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<br />Pool depth al so can provide cover. Dwarf suckers frequently congregate .. <br />in deeper pools when not spawning (Oence 1948). Suckers are generally easily ,.,J <br />disturbed and quickly retreat to. pools (Stewart 1926). Propst (1982b) <br />co 11 ected whi te suckers at water depths of 21 to 110 cm, but most common <br />depths were 61 to 90 cm. Thompson and Hunt (1930) observed white suckers in <br />slow water habitat at depths of 15 to 240 cm. The value of pools as whi te <br />sucker habitat is greatly improved when logs, brush, or other types of cover <br />are present (Oence 1948). <br /> <br />Symons (1976) demonstrated that cover-seeking behavior increased signif- <br />icantly as stream velocity increased. Minckley (1963) reported that most <br />white suckers were in deeper pools, with fewer suckers in swifter, shallower <br />water. When white suckers were present in shallow water with an appreciable <br />current, they were usually in the shelter of vegetation. When vegetation was <br />lacking in the stream current, the suckers were in the deepest pools. When <br />white suckers were found in smaller, shallow pools there was an accumulation <br />of debris and overhanging riparian vegetation for cover. <br /> <br />Pool s provide habitat with a slower current in which fi sh can rest. <br />Suckers recuperate in pools after negotiating a difficult stream obstruction <br />during migration or after breeding (Stewart 1926; Dence 1948). Optimum white <br />sucker habitat is assumed to have a pool to riffle/run ratio of 1:1. <br /> <br />White suckers avoided areas in reservoirs where the dissolved oxygen (DO) <br />was ~ 2.4 mg/l (Oence 1948), but specific information on adult and juvenile <br />dissolved oxygen (DO). requirements generally are lacking. Siefert and Spoor <br />(1974) reported that embryos could not survive DO levels ~ 1.2 mg/l and that f\\ <br />the growth of fry was reduced at DO levels < 2.5 mg/l. Minckley (1963) <br />described an abundant white sucker population in a portion of a stream that <br />had yearly DO values rangi ng from 4.3 mg/l to an occas i ona 1 average super- <br />saturation level of 14.79 mg/l. <br /> <br />White suckers have been collected from areas with a pH as low as 4.3 <br />(Dunson and Martin 1973), but Beamish (1974) reported sharp declines in white <br />sucker populations in Canadian lakes when the pH was lowered to 4.5 to 5.0 as <br />a result of acid precipitation. Laboratory studies on the effects of pH on <br />white sucker growth and survival indicated that feeding stops at a pH of 4.5 <br />and death occurs at a pH of 3.0 to 3.8 (Beamish 1972). Maximum successful <br />reproduction occurs at a pH above 5.8 (Trojnar 1977). The pH range which is <br />generally considered not harmful to fish is 5.0 to 9.0; the further the pH <br />varies from this range, the lower the water quality. Laboratory data indicate <br />that a pH between 9 and 10 may be harmful to some fish species, and that a pH <br />above 10 usually is lethal to all species (EIFAC 1969). <br /> <br />White suckers can survive in turbid waters, but they are more common in <br />clearer streams (< 50 JTU) and lakes (Pflieger 1975) and prefer relatively <br />clear spawning streams (Raney and Webster 1942). Young-of-the-year, juveniles, <br />and adults have been reported in the Missouri and James Rivers (North Dakota) <br />at relatively consistent turbidities of 50 to 135 JTU's. Smaller numbers of <br />white suckers occur in the Colorado and Yampa Rivers (Colorado) at more <br />variable turbidities of 85 to 100 and up to 350 JTU's (R. Muth, pers. comm.). <br />Pflieger (1971) stated that white suckers are uncommon in large turbid rivers. <br /> <br />4 <br /> <br />e <br />