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Growth rates of larval humpback at Willow Beach NFH differed. At a mean temperature of 20.1°C for 56 days Little Colorado humpback larvae grew from a mean total length of 7.1 to 3b.9 mm at a rate of 0.536 mm/day. At a higher mean temperature of 22.0°C, also for 56 days, Black Rocks humpback larvae grew from 6.9 to 47.5 mm at a rate of 0.855 mm/day. However, the Black Rocks fish not only had the advantage of warmer water but were less crowded having about 10,000 fish per unit compared to 20,000 fish per unit for the Little Colorado River fish. ~~~" G Bulkley et a1,Ri-~tti~~~ using juvenile humpback chub determined they ~` . preferred a temperature of 24°C with a range from 21 to 24.4°C. Schooling behavior and favoring fixed postions in the test chamber made the data variable and skewed. However, the preferred temperature calculated for the humpback chub (24°C) was close to that of the juvenile squawfish (25.5°C). The swimming ability of the humpback juveniles was greater than that of the Colorado squawfish juveniles, particularly at 26°C. Figure illustrates that while the squawfish's ability to swim is lessened by a 6°C increase in temperature from 20 to 26°C, the humpback's ability increases. This may indicate that the humpback tolerates higher temp- eratures than does the squawfish. In comparison with bonytail chub juveniles at 26°C the two fish are nearly equal in their ability to swim, but at 20 and 14°C the bonytail chub seems to a slightly better swimmer, particularly at 14°C. The razorback, on the onther hand, is by . far the worst swimmer among the target species. Temperatures between 16 and 26°C are critical to the spawning, egg and larval development and growth of the humpback chub. The best temp- eratures appear to be 1b-18°C for spawning, 20-26°C for egg and larval development and 24-26°C for growth. Temperatures to be avoided are those less than 18°C for eggs and larvae and temperatures during the growing season (June-October) below 20°C and above 28°C. Total Dissolved Solids - A strong schooling behavior interfered with tests to determine TDS preference in YOY and juvenile humpback chub (Bulkley et al 1981). Tests showed their preferred TDS concentration ranged from 1,000 to 3,500 mg/1 (specific conductance = 1,300 to 3,000 umhos). Bulkley observed their TDS avoidance level was higher than 11,600 mg/1 (specific conductance 8,500 umhos). Specific conductance levels in the Colorado River ranged from 280 to 1,720 umhos (TDS 380 to 1,220 mg/I) during recent studies (Valdez 1981), while TDS concentrations in the Little Colorado River ranged from 2,500 to 3,570 mg/1 (University of Arizona). The humpback appears to choose high TDS levels compared to those found in the Upper Colorado River system. However, their preference level coincides with TDS concentrations found in the Little Colorado River. An avoidance TDS level has not been determined for the humpback but is apparently is very high ( 11,600 mg/I), at least in the short term (24 hr) tests run in the laboratory (Bulkley et a1 1981). The avoidance levels for the bonytail and squawfish ( 5,100 umhos) were determined and were much lower, which may be one reason why the Little Colorado River contains no bonytail chub, roundtail chub or Colorado squawfish while humpback chub are abundant. Still based upon field ob- servation on humpback chub in the Little Colorado River and controlled test results mentioned above, we believe that TDS is not a limiting factor in the distribution of humpback in the Upper Colorado River.