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BIOLOGP OF HUMPBACK CHUBS IN "rHE GRAND CANYON 589 TABLE 4.-Mean catch rates for age-0 humfiback chubs collected by~ seine duri~ag daylight from. the Little Colorado River and its confluence with the Colorado River. River and reach Little Colorado Confluence Date and measure LC 2 LC 3 LC 4 C 4 July 1980 Mean fish/100 m$ (SD) 13.3 (2.3) 4,7 (1,6) 1.9 (0.4) 4.1 (0.6) Total effort (m4) 574 1,324 1,344 266 August ]987 Mean fish/100 my (SD) 0.0 (0.0) 0.9 (0.1) 0.5 (0.1) 0.7 (0.1) Total effort (ms) 1,fi78 2,644 2,30G .2,538 the highest mean values of G/f were from LC 2 and 3; however, there was no consistent re- lationship between catch rate and river reach within the Little Colorado. Mean G/f from reaches LC 3 and 4 increased in all diet periods between February and April-May 1981 (Fig. 9), perhaps because vulnerability to capture was greater during the spawning period or because mature fish were moving from the Colorado into the Little Colorado to spawn. The distribution of mean G/f from the reach- es in the Colorado followed abell-shaped curve about the confluence (Fig. 10). Adult humpback chubs from the Colorado are probably associ- ated with the confluence because of the impor- tance of the Little Colorado for reproduction. Interspec fie Associations Fourteen fish species and one hybrid were found in the study area. Seines and minnow traps collected all ages of fathead minnows, speckled dace Rhinichthy~s osculus, and plains kil- lifish Fundulus zebrintics, and age-0 and juveniles of other species that attain relatively large size at maturity. Adults of the species of large fish were collected most often by trammel net (Fig. 11). No Colorado squawfish, razorback suckers, or bonytails were encountered. Nearly all fat- head minnows, plains killifish, and juvenile bluehead suckers Catostomus discobolus in the Colorado were collected below the confluence, suggesting that. they reproduce in the Little Col- orado. Distributions for speckled dace and $an- nelmouth sucker Gatostomus latipinnis suggested some reproduction in the Colorado or its trib- utaries upstream. Of the salmonids collected, 86% were rainbow trout Salrno gairdneri. Cut- throat trout Salmo clarki, hybrid rainbow trout X cutthroat trout, brown trout Salmo trutta, and brook trout Salvelinus fontinalis also were found. Only a few redside shiners Richardsonius bal- teatus,4black bullheads Ictalurus melas, channel catfishlrtalurus~uncta.tus, and common carp C~~- ~rinus carpio were collected, although angling and electrofishing suggested that channel cat- fish and common carp are abundant in some areas of the Colorado. The small catch of these species prevented their inclusion in the graphic presentation (Fig. 11). Although each of the species collected from the study area is a potential colonizer, only humpback chubs, speckled dace, bluehead suck- ers, and fathead minnows were found in the Little Colorado in large numbers. The large travertine dams separating LC 1 and 2 may have prevented upstream fish movement. Only speckled dace and fathead minnows lived in LC 1, and these species plus humpback chubs and bluehead suckers were found immediately downstream in LG 2. Species diversity in the Little Colorado increased with proximity to the confluence. No significant linear relationships were found between mean C/f for humpback chubs and those for individual sympatric species, which suggests that the occurrence of another species does not significantly affect the relative abun- dance ofhumpback chubs within the study area. However, adult humpback chubs from the Lit- tle Colorado sometimes exhibited apparent channel catfish bite marks-similar crescent- a This represents a range extension for redside shiner. Voucher specimens have been deposited in [he museum at the USFWS National Fish and Wildlife Laboratory, 1300 Blue Spruce Drive, Fort Collins, Colorado, 80524.