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8 greatest impact on humpback chub mortality. Mainstem and tributary control actions would target non-native species using a variety of methods. Monitoring of native and non-native fish species must be able to detect changes in these populations that may result from management actions, e.g., non-native control efforts and the wanning of dam releases. Development of a stocking plan for non-native fish similar to the agreement between the Fish and Wildlife Service and the states of Utah and Colorado would help to prevent further incursions of predators and competitors into Grand Canyon. (3) Use experimental dam releases to reduce mortality of young- of-year (YOY) humpback chub leaving the LCR, particularly prior to operation of the TCD. These actions may include releases that would impound the LCR during periods when young humpback chub are leaving the LCR, stabilize habitat near the LCR confluence, reduce non-native spawning and recruitment, displace or disadvantage non-native fish, and maintain levels of turbidity that reduce predation by sight- feeding non-natives in the mainstem. Following construction of the TCD, the focus of dam operations might change to improving spawning and rearing habitat for native fish in the mainstem and controlling the spread of non-natives and parasites. (4) Control parasites and diseases. Additional monitoring and research is needed to determine the level of infestation and to develop control methods. Warmer dam releases may increase the spread or impact of parasites on humpback chub in the mainstem. (5) Use other management actions such as sediment/turbidity augmentation (to disadvantage non-native fish and provide cover for native species), invasive species management plans, and reduction of impacts from scientific and recreational uses. 4.3 Rcducing the Threat of Catastrophic Evcnts There is a risk of extirpation from catastrophic events in the LCR because it is currently the principle spawning location for humpback chub in Grand Canyon and is occupied by much of the population at any given time. Also, operation ofthe TCD and other management actions intended to benefit the humpback chub carry the risk of unintended consequences that may negatively affect the humpback chub population. The Science Advisors are currently evaluating the risks associated with the TCD, and the NEP A evaluation of the TCD will include actions that would be taken if negative consequences occur. The top priorities to protect against such risks are (I) expand the range of the population both above and below the LCR confluence (both mainstem and tributaries) so that a catastrophic event is less likely to negatively affect the population, (2) develop and implement an action plan to alleviate threats that originate in the LCR watershed, and (3) establish a captive breeding population for restoring the Grand Canyon population in case of extirpation. A genetics management plan should be prepared that guides preservation of the genetic diversity of the humpback chub in Grand Canyon. Developing a captive breeding population needs to follow this peer-reviewed comprehensive plan as well as USFWS .