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1 Att t bli h emp s to reesta s razorback sucker in the Verde and Salt Rivers of Arizona using hatchery-reared fish were unsuccessful. Between June 30, 1981, and December 1, 1989, 10,334,498 razorback suckers <100 mm in total length (TL) and 120,865 between 100 to 287 mm TL were stocked into the Verde and Salt Rivers and their associated tributaries (Hendrickson 1993). Recapture data showed that very few of these razorback suckers survived more than 1 year. Those that did survive were found to be in poor condition (lost weight) and harbored extremel dense infestations of the arasitic co e od Lernaea y P Pp cyprinacea (Creef and Clarkson 1993). Lernaea is an exotic parasite first known to inhabit Arizona waters as early as the 1930s (James 1968). It imbeds itself on the dermal surface, predominantly fin bases, offish and subsequently feeds upon them. Lernaea infected fish in the upper Verde River with the highest incidence rate (~54% of all catostomids) in Arizona (James 1968). Hendrickson (1993) found that razorback suckers infested with Lernaea were more likely to perish when caught in 3-hour trammel net sets than were other fish species (zero mortalities, with or without Lernaea) and alluded to parasitic loads, predation from exotic fishes, and stress related to transport as factors that may have contributed to the low survival rates of stocked razorback suckers. Current efforts to artificially enhance razorback sucker numbers in Lake Mojave have been attempted by placing the larvae into predator-free enclosures and allowing them to grow to 250 mm TL before release (Mueller and Burke 1999, Mueller 1995). Between 1993 and 1995, 640 razorback sucker had been stocked into Lake Mojave in this fashion and have yielded 15 recaptures. These 15 fish (28 to 53 cm) represented the largest assemblage of young adult razorback suckers collected from Lake Mojave in nearly 20 years. During the past 5 years, 20,000 additional razorback suckers have been stocked and have resulted in over 100 recaptures (Marsh and Pace 1999). Stocking efforts to enhance razorback populations in the upper Colorado River Basin have just recently begun in earnest and have also resulted in low survivorship rates, except for perhaps stockings made into the San Juan River (Dale Ryden, FWS, personal communique). Twenty razorback suckers were released with 4.5-year radio tags in the upper Colorado River [river kilometer (RK) 339.0 to 367.0] during April of 1994 by U.S. Fish and Wildlife Service (USFWS) personnel (Burdick and Bonar 1997). These fish were all small adults (451 to 534 mm TL) and experienced relatively high post-release mortality for fish of this size during the first 150 days (7 of 10 confirmed or presumed dead, 10 missing). The same researchers also released 25 similarly sized razorback suckers (455 to 535 mm) into the Gunnison River during the same time period. Razorback sucker stocked into the Gunnison River also experienced high mortality rates during the first 150 days (19 of 22 confirmed or presumed dead, 3 missing). In October 1994, 24 small adult razorback suckers (325 to 405 mm TL) outfitted with radio tags and 656 pit-tagged razorback suckers of similar size (200 to 440 mm), were ~, released into the San Juan River between RK 128 and 219 (Ryden and Pfeifer 1995). The 2 1