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P Marry) 7aaa THE SOUTHWESTERN NATURALIST 34(2):188-195 JUNE 1989 PREDATION BY ICTALURID CATFISHES AS A DETERRENT TO RE-ESTABLISHMENT OF HATCHERY-REARED RAZORBACK SUCKERS PAUL C. MARSH AND JAMES E. BROOKS ABSTRACT-Juvenile razorback sucker (Xyrauchen texanus), 45 to 168 mm standard length, rein- troduced within their native range into the Gila River, Arizona, suffered intensive predation by two, non-native ictalurids, channel catfish (Ictalurus punctatus) and flathead catfish (Pylodictu olivaM). Estimated losses in the 2.5-km study reach over a 2-day post-stocking period were up to 900 individuals/ km in autumn. If typical, predation may be high enough to preclude local re-establishment of the species by juvenile stockings at that time of year. Predation in autumn was lower when average size of stocked fish was increased from 68 to 113 mm standard length. Extrapolated total loss to predation in winter, when channel catfish did not feed actively, was about one-sixth that in autumn. Cold-weather planting of larger razorback suckers, in the range of 300 mm long, may be one way to enhance post- stocking survival. Predation by resident piscivores has often been implicated as a major factor limiting survival of stocked, hatchery-reared fishes of several species (Wales and German, 1956; Axon, 1971; Laarman, 1978; Murphy and Kelso, 1986). In recent years, recovery plans for fishes recognized as threatened or endangered often have included recommendations for stocking to re-establish or augment depleted populations. While predation impact may be an acceptable liability of sport fishery management programs, recovery efforts for imperiled fishes must focus on methods to reduce these losses. Reintroduction of imperiled razorback sucker (Xyrauchen texanus) into its historic range has been ongoing in Arizona since the 1981 implementation of an agreement between the Arizona Game and Fish Department (AZGFD) and the U.S. Fish and Wildlife Service Region II (USFWS; Johnson, 1985). Field studies in autumn 1984 and 1985 and winter 1986 were designed to evaluate dispersal of newly-stocked, juvenile razorback sucker in a river in eastern Ar- izona, and assess predation by two ictalurids, channel catfish (Ictaturus punctatus) and flathead catfish (Pylodictis olivaris). The catfishes are major non-native piscivores now established throughout much of western North America. MATERIALS AND METHODS-The study site was a 10-km reach of the Gila River (Colorado River drainage) near Clifton, Greenlee Co., Arizona. Average stream width ranged from 12 to 22 m, and mean depth ranged from 0.5 to 0.9 m at discharges of 1.1 to 4.0 m3/s in 1984 and 1986, respectively. oe Current velocity averaged near 0.2 m/s during each site visit. Substrates were mostly sand and gravel, with coarser materials in shallow runs (0.5 m deep) and short riffles (less than 10 to long). Pools were rare, but depths locally exceeded 1 to along canyon walls. Aquatic vegetation was sparse, and riparian r. vegetation was generally lacking along wetted stream margins. About 5,000 hatchery-reared, juvenile razorback suckers (82 to 168 mm standard length, SL; X = 113 ± 19 mm; n = 25) were released in aggregate at 0600 h on 21 September 1984; 25,875 (45 to 142 mm SL, X = 68 ± 17 mm, n = 100) were similarly stocked at 0800 h on 13 September 1985; and 4,600 fish (73 to 156 mm SL, X = 92 ± 19, n = 83) were planted at 1200 h on 8 January 1986.