| Abstrakt: |
AbstractWe describe the response of smallmouth bass Micropterus dolomieudensity, year-class strength, consumption of juvenile salmonids Oncorhynchusspp., mortality, relative weight, and growth to sustained removals of northern pikeminnow Ptychocheilus oregonensisin the lower Columbia and Snake rivers. Although fishery exploitation of northern pikeminnow (250 mm fork length and larger) averaged 12.1% annually from 1991 to 1996, we detected no response by smallmouth bass. Density of smallmouth bass varied among years, but we found no evidence of increased density concurrent with removals of northern pikeminnow. Year-class strength of smallmouth bass also varied, and variations were similar among areas; however, we found no trend of increasing year-class strength. Consumption of juvenile salmonids by smallmouth bass was zero for 74 of our 104 estimates and did not appear to increase over time. Estimates of mortality varied among areas, but we found no differences among years for any area. Relative weight and growth also varied among years, but we found no increases corresponding to removals of northern pikeminnow. Our estimates of density, consumption, mortality, and growth were similar to estimates made prior to northern pikeminnow removals. Spatial separation of smallmouth bass and most juvenile salmonids may limit increases in predation, even if exploitation of northern pikeminnow increases. The lack of response by smallmouth bass increased confidence in the hypothesis that sustained removal of northern pikeminnow increases survival of juvenile salmonids. |
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