Although northern pikeminnow are a native species and have always preyed upon juvenile salmonids, development of the Columbia River hydropower system has likely
increased the level of predation. Dams have slowed water velocity and decreased turbidity, effects which have increased exposure time of juvenile salmonids to predators and probably also increased predation success. Development of the hydropower system has also resulted in increased water temperatures, and therefore increased predator activity and consumption. Dams concentrate prey in forebay and tailrace areas, further increasing the likelihood of predation. Juvenile salmonids in dam tailraces are likely disoriented from passage through or around turbines, spillways, or bypass systems, increasing their vulnerability to predation.
In "natural" systems where northern pikeminnow or related pikeminnow speciescoexist with anadromous salmonids, Brown and Moyle (1981) found that predation by
pikeminnow in streams was minimal except near dams and other structures, and Buchanan et al. (1981) found that predation on salmonids by northern pikeminnow was
minimal in free-flowing reaches of the Willamette River. Beamesderfer and Rieman (1991) and Ward et al. (1995) confirmed that northern pikeminnow densities were
highest near dams, and Vigg et al. (1991) and Ward et al. (1995) confirmed that consumption rates are also highest near dams. Together, these studies suggest that the
predation impact of northern pikeminnow in the Columbia and Snake rivers today is likely much greater than what it may have been prior to construction of dams.
