The Effects of Increased Water Temperature and Dam Construction on the Spatial Distribution and increased abundance of Invasive American shad in the Columbia River Basin

On the Columbia River, at the Bradford Island Visitor’s Center of Bonneville Dam, visitors peer through windows into a fish ladder to witness the pageant of native Pacific salmons. In early summer, however, visitors instead see large schools of non-indigenous American shad, an anadromous clupeid fish native to East Coast rivers that first invaded the Columbia River in the 1870s. Tallies of adult shad on the Columbia River have been available since the U.S. Army Corp of Engineers began counting them in the fish ladders of Bonneville Dam (river km 235) in 1938. The ladder counts show that after The Dalles Dam (river km 309) was built in 1957, the adult shad population passing through Bonneville Dam increased dramatically. By 1977, adult shad at Bonneville Dam outnumbered adult fish of all native salmon species combined. Since it was known from pioneering work by William Leggett in the early 1970s that the timing of the spawning migration of shad corresponds closely to temperature, we hypothesized that the successful upriver colonization of the Columbia River was in part dependent on temperature. To test this hypothesis, we regressed the percentage of adult shad population that migrates beyond McNary Dam (river km 471) against temperature measured at Bonneville Dam. Statistical regressions showed that the upriver distribution of shad was significantly related to temperature averaged over the months of May-August. In warmer years, larger proportions of adult American shad migrate to spawning areas upstream of McNary Dam. When average temperature rose by 1 degree C, the regression model predicted that roughly 8% more of the shad population counted at Bonneville dam migrated to points upstream of McNary Dam. This suggests that the gradual conversion of the Columbia River into a series of reservoirs has altered temperatures regimes in favor of a wider shad distribution, possibly increasing shad reproductive success, and partially explaining trends in abundance observed for this invasive species.