Index of predation on juvenile salmonids by northern pikeminnow in the lower Columbia River Basin, 1994-1996

We estimated relative abundance of northern pikeminnow Ptychocheilus oregon ensis and relative consumption of juvenile salmonids Oncorhynchus spp. by n orthern pikeminnow at standardized sites in the lower Columbia and lower Sn ake rivers from 1994 to 1996. Indexes of abundance and consumption were com pared with indexes measured from 1990 to 1993 to evaluate changes in predat ion concurrent with a predator control program in the lower Columbia basin. Reductions in indexes of northern pikeminnow abundance, consumption, or bo th resulted in mean 1994-1996 predation index values that were 44-91% lower than mean 1990-1993 values throughout the lower Columbia basin. Consumptio n of juvenile salmonids by surviving northern pikeminnow has not increased in response to predator control efforts. Spatial patterns were consistent a mong years, being greatest downstream from Bonneville Dam, intermediate in lower Columbia River reservoirs, and lowest at Snake River sites. Reduction s in relative predation were consistent with changes in northern pikeminnow population structure associated with harvest, although annual variation in river flow, dam spill, and juvenile salmonid passage may have magnified re ductions in predation.