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Other fishes occupy humpback chub habitats, and there is some anecdotal information that suggests that the chub may have been displaced from some habitats due to interactions with channel catfishes and other introduced fishes (Tyus 1998). Direct ~ predation has been observed in Grand Canyon, where introduced catfish and trout consumed large numbers of the fish (Valdez and Ryel 1995; Marsh and Douglas 1996) Bonytail ~ Life history information about the bonytail is scant (USFWS 1990a), and its habitat requirements are virtually unknown. Very few fish have been reported in the UCRB and it is not known if the fish was ever abundant there (Tyus et al. 1982; but see photo in Quarterone 1993, which suggests that the fish may have been abundant locally). The last bonytail reported from the UCRB was captured on 17 July 1984 near Black Rocks ~ (Kaeding et al. 1986), Some bonytail have persisted in large reservoirs of the lower basin (e.g., Lake Mohave and Lake Mead), indicating an ability to live in lacustrine habitat (Minckley 1973, Valdez and Clemmer 1982, USFWS 1990a). Results of a radiotracking study of adult bonytail chub introduced into the upper Green River in 1988 and 1989 indicate that the fish exhibit crepuscular movements, and are relatively quiescent during the day and night (S. Cranney, Utah Division of Wildlife Resources, ~ pers. comm.). Studies are in progress to determine basic ecological requirements that may be needed for successful reintroduction (Growl et al. 1996). ~ PART 3. ENVIRONMENTAL FACTORS LIMITING RECOVERY General The abundance of a natural population is determined by the balance of individuals ~ gained through reproduction and those lost to mortality. If recruitment to the breeding population does not equal or exceed loss to all sources of mortality, other factors being equal, the population will decline. For endangered species, it is a foregone conclusion that loss has significantly exceeded recruitment in the past. Successful recovery will depend on enhancing recruitment relative to loss (or reducing loss relative to ~ recruitment). The factors contributing to recruitment and loss may be abiotic (physical or chemical), biotic, or both. Physical factors could include the quality or abundance of habitat required for one or more life history stages. For example, loss of habitat through ~ channelization, or degradation of substrate by sediment accumulation, will reduce the number of larvae produced. Other things being equal, a drop in production of larvae would decrease recruitment. The condition of the physical habitat also is strongly influenced by the hydrologic regime because of relationships between flow and extent of habitat, or between flow and sediment transport, for example. Water quality, another abiotic factor, could cause mortality via pollutants, or reduce recruitment by more subtle 23