Diet of yearling chinook salmon and feeding success during downstream migration in the Snake and Columbia Rivers

The objectives of this study were to characterize and compare the stomach contents and feeding success of yearling chinook salmon smolts (Oncorhynchus tshawytscha) during their downstream migration at three sites in the Snake and Columbia Rivers. From 1987 to 1991, 26 to 38% of the yearling chinook salmon smolts sampled as they passed Lower Granite Dam, the first dam encountered by migrants in the Snake River, had empty stomachs. In 1991, smolts were sampled further downstream at McNary and Bonneville Dams on the Columbia River on the same sample dates. Empty stomachs occurred in 3% and 5% of these fish, respectively, and overall stomach fullness values were significantly higher. Smolts ate primarily dipterans (chironomids) at Lower Granite Dam; cladocerans, homopterans, and dipterans at McNary Dam; and amphipods and dipterans at Bonneville Dam - taxa typical of impounded waters. A series of dams on the Snake and Columbia Rivers has altered the conditions and habitat available for migrating juvenile salmonids and contributed to their decline. Large numbers of hatchery smolts, long residence times, altered food resources, and reservoir morphology may contribute to poor feeding success near Lower Granite Dam and could lead to reduced smolt survivalMuir and Coley "Diet of yearling chinook salmon and feeding success during downstream migration in the Snake and Columbia Rivers." Northwest Science. 1996; 70(4): 298-30