Management of Northern Pikeminnow and Implications for Juvenile Salmonid Survival in the Lower Columbia and Snake Rivers

Abstract

Predation by large northern pikeminnow (formerly northern squawfish) Ptychocheilus oregonensis is a major source of mortality for juvenile salmonids Oncorhynchus spp. in the lower Columbia and Snake rivers. Large-scale, agency-operated fisheries have been implemented in this area since 1990 to harvest northern pikeminnow with a goal of 10–20% exploitation. We used indirect methods to analyze the success of the fisheries, and examined benefits to salmonid populations. From 1991 to 1996, three fisheries (sport-reward, dam-angling, and gill-net) harvested approximately 1.1 million northern pikeminnow that were 250 mm in fork length or longer, with the sport-reward fishery contributing 86.5% of the total catch. Total exploitation averaged 12.0% (range, 8.1–15.5) for 1991–1996 and met program goals in all years except 1993. Gill-net and dam-angling fisheries harvested larger northern pikeminnow (which consume a greater number of juvenile salmonids) than the sport-reward fishery. Modeling results indicate that potential predation on juvenile salmonids by northern pikeminnow has decreased 25% since fishery implementation. The relative benefits of a given exploitation rate decreased with time as the number of large northern pikeminnow was reduced; however, additional reductions in potential predation are probable if exploitation is maintained at mean 1994–1996 levels. We estimate a reduction in potential predation of 3.8 million juvenile salmonids (representing 1.9% of the total population) if exploitation rates are maintained at mean 1991–1996 levels. Continued monitoring of predator populations is prudent, and we recommend that restoration and enhancement of Columbia River basin salmonids not rely solely on any one management approach.