Factors Affecting Condition of Flannelmouth Suckers in the Colorado River, Grand Canyon, Arizona

—The impoundment of the Colorado River by Glen Canyon Dam, Arizona, in 1963 created a highly regulated environment in the Grand Canyon that altered the native fish populations, including the flannelmouth sucker Catostomus latipinnis. Flannelmouth suckers were sampled from 1991 to 2001 to determine seasonal, annual, and spatial trends in fish condition (i.e., relative weight [Wr]). Mean Wr peaked during the prespawn and spawning periods and was lowest in summer and fall, but it was never lower than 93. Condition was variable throughout the Grand Canyon but was typically greatest at intermediate distances from Glen Canyon Dam, possibly because of the increased number of warmwater tributaries in this reach. Flannelmouth sucker condition in September was positively correlated with Glen Canyon Dam discharge during summer (June–August); this result may be due to the larger euphotic zone and greater macroinvertebrate abundance observed during higher water flows. Increased dam discharge that stimulates river productivity may provide benefits for this native fish. The Colorado River in the Grand Canyon historically had eight endemic fishes (Minckley 1991), of which three are now extirpated (Colorado pikeminnow Ptychocheilus lucius, bonytail Gila elegans, and roundtail chub Gila robusta) and two are federally endangered (humpback chub Gila cypha and razorback sucker Xyrauchen texanus). The flannelmouth sucker Catostomus latipinnis, also endemic to the Colorado River, is relatively common in the Grand Canyon compared to other native fishes (Minckley 1991). Although previous research has suggested that the Grand Canyon population of flannelmouth suckers is relatively stable (Douglas and Marsh 1998), there is concern that recruitment may be declining because of the effects of lower water temperature caused by hypolimnetic releases from Glen Canyon Dam (Weiss et al. 1998; Clarkson and Childs 2000; Ward et al. 2002), loss of suitable rearing habitat (Thieme et al. 2001), blockage of spawning migrations (Chart and Bergersen 1992; * Corresponding author: [email protected] 1 Present address: U.S. Geological Survey, Biological Resources Division, Kansas Cooperative Fish and Wildlife Research Unit, Division of Biology, 205 Leasure Hall, Kansas State University, Manhattan, Kansas 66506, USA. Received May 9, 2003; accepted September 12, 2003 McKinney et al. 1999), and predation (Ward et al. 2002). Therefore, continued monitoring of flannelmouth sucker populations is needed to assess changes in the status of this native fish. Fish body condition is a common fisheries assessment tool. Measures of fish condition have been linked to general fish health (Coughlan et al. 1996), fat and lipid content (e.g., Neumann and Murphy 1992), prey availability (e.g., Paukert and Willis 2003), reproductive potential (e.g., Neumann and Murphy 1992), and environmental conditions (e.g., productivity [DiCenzio et al. 1995] and water level fluctuations [Johnson et al. 1992]). In general, higher condition is associated with higher energy (fat) content, increased food base or reproductive potential, or more favorable environmental conditions. Our objectives were to evaluate long-term (.10 years) temporal and spatial trends in flannelmouth sucker condition and to evaluate the biotic and abiotic relationships that might influence condition. The goal of the study was to assess condition on a large scale (i.e., canyonwide) to aid in the evaluation of trends in the flannelmouth sucker population in the Colorado River system.