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372 The Southwestern N¢tur¢list Tnst.E 1-Numbers and total lengths (mm, Jf ± SD) of larval razorback sucker remaining at the end of two separate experiments totalling six experimental exposures to odonate predators and four control setups with no predators. Initial numbers of razorback sucker were 100 in each setup far Experiment 1 and 75 in each setup for Experiment 2. Survivors Experiment, Number predator (percent) Total length Experiment 1 Damselfly 23 12.8 ± 0.7 Damselfly 14 12.9 ± 0.5 Dragonfly 28 12.9 ± 0.8 Dragonfly 19 13.1 ± 0.5 None 79 13.2 ± 0.7 None 96 13.1 ± 0.8 Experiment 2 Damselfly 37 (49) 18.1 ± 2.0 Dragonfly 33 (44) 17.2 ± 2.2 None 64 (85) 17.1 ± 1.5 None 59 (79) 17.8 ± 2.4 Mq~ntnr c nND METHODS-Two experiments, the first testing predation by odonate nymphs on 12-day- old razorback sucker larvae and the second on 35-day- old fish, were performed in continuously aerated, 40-1 aquaria under a 12L:12D photoperiod. Odonate nymphs were collected at night by light traps 1 to 3 days prior to experiments. Larval suckers were from eggs fertilized at Lake Mohave, incubated and hatched at Arizona State University, and returned to lake back- waters for growout. Experiment 1 used six aquaria, each containing 35 1 of unfiltered Lake Mohave water. Two aquaria con- tained fish plus damselfly nymphs, two had fish plus odonate nymphs, and two served as controls (fish only). Water temperature was maintained at ca. 19°C. One-hundred, 12-day-old larval razorback suckers (mean total length [TL] = 11.8 ± 0.7 [SD] mm, n = 12) were placed in each aquarium on 10 March 1992 and acclimated for 24 h; fish that died during this period were replaced. Larval razorback sucker were fed newly hatched brine shrimp nauplii (Artemis) (AU- RUM Aquaculture) ad libitum before introduction into aquaria and again mid-way through the 7-day exper- iment. At 1000 h on 11 March 1992 (day 1), 20 dam- selfly or dragonfly nymphs were added to each aquar- ium. The experiment ran un-monitored until 18 March 1992 (day 8) when and all surviving odonates and fish larvae were preserved in 5% buffered formalin. The second experiment was conducted using 35-day- old razorback sucker larvae (initial length not mea- vol. 39, no. 4 sated; estimated mean 14 to 15 mm TL, range 11 to 17 mm). Experimental design (photoperiod, duration, water) was similar to experiment 1 with two excep- tions: (1) treatments were not replicated and (2) num- ber of fish per aquarium was reduced from 100 to 75. Water volume was reduced to 26 1 to keep fish density (2.9 fish/1) as in experiment 1. Razorback sucker larvae were placed in aquaria on 7 April 1992 and allowed 24 h to acclimate. At 0845 h on 11 April 1992 (day 1), 20 damselfly nymphs were added to one aquarium and 20 dragonfly nymphs to the other; two aquaria served as controls (fish only). Water temperature was 23°C. Fish were fed ad libitum twice, once at startup and again on day 6. The first feeding was brine shrimp nauplii, the second of mixed zooplankton filtered (80 micron) from Lake Mohave. The experiment was terminated on 18 April 1992 (day 8); all fish larvae and odonates were preserved in 5% buffered formalin. Fish larvae were counted and measured (nearest 0.1 mm TL). Tukey's multiple comparison test (Neter et al., 1985) was used to compare mean number of sur- viving razorback sucker larvae in each Experiment 1 treatment. Survival in Experiment 2 was evaluated using atwo-sample t-test (Snedecor and Cochran,1967). R~sU1.TS-Larval razorback survivorship dif- fered significantly (F = 39.4; df. = 2,3; P = 0.0075) among treatments in Experiment 1, with greater survivorship in controls than in either dragonfly or damselfly treatments (q = 10.44; df. = 3,3; P = 0.01; q = 11.24, df. = 3,3; P = 0.009, respectively). There was, however, no difference between dragonfly and damselfly treatments (q = 0.82; df. = 3,3; P > 0.5). Because razorback larvae survival did not dif- fer in dragonfly and damselfly treatments for Ex- periment 1, results of the two treatments in Ex- periment 2 were pooled to conduct a t-test. Mean survival was significantly greater (t = 5.43, df. = 2; P < 0.05) in controls (82%) than in pooled odonate treatments (47%). Average fish size among aquaria at the end of Experiment 1 ranged from 12.8 to 13.2 mm TL, and those in Experiment 2 from 17.1 to 18.1 mm TL (Table 1); there was no difference (F = 1.01; df. = 5,107; P > 0.5, Experiment 1; F = 0.84; df. = 3,76; P > 0.5, Experiment 2) in final mean TL of fish among aquaria within either experi- ment. Average damselfly head width varied from 3.5±0.2mmto3.6±0.3mmandTL14.2± 1.1 mm to 14.4 ± 1.1 mm across all aquaria, while average dragonfly head widths ranged from 5.8±0.8mm to 6.8±0.9mm and body TL from15.2±2.4mmto19.4±3.5mm. 1