title: Factors influencing the survival of outmigrating juvenile salmonids through multiple dam passages: an individual-based approach.
abstract: Substantial declines of Pacific salmon populations have occurred over the past several decades related to large-scale anthropogenic and climatic changes in freshwater and marine environments. In the Columbia River Basin, migrating juvenile salmonids may pass as many as eight large-scale hydropower projects before reaching the ocean; however, the cumulative effects of multiple dam passages are largely unknown. Using acoustic transmitters and an extensive system of hydrophone arrays in the Lower Columbia River, we calculated the survival of yearling Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) passing one, two, or three dams. We applied a unique index of biological characteristics and environmental exposures, experienced by each fish individually as it migrated downstream, in order to examine which factors most influence salmonid survival. High outflow volumes led to involuntary spill in 2011 and created an environment of supersaturated dissolved gas concentrations. In this environment, migrating smolt survival was strongly influenced by barometric pressure, fish velocity, and water temperature. The effect of these variables on survival was compounded by multiple dam passages compared to fish passing a single dam. Despite spatial isolation between dams in the Lower Columbia River hydrosystem, migrating smolt appear to experience cumulative effects akin to a press disturbance. In general, Chinook salmon and steelhead respond similarly in terms of survival rates and responses to altered environmental conditions. Management actions that limit dissolved gas concentrations in years of high flow will benefit migrating salmonids at this life stage.
authors.affiliation: Department of Environmental Science and Management Portland State University PO Box 751 Portland Oregon 97207.
authors.forename: Timothy
authors.initials: T
authors.lastname: Elder
authors.affiliation: Environmental Laboratory US Army Engineer Research and Development Center 3909 Halls Ferry Rd Vicksburg Mississippi 39180.
authors.forename: Christa M
authors.initials: CM
authors.lastname: Woodley
authors.affiliation: Pacific Northwest National Laboratory 390 Evergreen Dr. PO Box 241 North Bonneville Washington 98639.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Weiland
authors.affiliation: Department of Environmental Science and Management Portland State University PO Box 751 Portland Oregon 97207.
authors.forename: Angela L
authors.initials: AL
authors.lastname: Strecker
keywords: Acoustic transmitters; Chinook salmon; dissolved gas; involuntary spill; random forest; steelhead trout