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January 1985 LAMARRA ET AL.: SQUAW FISH 131 :- !- A comparison between Transect 2 (pool) and Transect 3a-3b (riffle) indicated marked differences in these two habitat types. The major abiotic differences between these two transects were the dominant and interstitial substrate sizes. Transect 2 had sand and silt as the dominant substrate, with interstitial sub- strates less than 0.5 mm in size. At Transect 3a-3b, the dominant substrate was cobble, with 88% of the interstitial substrates greater than 4 mm. The cobble and absence of silt on this transect provided substantial interstitial voids. CRFP (1982) observed Colorado squawfish moving from the area of Transect 2 into and out of the cobble area of Transect 3b in ap- parent spawning behavior (hashed area in Fig. 2). Dominant substrate type and inter- stitial characteristics at Transect 3a-3b may be critical in the apparent preference of this habitat by spawning Colorado squawfish. At river mile 18.0, a physically similar habitat was found (Transect 5 in Table i). However, this transect had much higher velocities (>4.5 m/sec) as compared to Transects 3a- 3b (2.1-2.4 m/sec), which may have pre- eluded this area from being used £or spawn- ing activities during 1981. Seining Sites Five locations at river mile 16.5 were sam- pled intensively for fish, larval fish, macroin- vertebrate drift, and benthic macroinverte- brates (Fig. 2). Each of the five seining locations were sampled five times at four- hour intervals. At river mile 18.0, seven loca- tions were sampled twice over an eight-hour period. A comparison of the physical and bio- logical data collected at the seining locations (Table 2) indicates that the ranges of these parameters fall within the ranges observed on the transects (Table 1). Nine species of fish were captured during this survey (Table 3). Three species are endemic, two are native, and the remaining five are nonnative. Among the endemics, the most abundant were round- tail chubs (39% of the total). One fish tenta- tively identified as a humpback chub was captured. Redside shiners were the most abundant nonnative (15°70), followed by red shiners (13%). The ratio of endemics and na- TASt,E 2. Physical and biological data collected at five seining sites from river mile 16.5 on the Yampa River 24-2G July 1981. Area Dominant Substrate Macroinvertebrates Locat ion Time sampled Depth Velocity substrate organics Density Biomass RM SH (hours) (m~) (meters) (m/sec) (ems) (gmshn~) (#'s/m~) (gms/m~) 16.5 SHi 1620 60 0.44 0.02 2.5 0.26 50 1252 2030 0000 0910 1435 16.5 SH, 1Fi40 36 0.88 0.23 7.5 1.05 312 18.43 2100 0020 0935 1510 16.5 SH,i 1700 30 0.38 1.Oi 0.75 O.OG 777 24.599 2100 0040 0955 1535 16,5 SHE 1715 45 0.78 0.14 0.75 1.55 91 5.952 2125 0100 1010 1605 16.5 SH; 1745 45 0.67 0.30 7.5 0.25 11 0.01 2140 0125 1030 1630