Abstract

Hydroelectric development on the mainstem Columbia River has created a series of impoundments that promote the production of native and non-native piscivores. Reducing the effects of fish predation on migrating juvenile salmonids has been a major component of mitigating the effects of hydroelectric development in the Columbia River basin. Extensive research examining juvenile salmon predation has been conducted in the lower Columbia River. Fewer studies of predation have been done in the Columbia River upstream of its confluence with the Snake River; the most comprehensive predation study being from the early 1990s. The Public Utility District No. 2 of Grant County, Washington initiated a northern pikeminnow removal program in 1995 in an attempt to reduce predation on juvenile salmonids. However, there has been no assessment of the relative predation within the Priest Rapids Project since the removal program began. Further, there is concern about the effects of piscivores other than northern pikeminnow (Ptychocheilus oregonensis), such as channel catfish (Ictalurus punctatus), smallmouth bass (Micropterus dolomieu), and walleye (Sander vitreus, formerly Stizostedion vitreum). The Public Utility District No. 2 of Grant County, Washington and the Priest Rapids Coordinating Committee requested that the U.S. Geological Survey, in collaboration with the Washington Department of Fish and Wildlife, assist them in evaluating the effects of native and introduced predatory fish on migrating juvenile salmon. From 2009 to 2010, we conducted sampling in the 103 kilometers (64 river miles) of the Columbia River from the tailrace of Rock Island Dam downstream to the tailrace of Priest Rapids Dam. To assess predation, we used electrofishing to collect northern pikeminnow, smallmouth bass, and walleye to analyze their diets during 2009 and 2010. In 2009, we used methods to allow comparisons to a previous study conducted in 1993. During 2009, we also used an alternate sampling strategy using habitat data and geographic information system software to select sites and allocate samples. In 2010, we used the data collected during 2009 to further refine our sampling design, with the intent of using the data collected during 2010 to formulate a design strategy for implementation during 2011. Based on the results of 2011, we would then propose a strategy for future studies. However, during 2011, our efforts were redirected to specifically address factors that may be affecting steelhead trout survival in the Priest Rapids Reservoir, Columbia River.

We used the catch and diet data collected in 2009 and 2010 to estimate relative abundance, consumption, and predation indices for northern pikeminnow and smallmouth bass. Despite extensive sampling in the study area in 2009 and 2010, very few channel catfish and walleye were captured. The mean total lengths of northern pikeminnow were much lower than those observed in 1993; suggesting that efforts to remove northern pikeminnow in the study area may be shifting the population towards smaller fish. The northern pikeminnow predation index values were lower in 2009 than in the 1993 study. The reduced predation levels observed may be due to the prevalence of smaller pikeminnow in our catches than in catches reported in 1993. Predation by smallmouth bass was lower in 2009 than in 2010, and generally was greater than predation for northern pikeminnow. Predation for northern pikeminnow was concentrated in the tailrace areas of Priest Rapids, Wanapum, and Rock Island Dams; predation for smallmouth bass was concentrated in the forebay and mid-reservoir sections of the study area. Our results indicate areas where control measures for smallmouth bass could be concentrated to reduce predation in the Priest Rapids Project.

First posted June 21, 2012