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rr \ I I Ya.lda .c:+ Q I. Iq~5 fIb f,<,,/ j' ;!~ "1334 133:~ tJ J "-. c+ q I. .-- -" DRIFT OF LARVAL FISHES IN THE UPPER COLORADO RIVER Richard A. Valdez Jo hn G. Ca rter Ronald J. Ryel Ecosystem Research Institute Logan, Utah 84321 ABSTRACT Native as well as some non-native fishes in the Upper Colorado River were found drifting during their larval and early juvenile stages of development. Of the 13 and 14 species captured in each of two independent studies, about 35 river miles apart. the same 8 species were encountered in the drift. The five native fishes (flannelmouth sucker. bluehead sucker. roundtail chub. speckled dace, and mottled sculpin) dominated the drift with 84% and 90% of the catch. The only 3 non-native species found in the drift (white sucker, fathead minnow, and green sunfish) occurred in very low numbers. Si gnificant 1 y greater numbers of drift i ng fi shes were found along the sho-rel ine than in the midchannel surface zone. The midchannel benthic zone was not sampled because of high water velocities and excessively deep sample conditions. Larval fish were first caught in the drift, at both study sites, when the water temperature first reached 16 C during a rapid warming trend in mid to late July. About 71% and 61% of the drift recorded in the two studies occurred during a 2 to 3-week period when water temperatures ranged from 18 to 21 C. duri ng the descend i ng 1 imb of spri ng runoff. A diel pattern in drift densities was seen at both sites, with highest numbers of larvae caught between 2000 and 0400 hours, and lowest numbers between 1600 and 2000 hours. Thi s diel pattern suggests that many of the larval fishes of the Colorado River are photosensitive, and actively capable of entering and escaping the surface drift currents. INTRODUCTION The phenomenon of drift among fishes of the Upper Colorado River was onl.y recently described in studies of the federally-endangered Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila ~), and bony tail chub (Gila eleqans), as well as the candidate razorback sucker (Xyrauchen texanus). Intensive studies of fishes in this river system began in the early 1960.s (Banks 1964; Vanicek 1967), and it was believed that young Colorado squawfish moved little. from their spawning site (Holden 1977). A drift 'phenomenon for these fishes was not hypothesized in literature until the cOlI)pletion of the Colorado River Fisheries Project (CRFP) in 1981. In that report, Tyus et ale (1982) suggested the possibility of drift by larval Colorado squawfish from suspected spa~ning areas to confirmed nursery areas, located a considerable distance downstream. 171