Predation by Northern Pikeminnow on Hatchery and Wild Coho Salmon Smolts in the Chehalis River, Washington

Abstract

One explanation for the low smolt-to-adult survival rates of hatchery and wild coho salmon Oncorhynchus kisutch originating from the Chehalis River, Washington, is predation by northern pikeminnow Ptychocheilus oregonensis. To evaluate this hypothesis, we studied predation by northern pikeminnow on emigrating coho salmon smolts in the undammed, lower main-stem Chehalis River (river kilometers (rkm) 27-82) during Apr and May of 1988 and 1989. Where only wild coho salmon smolts were available, we estimated that northern pikeminnow ate 0.2% of the 1,100,000 wild coho salmon smolts migrating through this reach in 1989. In the reach where both hatchery and wild fish were present, northern pikeminnow ate about 0.9% of the hatchery and wild coho salmon available in this area in 1989. Thus, the total estimated loss of coho salmon smolts due to northern pikeminnow predation in the lower main-stem Chehalis River in 1989 was approximately 18,200 fish. This was about two orders of magnitude less than the loss that we predicted to occur if northern pikeminnow were primarily responsible for the low survival rates of coho salmon in this basin. Evidence suggested that most of the predation was occurring on hatchery origin fish below rkm 42. We conclude that northern pikeminnow were not responsible for the low survival rates of coho salmon smolts in the Chehalis River Basin, but they do have a more significant impact on the survival of hatchery-produced coho salmon than on the survival of wild coho salmon.