker in historic habitats (rivers) is unsuccessful, but <br />considerable information exists on spawning activities. Reviews of the ample data from Lake <br />Mohave found spawning usually occurs from January to April or May with water temperatures <br />varying from 10.5 to 21 .C, but usually above 15 .C over wave-washed, gravel and cobble <br />shorelines (Minckley et al. 1991; Minckley 1991). Larvae were produced but did not recruit <br />into juvenile stage until recent experimental actions of placing breeding adults into a predator- <br />free backwater was begun (Paul Marsh, Arizona State University, personal communication). <br /> <br />Sigler and Miller (1963) reported spawning to occur in water about 12 to 18 OC iri tributary <br />streams and reservoir shorelines over silt, sand, gravel, or rocks. Tyus and Karp (1990) <br />captured ripe razorback suckers in riffles on substrates of cobble, gravel, and sand during the <br />ascending and high limbs of the hydrograph of the Green River and Yarnpa River from mid- <br />April to early June. Spawning was associated with temperatures of about 14 .C, and migrations <br />of 30 to 106 !an were recorded for some individuals but not all. Some ripe female razorback <br />suckers were captured in flooded bottomlands with temperatures ranging from 17 to 19 .C, and <br />habitats and temperatures were found to vary with various water years experienced (Tyus 1987). <br />Marsh (1985) found greater success with experimental hatching of razorback sucker eggs at 20 <br />and 25 .C then at 15 .C and complete loss at 5, 10, and 30 .C, <br /> <br />Larvae and Juveniles <br /> <br />After the adhesive eggs hatch, the larvae settle in spaces between substrate material until swim- <br />up stage; accordingly, spaces free of silt or other sediment were deemed important to their <br />survival (Inslee 1981, Bozek et al. 1984). <br /> <br />Sigler and Miller (1963) reported the behavior of larval razorback suckers, approximately 25 <br />mm total length, as traveling in large schools along river and reservoir margins. They note that <br />two seine hauls on June 15, 1950, collected over 6,000 larval razorback suckers (10 to 35 mm <br />standard length) from the margin of the Colorado River in Nevada in temperatures ranging from <br />21. 7 to 24.4 .C while the mainstem river temperature was 14.4 .C. Tyus (1987) collected larvae <br />(10.6 to 13.6 mm long) (tentatively identified as razorback sucker) in quiet river margins of the <br />Green River downstream of areas where razorback suckers in breeding condition were previously <br />sampled. Extensive. collecting in LakeMohave has not found larvae greater than about 12 mm <br />total length (Minckley et al. 1991). <br /> <br />15 December 1994 final biological opinion 1-21-93-F-167 <br /> <br />14 <br />