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398 BERRY AND PIMENTEL TABLE i.-Swimming performance of three Colorado River fishes. Swimming velocity at 2 and 120 min is expressed as body lengths (BL) per second at which 50% of the fish were fatigued (FV50) and its equivalent absolute swimming speed; the 95% confidence interval is in parentheses. Number 2min 120min Temper- Total length of . ature (mm) velocities Number FV50 Absolute speed FV50 Absolute speed ("C) (mean :!: SD) tested offish (BL/s) (cm/s) (BL/s) (cm/s) Bonytait chub 14 100 :!: 7 5 44 4.9 (4.7-5.3) 49 (47-53) 4.7 (4.3-5.0) 47 (43-50) 20 97 :!: 7 6 59 6.4 (6.0-6.9) 62 (58-67) 5.4 (4.8-6.1) 52 (47-59) 26 99:t 8 7 65 6.3 (6.0-6.7) 62 (58-65) 5.8 (5.3-6.4) 57 (52-63) Humpback chub 14 92:t 5 9 90 a 4.4 (4.1-4.7) 40 (38-43) 20 93 :!: 5 7 71 6.8 (6.2-7.5) 63 (58-70) 5.5 (5.0-6.1) 51 (47-57) 26 99:t 8 8 85 7.1 (6.1-8.4) 70 (60-83) 5.7 (5.1-6.3) 56 (50-62) Colorado squaw fish (large) 14 411 :!: 31 4 19 2.6 (2.1-3.2) 107 (86-132) 2.3 (2.2-2.5) 95 (90-103) 20 451 :!: 28 4 15 2.4 (1.7-3.4) 108 (76-153) 2.lb 95b 26 433 :!: 19 4 14 2.4 (1.9-2.9) 104 (82-126) 2.0 (1.5-2.7) 87 (65-117) Colorado squaw fish (small) 14 88:t 10 10 76 4.4 (4.2-4.7) 39 (37-41) 4.0 (3.6-4.4) 35 (32-39) 20 121 :!: 17 6 60 4.1 (3.9-4.3) 50 (47-52) 3.9 (3.7-4.0) 47 (45-48) 26 104:!: 13 7 67 5.0 (4.7-5.3) 52 (49-55) 4.5 (4.3-4.7) 47 (45-49) a Only 40% of the fish were fatigued in 2 min at the highest velocity tested (5.7 BL/s). b No fatigue data between 0 and 100% were obtained. Thomas et al. (1964). The apparatus consisted of two 500-L reservoirs connected by two pipes, a return-flow pipe, and the swimming chamber, a plexiglas tube 2 m long and 20 cm in diameter. Blocking forces (Webb 1975) were probably in- significant because the largest Colorado squaw- fish's cross-sectional diameter was 9.9% of the diameter of the tunnel. An electromagnetic cur- rent meter recorded flow velocities, and a down- stream barrier was electrified with 5-1O-V alter- nating current to encourage tiring individuals to swim. The tunnel was wrapped with black plastic at the upstream end to isolate the fish but to allow the entrance of light from above. A standard testing protocol was used. The ap- paratus was filled with water at the acclimation temperature of the fish. One large or two small fish were acclimated to the tunnel for 5 min at a water velocity of 15 cm/s. Preliminary tests indicated no difference in prolonged swimming performance between fish acclimated for 5 and 20 min. Fish that did not swim in the tunnel during the acclimation period were excluded from the tests. After the acclimation period, fish were rapidly subjected to a selected test velocity when a stop-door was removed at the rear of the tun- nel. An observer shocked tiring fish as needed and recorded the time when fish became im- pinged on the electrified screen as the fatigue time. Tests continued for a chosen period of 120 min or until fish fatigued. Tests with two to four fish were first conducted at selected velocities to find a range over which the percent of fish fatigued at 2 and 120 min varied from 100% to 0%, respectively. Then, at selected fixed velocities within the range, sample size was increased to 9-14 small fish or 4-6 large fish, depending on the number available. Tested fish were measured, returned to holding tanks, and monitored for mortality. Tested fish were not reused. The method used for estimating swimming ability was basically that used by Brett (1967): the fatigue times of individuals in a group were determined at a constant water velocity. Analysis of covariance (covariate equals fish length) was used to evaluate the effects of water temperature and velocity on fatigue times. We used a "dose- response" curve to characterize the relation be- tween velocity and percent fatigued. The curve was plotted as straight-line connections of three- point moving averages. The FV50 (velocity at which half the fish fatigued) at 2 and 120 min was determined algebraically by linear interpo- l'