Fish Community Composition in the Mainstem Willamette River

We examined native and non-native fish communities in the mainstem Willamette River and adjacent sloughs in summers of 2011-2014. More than 90% of the total number of fish captured in the mainstem channel were native species. Community composition in sloughs differed greatly from mainstem channel habitats. Proportions of non-native species in sloughs increased from upper river to lower river. Species richness of native species decreased from more than 90% in mainstem of the upper river to approximately 70% in the lower river. Non-native fish species were more abundant than native fish in floodplain habitats isolated from the mainstem river. At the scale of 1-km reaches, large wood was not a significantly related to abundance or richness. However in more localized accumulations of wood, both abundance and richness were greater than similar habitats with low amounts of wood. Life history traits of native species exhibit substantial overlap with non-native species, with equilibrium life history types being most common. Non-native species with equilibrium life history types (greater fecundity and parental care) dominate communities in floodplain habitats with little connectivity to the mainstem river. Trends and life history patterns of native and non-native fish communities provide a framework for conservation and restoration approaches.