Smallmouth Bass Predation on Juvenile Salmonids in Lower Granite Reservoir on the Snake River

Predation by nonnative fishes has been identified as a contributing factor in the decline of juvenile salmonids in the Columbia River basin.  This study examined the diet composition of smallmouth bass Micropterus dolomieu and estimated their consumption of juvenile salmonids in Lower Granite Reservoir on the Snake River.  We examined 1,782 bass stomachs from April to September of 2013 in riprap and non-riprap habitats.  Salmonids were a significant portion of bass diets at certain times and locations, especially in non-riprap habitat where they comprised up to 48% of diet by weight and were found in 47% of stomachs in late May.  We estimated that 168,381 salmonids were consumed in our 18 km study area (49,275 in riprap and 119,106 in non-riprap habitat) of which 154,952 were Chinook salmon Oncorhynchus tshawytscha.  Length frequencies of consumed Chinook salmon suggested that 97% were subyearling fall Chinook.  Compared to a similar study conducted in 1997, our results indicate a 13-fold increase (6,823 to 90,318) in the number of salmonids consumed, despite a lower bass abundance.  Fall Chinook salmon may be particularly vulnerable to predation due to their small size, extended reservoir rearing, and migrating at times when bass abundance and metabolic rates are high.