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more than 100 years (Wiley et al. 1993). Considering the size or age class at which most hatchery fish are released, the magnitude of mortality is especially great compared to wild mortality rates (Maynard et al. 1995). Post-release survival rates for razorback sucker released in the San Juan River have been roughly estimated as high as -25% (F. Pfieffer, USFWS, pers. com.); however, these fish were grown to 400 mm. Additionally, growing fish to 200 mm may take additional space, entail an additional cost, and add an additional six months of growing time minimum (M. Ulibarri, DNFH & TC, pers. com.). It is estimated that in order to grow humpback chub much beyond 150 mm, more than a year will be needed. If fish are cultured on a yearly basis, it would be optimal to free space in time for the arrival of new fish. An important factor that could negate some of these concerns is that about half as many fish would need to be grown to 200 mm as to 150 mm in order to accomplish the same objective; that is to increase recruitment to sufficient levels. This holds at least three important implications. First, collecting fewer fish and growing them to a larger size (200 mm) would result in less annual cropping of the wild cohort. In addition, it may be preferable to annually release a smaller number of cultivated fish into the wild. Second, it would be logistically easier to collect fewer fish. It is unlikely that this would reduce hatchery space requirements or costs, since growing fish to 200 mm will result in holding multiple cohorts on station. In summary, we recommend that fish are grown out to a minimum of 150 mm for initial efforts. If problems with survivorship rates upon release at this size appear to be insurmountable in achieving augmentation, efforts may need to be taken to grow fish to 200 mm before release. Our rationale for recommending 150 mm is based on 1) a need to PIT tag and monitor these fish once released, and 2) minimizing the chance for hatchery mortality and domestication issues. How many fish will need to be released into the wild to sufficiently supplement the population of humpback chub in Grand Canyon? We have investigated an Age Structured Mark-Recapture (ASMR) population model (designed by C. Walters,UBC) for humpback chub. This model provides an estimate of the number of fish that may be needed to arrest a continuing decline or to augment the population of humpback chub in Grand Canyon via the use of supplemental stocking using wild caught fish. The model can be set up using alternative assumptions about wild survivorship rates. For example, the model can use the age at length and estimated survival rates from ages 1 to 30 (partially shown in Table 1), and with the use of historical data, run from 1989 to 2020. The model can operate by inputting the number of aged fish that are stocked (e.g., ages-1, 2, 3, etc.) on top of the estimated average wild recruitment between 1994 and 1999. Since the model uses annual age at length data, it will not predict exactly how many humpback chub at 150 mm would need to be stocked (i.e., it can only input the age specific lengths - for example age-3 fish at 171 mm). In this report, the model is not intended to provide a final definitive number; it is dynamic and is subject to assumptions and future refinements that mayor may not be considered necessary. The model accounts for removal of age-O fish from the wild cohort (i.e., cropping), and assumes a 20% mortality rate in the captive phase (as based 28