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Introduction 1 1 1 Native fish faunas around the world are increasingly at risk due, at least partially to the introduction of nonnative fish predators. Recent reviews suggest that virtually every continent now has documented cases of species extinctions due to nonnative introductions (Grossman 1991, Fernando 1991, Growl et al. 1992, Townsend and Winterbourn 1992, Allan and Flecker 1993, Ng et al. 1993). For many freshwater systems with newly introduced predators, at least some species are hypothesized to be at risk, though data are lacking on long-term population trends (Miller et al. 1989). In systems that have undergone multiple invasions by potential nonnative predators, assigning potential importance is often methodologically difficult. This situation is exacerbated when species of concern are already rare and/or considered endangered. Quantifying nonnative predator effects has traditionally been accomplished by three methods. The simplest and least certain approach involves mapping spatial and temporal distributions of species. The underlying premise is that if a predator overlaps in space and time with target prey species, predation is a likely outcome. This approach does not allow direct quantification of predation pressure. A second method involves estimating volume or biomass of stomach contents and the rate of gastric evacuation to compute daily rations of predators for specific prey items (Carline 1987). This method is labor-intensive in that large numbers of predators are required over long- time periods to get good estimates. The third method involves direct experimentation using ponds, cages or some form of enclosure/exclosure design to directly measure predation rates on specific prey types (Cooper et al. 1991). This method has the distinct advantage in that it allows 1