Drift of Larval Fishes in the Upper Colorado River

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Journal of Freshwater Ecology



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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. Five native fishes (flannelmouth sucker, bluehead sucker, roundtail chub, speckled dace, and mottled sculpin) dominated the drift with about 87 percent 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. Significantly greater numbers of drifting fishes were found along the shoreline than in the midchannel surface zone. Larval fish were first caught in the drift when the water temperature reached 16°C during a rapid warming trend in mid-to-late July. About 65 percent of the drift recorded in the two studies occurred during a two to three week period when water temperatures ranged from 18 to 21°C, during the descending limb of spring 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. This 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.