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i .• z~ January 1985 LAMARRA ET AL.: SQUAW FISH 139 isms: avoidance of sand (0.25 cm) and selec- tion for either small substrate (<0 .075 cm) or larger substrate (2.5 to 10 cm). Exam- ination of the interactions between these biotic components and substrate size appears to be consistent with the findings of Cum- mins and Lauff (1969). The distribution of fish across the various substrate sizes differed by species. The en- demic fish appeared to avoid sand and gravel and select from intermediate to large sub- strates. Within the nonnative species, sand shiners appeared to prefer environmental conditions associated with the smallest sub- strate, whereas red and redside shiners fa- vored intermediate (2.5 to 7.5 cm) substrates. All species avoided sand. The mechanisms causing the above observed distributions of endemic and nonnative fishes are unknown; however, the similar distributions o€ the fish, periphyton, and macroinvertebrates with substrate size indicate important interactions. As noted previously, the density of the riv- er biocenosis changed markedly over differ- ent substrate types; however, as Carlson et al. (1979) have stated, the distribution of organ- isms may be more influenced by com- binations of the effect of physical and biotic characteristics than by physical factors alone. Our data indicate these types of inter- relationships are important in the Yampa River. For example, no statistical relationship was found between the biomass of macroin- vertebrates and the biomass of periphyton or detritus; however, a significant relationship was found between water velocity and mac- roinvertebrate biomass (rz = + .45; p = .Ol), This relationship was believed to be indirect because the dominant benthic macroinverte- brate functional group was found to be fine particulate collectors or filter feeders (989b of the total biomass) that may be responding to areas of high food availability (high flow). An attempt has been made in this report to quantitatively describe the physical and bio- logical environment at a suspected spawning site for Colorado squawfish. The Yampa Riv- er at mile 16.5_-was dominated by loose cobble substrate in association with large sandy pools. This site was also found to be free of small interstitial particles and organic material. This area appeared to be heavily used by several species of larval fishes and actively feeding adults. The combination of well-washed coarse substrate (cobble), abun- dant food (both drift and benthos), and adja- cent areas with slow, uniform laminar flow may be critical factors in determining pre- ferred reproductive sites of Colorado squawfish. CONCLUSIONS L. A dominant abiotic factor appeared to be substrate type and associated inter- stitial sediment size fractions. a. Yampa River mile 16.5 was domi- nated by cobble substrate with in- terstitial voids containing little or no organics. b. Associated with these cobble areas were sand-substrate pools. c. Periphyton, macroinvertebrates, and fishes were found in lowest densities in or on sandy substrates. d. Highest densities of periphyton, macroinvertebrates, and fishes were found over substrates smaller than boulders (except sand). e. Roundtail chubs were dominant among the endemic fishes and pre- ferred cobble substrate. f. Nonnatives were dominated by redside shiners and red shiners, and both species were most abun- dant over cobble substrates. g. Sand shiners were the only species to demonstrate a preference for silt substrates. 2. Samples showed nocturnal hours to be important periods of biological activity. a. Periods of highest macroinverte- brate drift occurred between 1500 and 0200 hours. b. Drifting larval fish were collected only after 2000 hours, with the peak occurring at 0100-0125 hours. c. Drifting larval fish appear to have originated in a cobble substrate .with substantial interstitial voids. d. Maximum feeding intensity of fish captured occurred between 1950 and 2140 hours.