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Rnal Report downstream into areas with large numbers of brown trout. Brown trout are not presently stocked in the system and spawn primarily in Bright Angel Creek and the adjacent inflow. Rainbow trout also appear to be important predators of humpback chub and could consume up to 27,375 fish per year. Rainbow trout that are sympatric with humpback chub are primarily progeny of local natural reproduction from tributaries such as Nankoweap Creek, Clear Creek, Bright Angel Creek, Shinumo Creek, Tapeats Creek, Deer Creek, and Havasu Creek. The area of highest potential predation (i.e., LCR to Bright Angel Creek) was supplied by fish that probably originated primarily from Nankoweap Creek, Clear Creek, and Bright Angel Creek. The rainbow trout that are sympatric with humpback chub downstream of Nankoweap Creek did not appear to originate from the tailwater fishery; only three of approximately 151,000 catchable rainbow trout (marked with coded wire nose tags) released by AGF (1992 -1993) between the dam and Lees Ferry were recaptured downstream of Lees Ferry, at RM 2.9, RM 3.2, and RM 3.2. Rainbow trout may also compete for food with humpback chub, since favored food items of both species . were simuliids, amphipods, and midges. Limited food resources may be limiting native fish population size in western Grand Canyon. Channel catfish are apparently primarily mainstem inhabitants that aggregate annually for spawning in warm tributaries, primarily in the LCR, where they have been reported for years (Kaeding and Zimmerman 1983, Gorman et al. 1994). Channel catfish have not been reported in Kanab Creek (AGF 1993, 1994), and were not reported in recent surveys of the Paria River (Weiss 1993), Shinumo Creek, or Bright Angel Creek (Otis 1994). The effect of striped bass migrating annually from Lake Mead into Grand Canyon has not been fully evaluated. Although the numbers ascending annually from Lake Mead appear small, this species is a voracious predator and even small numbers could account for substantial mortality of native fishes. The numbers of striped bass in the Lake Mead inflow (i.e., downstream of Bridge Canyon) in spring are high (Valdez 1993, 1994, 1995), and suggest that greater numbers of striped bass could ascend the Colorado River into Grand Canyon given Executive Summary . 17 more suitable conditions such as warmer temperatures and higher turbidity. Small non-native forms, such as fathead minnow and plains killifish, are presently low in numbers but could become numerous with changed conditions, such as warmer mainstem temperatures. These species are relatively weak swimmers and, in their native waters, inhabit low-velocity areas. Fathead minnows thrive in flood bottomlands and backwaters in the Mississippi and Missouri rivers and are often the most resistant species to low oxygen, high temperature, and high turbidity (Pflieger 1975). Plains killifish are typically inhabitants of small to medium streams and prefer low velocity areas. These warmwater species are likely to become transported downstream and stressed with high flows that flood sheltered shoreline habitats such as backwaters. These species are likely to remain in the system, since they inhabit many tributaries from which they can reinvade the mainstem. Red shiners, a known predator and competitor of native stream fishes, are rare upstream of the Lake Mead inflow, but could become numerous if suitable conditions (primarily warmer temperatures) are available (Valdez et al. 1995). Biomass estimates indicate that fish biomass in the mainstem is dominated by non-native species. If food is limiting, removing potential predators may also benefit native species by availing greater supplies of food. Two species of alien parasites presently infect the humpback chub in Grand Canyon, including the parasitic copepod (Lernaea cyprinacea) and the Asian tapeworm (Bothriocephalus acheilognathi)' Parasitic copepods were first reported from this population by Carothers et al. (1981), and the Asian tapeworm was first reported in 1989 by D. Hendrickson (Angradi et al. 1992). These warmwater parasites are unable to complete their life cycles in the cold mainstem. Lemaea cyprinacea and Ii. acheilognathi were in found in 0.13% and 3.6% of humpback chub examined, and were not considered a major threat in the mainstem, Of four native species captured in the mains tent Colorado River in Grand Canyon, flannelmouth suckers and bluehead suckers showed the weakest age structures for population stability, i.e., young