Effects of Temperature, Fish Length, and Exercise on Swimming Performance of Age-0 Flannelmouth Sucker

—The flannelmouth sucker Catostomus latipinnis is one of the few native fish that persists in the lower Colorado River basin. Little is known about the effects of hypolimnetic releases of cold, swift water from Colorado River dams on flannelmouth sucker populations. We conducted fatigue velocity tests on age-0 flannelmouth suckers in the laboratory to evaluate the effects of water temperature and fish size on swimming ability. Fish of 25–114 mm total length (TL) were subjected to incremental increases in water velocity until the upper limit of their swimming ability was reached. Swimming tests were conducted at 10, 14, and 208C. Swimming ability increased with fish length and was directly related to water temperature at all fish sizes. A decrease in water temperature from 208C to 108C resulted in an average decrease in swimming ability of 40%. Mean swimming ability of wild-caught flannelmouth suckers was 7 cm/s higher than that of captivereared flannelmouth suckers of similar size at 208C and 148C. Flannelmouth suckers subjected to an abrupt 108C temperature drop did not have significantly different swimming ability than flannelmouth suckers acclimated to 108C over 4 d. The construction and operation of hydroelectric dams have transformed the Colorado River from a warm, turbid, and highly dynamic stream into a system dominated by large reservoirs and cold tailwaters. These changes have dramatically affected the abundance of many native fishes, including the flannelmouth sucker Catostomus latipinnis. Reduced summer water temperature has been implicated in the decline of native fish in Glen and Grand canyons (Kaeding and Zimmerman 1983; Childs and Clarkson 1996; Robinson et al. 1998; Clarkson and Childs 2000). After completion of Glen Canyon Dam in 1963, maximum summer river temperatures were reduced from 25–308C to a nearly constant temperature of 108C, and nonnative rainbow trout Oncorynchus mykiss and brown trout Salmo trutta were introduced. As a result, native fish now experience colder water temperatures and predation from the introduced trout, conditions that may result in a low survival rate for young, native fish. * Corresponding author: [email protected] 1 Present address: Arizona Game and Fish Department, Research Branch, 2221 West Greenway Road, Phoeniz, Arizona 85023, USA. Received December 21, 2000; accepted November 9, 2001 Flannelmouth suckers spawn in the Paria River, a tributary of the Colorado River located 25 km below Glen Canyon Dam (Weiss 1993). Young flannelmouth suckers use the mouths of tributaries as nursery areas until they move into the Colorado River main stem in late summer (Thieme et al. 2001). Large numbers of age-0 flannelmouth suckers are commonly captured at the mouth of the Paria River (Hoffnagle 1999), but catch data from a variety of sampling methods indicate that low numbers of juvenile flannelmouth suckers are present in the Colorado River in Glen and Grand canyons (Valdez and Ryel 1995; McKinney et al. 1999a). This lack of recruitment may lead to population declines and extirpation of flannelmouth suckers below Glen Canyon Dam. Three other species of fish endemic to the Colorado River—the Colorado pikeminnow Ptychocheilus lucius, the bonytail chub Gila elegans, and the razorback sucker Xyrauchen texanus—have already been extirpated or are exceedingly rare downstream of Glen Canyon Dam (Minckley 1991). Probable explanations for the rarity of age-0 and juvenile flannelmouth suckers in the Colorado River in Glen and Grand canyons include (1) a cold shock that causes the direct mortality of age-0 flannelmouth suckers as they exit warm tributaries and 493 SUCKER SWIMMING PERFORMANCE enter the cold, swift water discharged from Glen Canyon Dam, (2) predation by introduced fishes in the main stem of the Colorado River, and (3) an impaired swimming ability caused by cold-water discharges that results in age-0 flannelmouth suckers being displaced downstream. The Bureau of Reclamation has proposed the installation of a modified intake structure on Glen Canyon Dam that could raise summer water temperatures downstream by 48C (USDI 1999). The effects of these proposed modifications on native fishes, however, are unknown. We tested age-0 flannelmouth suckers in the laboratory to examine the effects of temperature, fish size, and exercise on prolonged swimming ability. The objective of this study is to test the swimming performance of age-0 flannelmouth suckers over a range of sizes and temperatures to assess their response to a change in Colorado River water temperature.