Climate regimes and water temperature changes in the Columbia River: bioenergetic implications for predators of juvenile salmon

We examined how climatic regime shifts may have affected predation rates on juvenile Pacific salmonids (Oncorhynchus spp.) by northern squawfish (Ptyc hocheilus oregonensis, also called northern pikeminnow), smallmouth bass (M icropterus dolomieu), and walleye (Stizostedion vitreum) in the Columbia Ri ver. During 1933-1996, oceanic, coastal, and freshwater indices of climate were highly correlated, and an index for the Columbia River Basin suggested that climate shifts may have occurred about 1946, 1958, 1969, and 1977. Su mmer water temperature varied as much as 2 degreesC between climate periods . We used a bioenergetics model for northern squawfish, the most important piscivore, to predict that predation on salmonids would have been 26-31% hi gher during two periods with relatively warm spring-summer water temperatur es (1933-1946, 1978-1996) than during an extremely cold period (1947-1958). Predicted predation rates of northern squawfish were 68-96% higher in the warmest year compared with the coldest year. Predation rates of smallmouth bass and walleye on juvenile salmonids varied among climate periods similar to rates predicted for northern squawfish. Climatic effects need to be und erstood in both freshwater and nearshore marine habitats, since growth rate s of salmon populations are especially sensitive to mortality during early life stages.