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t Adult. White suckers have broad temperature tolerances, and optimum temperatures vary geographi ca lly. Whi te suckers occur in III i noi s headwater streams with summer temperatures up to 320 C (Thompson and Hunt 1930). A preferred temperature range of 19 to 210 C was reported for a Colorado reservoir (Horak and Tanner 1964). Experimental evidence suggests an optimum summer water temperature of 240 C (Reynolds and Casterlin 1978). Reutter and Herdendorf (1976) reported a critical thermal maximum for white suckers of 31.60 C. Brett (1944) reported an upper lethal temperature of 31.20 C for suckers acclimated at 260 C. Specific minimum temperatures have not been reported, but the wide distribution of white suckers indicates that they can survive temperatures as low as 1 to 20 C. For example, Minckley (1963) reported average January temperatures between 1.1 and 2.20 C in Doe Run, Kentucky, where white suckers occurred. Embryo. Embryo deve 1 opment is temperature dependent (Raney and Webster 1942; Geen et al. 1966). Fuiman (1978) collected eggs in streams with water temperatures ranging from 11 to 160 C. McCormick et al. (1977) reported maximum hatching success at 150 C. Hatching success diminished significantly at temperatures < 90 C or > 170 C, and upper and lower 1 etha 1 1 imits were 240 C and 60 C, respectively. t Larval. White sucker larvae apparently prefer water temperatures of 23 to 250 C, but occur in water temperatures of 13 to 250 C (Marcy 1976). The greatest growth was obtained experimentally in water that was 270 C, and the upper lethal limit was 30 to 320 C (McCormick et al. 1977). Krieger (1980) reported that the highest larval densities in lacustrine habitats were in :shoreline areas with sand and sand/gravel substrate combinations. Few larval fish occurred in areas with a rock substrate, and no larval fish occurred in areas with silty sand or boulders. Thompson and Hunt (1930) usually found young suckers in streams where the substrate was a mixture of sand and gravel. White sucker fry prefer moderate currents and do not occur in rapids or still pools, although they may be present in intermediate situations where the stream enters deep, quiet stretches (Stewart 1926). Young suckers in the surface-feeding stage appear to congregate in eddies and backwaters in response to gentle currents. Juvenile. Upper lethal temperature 1 imits for juvenile white suckers were 26 to 310 C at acclimation temperatures of 5 to 250 C (Brett 1944; Carlander 1969). At acclimation temperatures of 20 to 250 C, the reported lower lethal temperatures were 2 to 60 C (McCormick et al. 1977). Small white suckers (< 150 mm TL) have been collected from shallow backwaters, riffl es with moderate water velocity (approximately 50 cm/sec), and sand-rubble bottom runs (Propst 1982b). t 5