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CHANGES IN FISH ASSEMBLAGE STRUCTURE IN THE MAINSTEM WILLAMETTE RIVER, OREGON
Citation:
Hughes, R M., C. Andrus, R. Wildman, AND S. V. Gregory. CHANGES IN FISH ASSEMBLAGE STRUCTURE IN THE MAINSTEM WILLAMETTE RIVER, OREGON. Presented at American Fisheries Society annual meeting, Phoenix, AZ, August 17-23, 2001.
Description:
The Willamette River has a mean annual discharge of 680 m3s-1. In the 1940s it was polluted by organic wastes, resulting in low dissolved oxygen concentrations and floating and benthic sludge deposits that hindered salmon migration and navigation. Following basin-wide secondary waste treatment and low-flow augmentation, water quality markedly improved, salmon runs returned, and recreational uses increased. However diffuse pollution remains a problem as do continued physical habitat alterations and introduced species. Fish assemblages in the mainstem Willamette were systematically sampled in the summers of 1945, 1983, 1992, and 1998. In the past 55 years, tolerant species decreased and intolerant species increased. In the past 20 years introduced centrarchids have expanded their ranges in the river. We associate these changes with improved water quality, altered flow regimes, and fish migration.