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II more accurately represented field conditions in which an array of potential prey items would be available to fathead minnows. While resulting predation rates on sucker larvae were lower than in previous trials, fathead minnows continued to consume sucker larvae despite the presence of readily accessible alternate prey items. This result suggests that fathead minnows m the wild may consume sucker larvae despite the availability of alternate prey. Management Implications Larval Lost River and shortnose suckers enter shoreline nursery habitats in the lower Williamson River and Upper Klamath Lake during May and June soon after swimup and emigration from spawning areas (Buettner and Scoppettone 1990). Historically, these nursery habitats were structure-rich Aerial photographs taken in 1941 show 9.7 km of lower Williamson River channel meandering through 1,930 ha of emergent marsh, with dense growths of willows Salix sp. and cottonwoods Populus sp. lining the riverbanks. Shorelines in Upper Klamath Lake for many kilometers on both sides of the river mouth were uninterrupted edges of emergent marsh before conversion of the entire marsh to agricultural use. Both river and lake shorelines of the Williamson River delta are now dikes constructed with materials dredged from the river and lake bottoms. These dikes provide shoreline habitats ranging from narrow strips of emergent macrophytes (Scirpus, Sparganium, or 7ypha), to bare expanses of sand or riprap. In addition to the vegetative changes resulting from dredging and diking, 4.5 km of the lower Williamson River were lost through channelization. The net result of human activity has been to greatly reduce the quality and quantity of available nursery habitat for larval suckers in the lower Williamson River and Upper Klamath Lake. Availability of any existing emergent cover to larval suckers in Upper Klamath Lake is determined by fluctuations in lake surface elevations, which are regulated by a dam constructed at the outlet in 1921. As water is removed from the lake for irrigation throughout the summer, emergent vegetation along much of the lake shoreline can be left above the waterline during the period when sucker larvae are entering and using shoreline nursery areas. Measurements taken at the mouth of the Williamson River (from a single location) indicate that when water elevations decrease to about 0.48 m below maximum water elevation, emergent vegetation will no longer be in the water. During the previous two decades (1970-1990) water elevations dropped 0.48 m below full pool before mid-July in 9 out of 20 years. While resulting impacts on interactions between larval suckers and their predators have not been measured in the field, my results indicate