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C? c:) _:-J 30 {.0 I:J1 forms (e.g., rainbow trout) when native prey are absent or greatly diminished in abundance. In the San Juan River sub-basin, 23 non-native fish species have been reported. In comparison to other Upper Colorado River Basin drainages, the San Juan has few common or widely distributed non- native fish species. Of these, only common carp and channel catfish are common in main channel habitats within warIDwater reaches and red shiner and fathead minnow are the most common non-natives in associated low-velocity habitats. Other non- native species in warmwater reaches are uncommon or occur sporadically. In main channel habitats, native fish species remain numerically dominant, but non-natives numerically dominate many low-velocity habitats. Although the impact of non-native fishes upon the native fish fauna has been known for some time, means to eliminate or reduce problem species are limited, expensive, and usually not feasible. An essential element of any control effort is accurate definition of the problem. Among non-native fishes inhabiting the San Juan, the mode of interaction of particular non-native species with specific native species (or life stages of each) is not clearly defined. For example, adults of channel catfish consume a variety of food items, including fish. However, the relative impact of their foraging habits upon particular life stages of Colorado squawfish or other natives is not known. In low- velocity habitats, young of Colorado squawfish may compete for food and habitat with several non-natives. The effect of this presumed competition upon recruitment of young of Colorado squawfishto larger size-classes is unknown. Although the San Juan River drainage is occupied by several warmwater non-native fish species, additional stocking of these species or introduction of new species will only exacerbate problems. In this sense, the best control is prevention of potential problems by eliminating stocking of non-native warmwater species. The high flows of 1987 demonstrated an effective method of control of non-native species. That year, successful reproduction by channel catfish was markedly lower than in subsequent years when spring run-off was much lower and of shorter duration. Other controls, such as chemical treatment, are biologically and logistically infeasible in almost all situations on the San Juan River. Some actions, while not strictly control, may be warranted in particular instances. Such actions might include habitat modifications to the detriment of non-native fishes, and presumably the benefit of native species. stocking of most non-native fish species in the San Juan Drainage