Index of predation on juvenile salmonids by northern squawfish in the lower and middle Columbia River and in the lower Snake River

We developed a predation index to describe the relative magnitude of predation on juvenile salmonids by northern squawfish Ptychocheilus oregonensis throughout the lower and middle Columbia River and lower Snake River. The predation index was the product of an abundance index and a consumption index. We evaluated various catch indices and found that catch per unit effort best reflected differences among northern squawfish abundances. Northern squawfish abundance was higher in the lower Columbia River than in the middle Columbia or lower Snake rivers and was highest in Bonneville Reservoir and the Columbia River downstream from Bonneville Dam. The consumption index was based on the concept of meal turnover time for a sample of northern squawfish. Variables needed to calculate the consumption index (CI) were water temperature (T), mean weight of the northern squawfish (W), mean number of salmonids in each gut (S), and mean weight of the gut contents (GW): CI = 0.0209·T ^1.60·W ^0.27·(S·GW^−0.61). Generally, northern squawfish consumption of juvenile salmonids was highest in tailraces downstream from dams and higher in the Columbia River than in the Snake River. Predation on juvenile salmonids was much higher in the lower Columbia River than in the middle Columbia or lower Snake rivers and was usually higher in summer than in spring. Predation was highest in the Columbia River downstream from Bonneville Dam and in John Day Reservoir. The predation index identified areas where predator control efforts can be most effective. Angling for northern squawfish at dams should be concentrated in tailraces. Removal efforts concentrated in the lower Columbia River would have the greatest effect on predation.