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10 Sucker larvae used in this study ranged in age from 40-52 days (about 26-32 days after swimup) and were in the late mesolarval to mid metalarval developmental stages [terminology after Snyder (1981)]. Wild suckers are generally <40 days old when they reach the lower Williamson River and Upper Klamath Lake as protolatvae or early . mesolarvae (Dunsmoor, unpublished data). Miller et al. (1988) demonstrated an inverse relation between larval developmental stage or size and susceptibility to predation, primarily resulting from ontogenetic increases in predator detection (vision and lateral line development) and avoidance (fin and muscle development) capabilities. Therefore, due to age-related developmental differences, wild sucker larvae just entering.their rearing habitats in Upper Klamath Lake may be even more susceptible to predation by fathead minnows than the larvae used in these experiments. Availability of sucker larvae as prey for fathead minnows in the wild is most likely determined by factors which influence their encounter frequency and/or ability to evade fathead minnows. If sucker larvae change habitat use patterns through ontogeny, encounter rates with specific predators may also change. When prey and predators coexist in the same habitat, complex habitats provided by macrophytes generally reduce foraging efficiency of fish predators (Boyle 1979; Cooper and Crowder 1979; Savino and Stein 1982; Heck and Crowder 1991). Consequently, the typical result of water drawdowns in reservoirs is an increase in predation as forage fish are forced to abandon refugia in littoral areas, becoming concentrated and more available to predators (Ploskey 1986). Results of trial six follow this general pattern, in which efficiency of fathead minnow predation on larval suckers was reduced in the presence of cover. The statistical analysis was complicated somewhat by an outlier, but the nonparametric statistical analysis showed significantly lower predation rates in the low water treatment when cover was present. Parametric ANOVA with the outlier excluded showed a significantly lower mean predation rate in the presence of cover regardless of water depth (P<0.06). Specifically, excluding the outlier, mean predation rates were 96.6% (11.3% SD) with cover absent, and 63.3% (18.4% SD) with cover present. The observed 33% decrease in mean predation rate resulting from the addition of cover to tanks is biologically significant because an influence of this magnitude on survival of the larval life stage could be influential in year class formation for the endangered suckers in Upper Klamath Lake. Another factor that can influence prey selection by a predator is the presence of alternative prey items (Hodgson and Kitchell 1987). In trial five of this study fathead minnows had access to sucker larvae, Daphnia, and dry food. Presence of alternate prey