US EPA

7343025 

Technical Report 

Upriver bright predation bottleneck. Final report to the Letter of Agreement – Chinook Technical Committee, Pacific Salmon Commission, under contract CTC2015-2 

McMichael, GA; James, BB 

2017 

English 

10.13140/RG.2.2.34523.16165 

Chinook salmon Oncorhynchus tshawytscha that spawn in the Hanford Reach of the Columbia River comprise the majority of the Columbia River Upriver Bright (URB) stock, which is a driver stock for several important commercial, tribal, and recreational fisheries. Researchers have reported that nearly two-thirds of tagged juvenile URB Chinook salmon released in Hanford Reach failed to survive to the first dam they encountered on their seaward migration (McNary Dam). Based on results from a recent cohort reconstruction, a 50% reduction in McNary Reservoir mortality would result in an increase in production by about 100,000 Hanford Reach fall Chinook salmon adults annually. The majority of the mortality of juvenile URB Chinook salmon between spawning areas and McNary Dam has been attributed to predation by native and non-native predator fishes. Our primary objective was to estimate the consumption rate of juvenile URB fall Chinook salmon by smallmouth bass Micropterus dolomieu and walleye Sander vitreus between McNary Dam and Priest Rapids Dam (Hanford Reach and McNary Reservoir) on the Columbia River. Adult predator fishes were collected by Oregon Department of Fish and Wildlife (ODFW) electrofishing crews on seven sampling trips between May 23 and June 21, 2016. Electrofishing crews provided Mainstem Fish Research LLC (MFR) with adult non-native predator fishes and stomach/gut samples were collected from a total of 274 smallmouth bass and 47 walleye. Diet samples were processed in the laboratory using published methods using diagnostic bones to determine genus or species and regressions were used to estimate size of ingested prey items based on dimensions of diagnostic bones. Published bioenergetics equations were used to generate daily consumption rate estimates for smallmouth bass and walleye. Smallmouth bass and walleye preyed heavily upon juvenile URB Chinook salmon during the spring of 2016 between McNary and Priest Rapids dams. Juvenile Oncorhynchus species were important prey items and accounted for 59.6% of the 274 prey fish found in the diet of smallmouth bass. Salmonids comprised an even larger portion of the diet in walleye, with 92.5% of the 157 prey fish found being juvenile Oncorhynchus species. The mean number of juvenile salmonids/predator in smallmouth bass diets was 0.60, while it was 2.87 in walleye. Nearly all of the juvenile Oncorhynchus species were the size of subyearling URB fall Chinook salmon and their mean length was smaller in smallmouth bass than in walleye. Smallmouth bass between McNary Dam and Priest Rapids Dam consumed an estimated average of 1.33 juvenile Oncorhynchus species per day while walleye consumed an estimated 2.52/day. A series of assumptions were made in the discussion section of the report in order to extrapolate loss estimates of juvenile subyearling Chinook salmon. Based on these stated assumptions, we estimated that about 24 million juvenile URB Chinook salmon may be lost to piscivorous fishes annually, accounting for an average of about 46% of the URB Chinook salmon presmolt population. There is some evidence that removal of significant numbers of northern pikeminnow through the Northern Pikeminnow Management Program since its inception 27 years ago is resulting in compensation by other predator species. New management approaches intended to reduce predation losses to juvenile salmonids should take into account all life stages of predator populations and evaluate compensatory responses.