Habitat relationships of native and non-native fishes of the Willamette River, Oregon

Graduation date: 2015The Willamette River flows north between the Cascade Mountains and Coast Range of western Oregon within a 29,728-km² basin. The fish community in the geologically young basin consists of 36 native species, but introductions of non-native fishes have added 33 additional species for a total of 69 fish species. During the summers of 2011-2013, fish distributions were sampled from the confluence of the McKenzie River downstream to the confluence of the Columbia River to assess: (1) fish community composition, (2) differences in community composition between mainstem river and slough habitats, and (3) relationships between native and non-native fish species and physical habitat characteristics. In addition, a 182-ha seasonally inundated floodplain habitat along the middle Willamette River was sampled during winter 2011 through spring 2012 to assess: (1) fish community composition, (2) spawning and rearing, and (3) timing of use and movement by fish species.\ud Throughout the three years of longitudinal sampling, 36,586 fish were collected comprising 41 species, 22 native and 19 non-native. Overall, native fish represented 93% of the total fish sampled. Higher numbers of fish were collected in the upper river, and higher proportions of those fish were native species. Though non-native fish were more common in slough habitats than mainstem sties, the majority of fish collected in both habitat types were native. The strongest environmental predictors of fish community were quantitative measures of longitudinal distance, velocity, depth, percent macrophyte cover and percent embeddedness. Habitat type (slough versus mainstem) was also a significant predictor. Native fish species\ud exhibited greater variability in habitat relationships than non-native fishes, though several native species were strongly associated with specific environmental attributes. Non-native species were more associated with slough habitats and sites with high amount of macrophyte growth and embedded substrates. Overall, 25 species had a high fidelity to mainstem or slough habitats. Equal numbers of native species showed a fidelity to either mainstem or slough sites, but non-native species were significantly related only to slough habitats.\ud In the seasonally inundated floodplain habitat, an estimated 43,000 fish were collected comprising 23 species; 12 native and 11 non-native. Of these 98.1% were native. The most common native species were threespine stickleback Gasterosteus aculeatus and peamouth Mylocheilus caurinus while the most common non-native species were bluegill Lepomis macrochirus and western mosquitofish Gambusia affinus. Several species used the inundated floodplain for spawning and rearing, particularly threespine stickleback, peamouth and western mosquitofish. With the exception of young-of-year threespine stickleback, native species were collected mostly in the beginning and middle of the sampling period while non-native species were collected in the latter portion of the sampling.\ud The Willamette River Basin has a long history of environmental modification. Understanding relationships of a broad range of species to their environment and to each other provides a technical foundation for ecosystem based management by state and federal agencies, local municipalities, and private landowners. The need for ecosystem based management is vital for future conservation and recovery of large river floodplain ecosystems