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C. Maintaining Genetic Diversity of Wild Endangered Fish in Controlled Environments. Stocking of larval or juvenile wild endangered fish into protected habitats has been suggested as a strategy to increase natural recruitment. Even though wild, naturally spawned endangered fish larvae will be stocked into protected habitats, protective custody is extensive aquaculture (i.e., captive propagation) where humans have intervened and the genetics risks are uncertain. Before this method is used, the management objectives, opportunities, and alternative strategies should be carefully considered to reduce potential genetic risks. For example, stocking equal numbers from all family lots of captive-reared endangered fish into protected habitats such as flooded bottomlands is a strategy with more control. Sections IV.C.2. and IV.C.3.. provide guidance for stocking larval endangered fishes into protected habitats and comparing the genetic characterization of these fish with wild stocks prior to release into any river in the upper basin. 1. Background. The major limiting factor regarding the endangered Colorado River fishes is low survival during the early life stages that results in low, or lack of, recruitment. When recruitment does not replace adult mortality, wild stocks decline in numbers. Colorado squawfish and humpback chub stocks are considered to be low but stable in the upper basin. To protect these stocks and increase the numbers of fish, fishways will be constructed to mitigate the loss of passage of the squawfish over dams, providing access into unoccupied historic habitats. In addition, streamflows needed to maintain vital habitats for both species will be defined. Recruitment of wild razorback suckers, however, is low or non- existent, although spawning has been documented. Therefore, attempts will be made to enhance flooded bottomland habitats with the goal of increasing survival of razorback suckers, and possibly bonytail, during the early life stages to aid natural recruitment. The evaluation of these sites as nursery areas for razorback suckers may require stocking naturally spawned larvae into protected, predator- free sites, an activity recommended by Dowling and Minckley (1993) to augment the razorback stock in Lake Mohave. 2. Guidelines for Maintaining Genetic Diversity in Wild Larval Endangered Fish to be Stocked in Protected Environments. Larval razorback suckers should be collected from or near a known spawning site near a suspected historic nursery area. Temporal sampling of larvae should be made throughout the natural spawning period (April through mid-June when average water temperature is about 14-15 C; Tyus and Karp 1991). For example, collections could be made weekly using light traps during this period. An attempt should be made to collect equal numbers of larvae during each collection that is not very likely. Standardized techniques should be used during all collections so that mortality of larvae during capture and handling remains similar among collections. Ideally, larvae from each collection should be marked. However, batch-marking techniques to 14 1 1 1 1 1 L 17