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~ ; ; ~ ;. ~ , , ~ ~ , , ~ , I:> , :) :) j j ~. :> j ~ ~ ~ :> :> :; :J :J , , ) .. ./ ~ ~. ..- '" -' .. -' . C~) c-) /-. CD 00 1-6. Platania, Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River 1991-1997. FINAL EXECUTIVE SUMMARY This study of the drifting fishes in the San Juan River from 1991-1997 was one component of the extensive biological research activities conducted under the San Juan River Recovery Implementation Program. The primary objectives of the passive drift-netting study were to 1) determine the temporal distribution of San Juan River ichthyoplankton in relation to the hydro graph, 2) provide comparative analysis of the reproductive success of San Juan River fishes, 3) attempt to characterize downstream movement of ichthyoplankton, and 4) attempt to validate the presumed spawning period of Colorado pikeminnow (Ptychocheilus lucius). A total of 13,683 drifting larval fish, representing eight taxa, were collected over the tenure of the seven year study. Overall larval fish catch rates were about equal at the upper (Four Comers, New Mexico) and lower (Mexican Bat, Utah) collecting localities. However, nonnative channel catfish (Iclalurus punclatus) numerically dominated the catch at the lower site while native speckled dace (Rhinichlhys osculus), flannelmouth sucker (Caloslomus lalipinnis), and bluehead sucker (Caloslomus discobolus) were most abundant at the upper site. The highest number ofJarval fish, at both sites, was collected in 1995. The most abundant fishes at the lower site during 1995 were nonnative red shiner (Cyprinella lUlrensis) and channel catfish while speckled dace, flannelmouth sucker, and bluehead sucker were the most abundant taxa at the upper site. Red shiner mean annual catch rates were very high at the upper site in 1991 (low flow year), moderate at both sites in 1995 and 1996, and low during other years sampled. Fathead minnow (Pimephales promelas) followed nearly the same annual abundance pattern as red shiner but was far less prevalent than the latter taxon. The highest mean annual catch rates of speckled dace were in 1993, 1994, 1995, and 1997 (high flow years) and lowest speckled dace mean annual catch rate occurred in 1996 (lowest flow year). Native suckers were also nearly absent during 1996 and generally paralleled abundance trends observed for speckled dace. Channel catfish abundance peaked at Four Comers in 1996 and was moderate to high at Mexican Hat for all years of the study. The correlation between the number of drifting larvae and occurrence of summer rainstorms was relatively consistent throughout the study period for all species analyzed. Rainstorms resulted in low water visibility and high levels of instream debris, conditions during which the majority of drifting larval fish (of all species) were collected. While temporal drift patterns were highly variable, most of the largest collections occurred in late July and early August when both water temperature and the incidence of rain events generally peaked. Increased water velocities, due to rainstorms, may have displaced larval fish from preferred habitats resulting in their passive downstream drift. Alternatively, drifting fish may have actively dispersed under conditions of high turbidity which favored reduced predation. Drifting Colorado pikeminnow were rare throughout the study period at both sites. Back- calculations, based on larval Colorado pikeminnow capture dates, indicated that the majority of spawning occurred as flow decreased in mid to late July following spring runoff. Mean water temperature during the back-calculated spawning dates of drifting larval pikeminnow (8 July to 18 July) ranged between 18.0'C and 18.5'C and had risen about 5'C several weeks before spawning. Collection of Colorado pikeminnow from the upper site in 1996 provided additional support for the hypothesized Mixer (RM 131.0-132.0) spawning area. Conversely, two Mexican Hat larval pikeminnow collected in 1995 appear to have originated from the same spawning bar and provide circumstantial evidence of a spawning location markedly downstream of the Mixer Reach. Downstream displacement of Colorado pikeminnow both below instream barriers and into unsuitable habitats and reaches, such as Lake Powell, appear to be a major factor for this species rarity. The hydrologic and behavioral components that contribute to the dispersal of Colorado pikeminnow need to be better understood and addressed before recovery efforts can be expected to achieve sustainable success. "~i;-: