Cold Water Refuges and Coastal Cutthroat Trout Movement in the Willamette River

Land use has directly and indirectly altered streams and rivers and warmed aquatic ecosystems throughout the Pacific Northwest, contributing to the decline of anadromous salmon and trout, resident salmonids, and other cold water species.  Distributions of native fish species will undoubtedly shrink and become disconnected as thermal regimes in river networks warm more rapidly due to human influences and climate warming.  More than two-thirds of the floodplain sloughs in the Willamette River were colder than the mainstem, and 50% were more than 2ºC colder.  Models of three sloughs indicated that residence time of water in the hyporheic flowpath was a major determinant of cold water refuges.  Sampling of fish communities with boat shocker, backpack shocker, and beach seines revealed that more than 60% of the fish sampled in cold floodplain sloughs were native, but 85% of the fish sampled in warm sloughs were non-native.   During this sampling with multiple gear types, we captured an average of 11.6 coastal cutthroat trout (Oncorhynchus clarkii clarkii) in cold sloughs and only an average 0.6 cutthroat trout in warm sloughs.  Based on radiotracking, we found that more than half of the coastal cutthroat trout released back into the Willamette River moved into cold water sloughs within two weeks and remained in these cold water habitats until late summer in 2008 and 2009.  Summer of 2010 was colder than previous years and high water persisted through late June.  Radiotagged trout did not show as strong an affinity for cold water habitats in 2010.  We implanted iButton temperature dataloggers in coastal cutthroat trout to determine the thermal environments used by cold water native species.  These trout used habitats that were 2 to 3ºC colder than the temperatures in the mainstem river, predominantly in the range of 13-16ºC.  In contrast, carp occupied temperatures ranging from 21-26ºC.  River conservation must protect cold water refuges if we are to maintain our native fish assemblages in the face of climate change, non-native species, and human population growth.  Actions that harden rivers, reduce natural flood events, and remove gravel from rivers must be reduced if we are going to restore dynamic channels that create the cold water refuges associated with floodplain dynamics and river channel change.