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EXECUTIVE SUMMARY The San Juan River is historic habitat for seven native fish species including three rare fish species: the Colorado squawfish (Ptvchocheilus lucius), razorback sucker (Xvrauchen texanus), and roundtail chub (Gila robusta). In 1991 and 1992, the U.S. Fish and Wildlife Service's Colorado River Fisheries Project (CRFP) office in Grand Junction, Colorado, in cooperation with other state and federal agencies conducted electrofishing surveys on the lower mainstem San Juan River. The primary responsibility of the CRFP office is to monitor the adult fish community in the river's main channel habitat. The river was electrofished from river mile (RM) 158.6 (Hogback Diversion, New Mexico) to Mexican Hat, Utah (RM 53.0) in 1991, during three sampling trips. Three trips in 1992 sampled from Farmington, New Mexico (RM 179.0) to Mexican Hat, Utah (RM 53.0). The most abundant species collected in 1991 was flannelmouth sucker (Catostomus latipinnis), followed, in descending order, by common carp (Cvorinus caroio), channel catfish (Ictalurus Dunctatus), and bluehead sucker (~discobolus). In 1992 the most abundant species was flannelmouth sucker, followed by bluehead sucker, channel catfish, and common carp. Fish community composition did not change markedly between the two years. Common carp and channel catfish accounted for 20% of the total catch in the first two years, while all other introduced species composed only 3% of the total catch. Native species, mostly flannelmouth sucker and bluehead sucker, composed 77% of all fish collected in 1991 and 1992. Colorado squawfish and roundtail chub combined composed less than 1% of the total catch in the first two years. Electrofishing capture rates were highest for flannelmouth sucker, bluehead sucker and speckled dace (Rhinichthvs osculus) in the reach of river from Hogback Diversion, New Mexico (RM 158.6) to Four Corners Bridge (RM 119.2). Catch rates for most species showed a downward trend as sampling proceeded downriver from Hogback Diversion. Twelve Colorado squawfish and five roundtail chub were collected in two years. No razorback sucker were collected. All Colorado squawfish were captured between RM 142.0 and 117.0. Four of five roundtail chub captures occurred between RM 147.9 and 135.3. Nine Colorado squawfish were surgically implanted with radio tags and tracked year-round from the air and ground. One hundred and thirteen radio contacts were made with these fish between June 1991 and January 1993. All radio contacts occurred between RM 142.0 and 117.0. During pre-spawn periods, May and early June, Colorado squawfish demonstrate an affinity for the section of river in and around the Mancos River confluence (RM 122.6). During estimated spawning periods, 79 percent of radio contacts with Colorado squawfish were made within the "Mixer" area (RM 133.4 to 129.8). The highest percentage of ground contacts (34.3%) were made with Colorado squawfish using run habitat. Mean maximum displacement (7.4 miles) and mean final displacement (3.99 miles) were very close to that of Colorado squawfish in the illS-mile reach" of the Colorado River near Grand Junction, Colorado. Total longitudinal movement averaged 8.94 miles. All of the above information suggests that the river reach between Hogback Diversion and Four Corners Bridge (RM 158.6-119.2) is important to the native fish of the San Juan River. Information to date is insufficient to assess the impacts of existing instream water diversion structures on rare fish movement. I I I I I I I I I I I I I I I I I I I