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Feasibility of Establishing a Supplemental Stocking Program Using Wild Caught Age-O Fish Objective and rationale Below, we investigate the feasibility of capturing wild age-O humpback chub from the LCR, transporting them to a grow-out facility, marking them with a unique identifier (such as a Passive Integrated Transponder (PIT] tag), and releasing them back into the Colorado River or its tributaries within Grand Canyon. Part of the rationale for investigating this approach is that this method is currently being used as a management action to conserve the razorback sucker population in Lake Mohave. Since augmentation of a population via the use of brood stock progeny is genetically risky, this document incorporates the concept of capture and grow out of wild age-O fish, and releasing them at larger sizes to augment the population of humpback chub in Grand Canyon. Many of the potential problems associated with inbreeding and reduction of Ne in the wild population should be avoided since there is no captive breeding of offspring. In addition, this method is expected to be more cost effective. Unfortunately, there appears to be a lack of literature concerning the capture, short-term grow-out, and release of wild progeny. Dowling et al. (1996) discussed direct capture and grow out of larvae in order to augment the population of razorback sucker (Xyrauchen texanus) in Lake Mohave. This approach, rather than supportive stocking via standard hatchery broodstock procedures, was suggested by Dowling et al. (1996) in order to maximize genetic variation. The assumption is that collection of wild larvae in a temporally and geographically spaced design will result in maximization of parental representation, and maximum genetic variability should be retained. Such an approach has been ongoing on Lake Mohave since 1993, and in 1999 repatriates from this program constituted approximately 12% of the adult population (Minckley et al. 2003). Hence, the decline that has been occurring in the Lake Mohave razorback population may be showing progress toward the goal of establishing a stock of no fewer than 50,000 repatriated fish (P. Marsh, ASU, pers. com.). The main purpose for establishing a program for the capture and grow-out of age-O humpback chub is to potentially increase the likelihood for survivorship to a larger size class (e.g., > 150 mm). Based on modeling, it is believed that recruitment failure is the main factor causing decline in the humpback chub population of Grand Canyon (C. Walters, Univ. British Columbia [UBC], pers. com.). Recruitment failure is thought to be caused by a myriad of factors, including mainstem Colorado River habitat degradation, predation (Minckley 1991, Valdez and Ryel 1995, Clarkson and Childs 2000), or parasites such as the Asian tapeworm (Clarkson et al. 1997). One of the main reasons that age-O and juvenile humpback chub appear to suffer high mortality in the mainstem is that once displaced into the mainstem, they lack growth and remain vulnerable to the effects of predation for a long period of time (Clarkson and Childs 2000). 24