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604 SCOPPEITONE TABLE 2.-Means (ISD) and niche breadths (B) for microhabitat variables associated with upper Muddy River fishes. Maximum breadth is the number of resource classes. Mean velocity Focal velocity Total depth Relative depth Statistic or Mean :t SD Mean :t SD Mean :t SD Mean :t SD life stage (cm/s) B (cm/s) B (cmls) B (cm/s) B N Maximum breadth 8.0 6.0 10.0 10.0 Moapa dace Larvae 1.0 :t 1.2 1.0 1.6 :t 1.4 1.0 31.1 :t 19.2 4.3 0.43 :t 17.8 4.9 199 Juvenile 6.8 :t 8.0 2.0 5.9 :t 6.6 1.4 42.3 :t 24.7 6.6 0.49 :t 23.2 6.7 148 Adult 24.8 :t 16.6 4.9 14.4 :t 9.1 3.6 58.9 :t 17.9 5.3 0.82 :t 11.S 3.4 452 Moapa White River springfish Larvae 1.1 :t 1.4 1.0 1.2 :t 1.7 1.0 9.5 :t 7.0 2.0 0.61 :t 30.6 5.7 72 Juvenile 55 :t 7.0 1.9 4.2 :t 6.0 1.3 32.6 :t 21.8 6.1 0.68 :t 22.6 5.3 116 Adult 11.2 :t 10.2 2.6 6.8 :t 6.8 1.7 33.7 :t 21.9 6.5 0.75:t 16.9 4.6 432 Shortfin molly Juvenile 5.1 :t 7.3 1.S 3.5 :t 5.1 1.2 24.4 :t 25.7 3.0 0.40 :t 23.1 6.8 478 Adult 10.7 :t 9.0 2.6 8.8 :t 6.7 2.1 46.8 :t 22.8 6.6 0.36 :t 20.2 6.4 843 Western mosquitofish Juvenile 1.6 :t 3.6 1.0 1.4 :t 2.4 1.0 15.0:t 12.0 2.7 0.16:t 19.4 3.2 463 Adult 3.8 :t 3.8 1.1 4.4 :t 4.0 1.1 24.0 :t 18.0 4.4 0.14 :t 16.4 2.7 311 younger life stages of the two natives, but it was my subjective evaluation that these stages were not influenced spatially by the nonnatives. Dietary Overlap Moapa dace, Moapa White River springfish, and shortfin mollies were captured in each of the three habitats included in the diet study, but western mosquitofish were collected only at one site. Com- TABLE 3. - Relative spatial overlaps (100 x Schoener's S) of life stages of Moapa dace, Moapa White River springfish, shortfin molly, and western mosquitofish. Interacting species Larvae Juveniles Adults Moapa dace Moapa White River springfish Larvae 23 24 9 Juveniles 19 43 23 Adults 18 39 33 Shortfin molIy Juveniles 30 31 \3 Adults 44 45 2 Western mosquitofisb Juveniles 26 22 0 Adults 25 22 0 Moapa White River springfish Shortfin molly Juveniles 47 31 21 Adults 19 19 21 Western mosquitofish Juveniles 36 \3 5 Adults 19 7 7 pared with the other species, Moapa dace had few items in their alimentary canals (Table 4). Their overlap in items consumed was 24% with shortfin molly, 30% with western mOSQuito fish and 31 % with Moapa White River springfish. Moapa White River springfish overlapped 63% with shortfin molly and 66% with western mosquitofish. All four species exhibited some degree of om- nivory. Moapa dace were the most carnivorous, containing by volume 75% invertebrates and 25% plant material.and detritus. Shortfin mollies were the most herbivorous, containing 93% plant ma- terialand detritus. Moapa White River springfish, the most omnivorous, contained 61 % plant ma- terial and 39% invertebrates. Moapa dace taken from the spring pool habitat, the only site from which western mosquitofish were captured, con- tained fewer food items than those from other habitats; two had empty guts and the remaining eight were virtually empty. The gut contents were 73% invertebrates and 27% vegetative matter. Shortfin monies had consumed 100% vegetative matter, Moapa White River springfish 97%, and western mosquitofish 65%. Western mosquito fish was the only species to have preyed on another fish; one had consumed a young shortfin molly. Predation When relative predatory aggressiveness was tested among the three atheriniforms, Moapa White River springfish consumed the fewest Ca- tostomus larvae. In the four experiments, the