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

Abstract

We estimated the consumption of juvenile salmon Oncorhynchus spp. and steelhead O. mykiss by smallmouth bass Micropterus dolomieu in the tailrace and forebay of the Lower Granite Dam and compared this consumption with that in the two major river arms of the upper Lower Granite Reservoir, Snake River, Idaho–Washington. We examined over 9,700 smallmouth bass stomachs from April through August during 1996 and 1997. Juvenile salmonids were not a major component of smallmouth bass diets by weight and number at any location in either 1996 or 1997. Of the approximately 8,600 stomach samples containing food items, only 67 had juvenile salmonid remains. Juvenile salmonids accounted for approximately 11% of smallmouth bass diets by weight in the forebay in 1996 and 5% in the Snake and Clearwater river arms in 1997, with smaller proportions at other locations. Crustaceans and nonsalmonid fishes were the dominant prey items by weight at all locations in 1996 and 1997 except for the Snake River arm in 1996, where macroinvertebrates were dominant. Monthly consumption rates (smolts/bass/d) of juvenile salmonids by smallmouth bass were highest in the forebay (0.02) in April 1996 and in the restricted zone of the forebay (0.05) in July 1997. We estimated that approximately 17,500 (1996: 6,728; 1997: 10,809) juvenile salmonids were consumed by smallmouth bass from April through August in 1996 and 1997. High flows and resulting lower water temperatures and higher turbidity in the Lower Granite Reservoir during our study probably affected salmonid predation rates by smallmouth bass. Management efforts directed at enhancing migratory conditions for juvenile salmonids (e.g., higher flows, lower water temperatures, and higher turbidity) will probably also reduce predation from smallmouth bass.