Baseline Food Web Interactions in Lake Kachess: Seasonal Predation by Northern Pikeminnow and Burbot on Prey Important for Bull Trout Adam Hansen, Jenny Gardner, Kristin Connelly, Matt Polacek, Dave Beauchamp Today I want to provide an update on our assessment of food web interactions in Lake Kachess; specifically, quantifying the predation impact of NPM and Burbot (the two other dominant predators in the lake in addition to the threated bull trout that occupy the lake) on prey that we know is important for bull trout. The purpose of this is to develop a baseline for evaluating the potential impacts of a new pumping plant that is being proposed to help irrigators during drought.

Why Quantify Food Web Interactions? Identify processes (predation, competition, food supply, temperature, distribution) that LIMIT or PROMOTE the growth and survival of key species (e.g., bull trout) Foundation for evaluating how different species respond to change Temperature regime Water level fluctuations Predator-prey abundance Why quantify food web interactions? Two primary reasons. First…. Second….. Affected by reservoir operations

Kachess Drought Relief Pumping Plant (Fixed or Floating Barge) Crest of spillway How will pumping affect food web interactions in Lake Kachess? 70 ft Current outlet Active: 239,000 ac-ft Proposed fixed pump intake Things could change with this new pumping plant Lower by additional 80 ft to access 200,000 ac-ft of inactive storage Implications for bull trout? 1850

Food Web Structure From Stable Isotopes Feeding history and position in food web (fish fin or muscle) Predators Top predators: large bull trout, burbot, northern pikeminnow Prey Kokanee and other pelagic prey important for bull trout and burbot Which food web interactions influence bull trout and need evaluation? As a fist cut… can provide insight and guidance toward which are the most important interactions to quantify Mean ± 2SE Northern pikeminnow eat mix of fish prey Pelagic Benthic

Predatory Threat Dependent on Size Burbot Predatory Bull trout Pikeminnow Plotting the data now for individual fish as a function of their length shows that all species eat more fish as they grow in length. This is based on the N signatures, whereby the most effective predators on fish are those greater than 300 mm in length. So, the take home here is that when estimating the predation impact of NPM and burbot on prey important for bull trout, we must account for the size-structure of their populations. Relative number of small individuals vs. large predatory individuals? Effective predators >300 mm Prey fish Fork length (mm)

How will pumping effect food web interactions and bull trout in Lake Kachess? Are NPM and BURBOT imposing undue mortality on prey important for bull trout (kokanee, pygmy whitefish)? So the question is how will new water management impact bull trout? Before we can assess potential impacts, we need to characterize the contemporary food web structure of these reservoirs as a baseline. Here, we’re really identifying and quantifying key predator-prey interactions as they relate to bull trout.

Bioenergetics Approach Sample fish within seasonal, depth, and size-structured framework Gill netting Hydroacoustics Midwater trawling To address this question we’re using what called a Bioenergetics approach. There is a number of dimensions to this which I’ll step through now. First is the field sampling componenet… Matt Polacek, LLRT Electrofishing

Bioenergetics Approach Collect fish samples within a seasonal, depth, and size-structured framework Fish sampling informs: Abundance, distribution, size-structure Age, growth, survival Diet composition Thermal experience Food web structure Gill netting SONAR Midwater trawling We can learn a lot by sampling fish within this type of framework including… this picture is a bit gruesome, but it shows quite well what we can learn from different types of samples that we take from fish…. Matt Polacek, LLRT Electrofishing

Bioenergetics Approach Collect fish samples within a seasonal, depth, and size-structured framework adf Consumption = Metabolism + Waste + Growth (in weight) M Fish samples inform: Abundance, distribution, size-structure Age, growth, survival Diet composition Thermal experience Food web structure Gill netting C SONAR Midwater trawling + - W G Matt Polacek, LLRT Electrofishing Inputs for a bioenergetics model of consumption Dependent on temperature and body size

Bioenergetics Approach Collect fish samples within a seasonal, depth, and size-structured framework adf adf Consumption = Metabolism + Waste + Growth Annual growth in weight Predator & prey energy density (J/g) Size and age-structure M Fish samples inform: Abundance, distribution, size-structure Age, growth, survival Diet composition Thermal experience Food web structure Gill netting Bioenergetics Model Seasonal diet composition C SONAR Midwater trawling Here is a diagram showing the specific information used as input or to inform the model. W G Seasonal thermal experience Matt Polacek, LLRT Electrofishing Consumption Estimate (g prey/d by age class) Inputs for a bioenergetics model of consumption Dependent on temperature and body size

Seasonal Diet Composition: NPM Diet varied by size and season (fish: 22-75% of diet) Predation on kokanee by large NPM: spring & summer (8-19%) No pygmy whitefish detected SPRING SUMMER LATE SUMMER (FALL) N = 2 N = 4 N = 4 N = 9 N = 7 N = 13 N = 3 N = 3 N = 7 NPM; Suckers; Dace Cutthroat; M. Whitefish

Seasonal Diet Composition: BURB Diet composed largely of fish, including salmonids (fish: 42-78%) SPRING SUMMER Could not be Identified FALL-WINTER N = 9 N = 4 N = 2 Samples collected spring 2016 Could not be identified No data

Predator-Prey Size Relationship Which size or age-classes of prey are most vulnerable to different size or age-classes of predator? Numerical predation rates Age-2 and older kokanee not vulnerable to NPM Age-1 kokanee Assuming majority of predation on kokanee by NPM focused on age-1

Age & Growth: NPM and BURBOT Annual growth in length estimated from model that describes length as a f(age) 17 years 15 years N = 84 (cm) N = 25 Opercles Otoliths From here, easy to convert length at age to weight at age

Survival and Age-Structure: NPM Total abundance unknown: estimate predation on key prey by age-structured population unit of 1,000 NPM ≥150 mm or ≥age-3 Catch Curve Analysis Shifts in diet, consumption, thermal experience, growth with predator size or age Fully recruited to sampling gear Useful metric for gauging extent of predation mortality and quantifying baseline food web interactions Need estimate of annual survival to develop age-frequency distribution Survival = 69%

Annual Consumption by NPM Unit population of 1,000 NPM ≥ age-3 Age-1 Total consumption of kokanee: 48 kg ~6,200 age-1 kokanee ~3.0% of age-1 kokanee at large August 2014 (206,000) Age-structured unit of 100 bull trout: 22,000; 10.7%

Summary Modest predation on kokanee by NPM Numerically, sufficient kokanee to support age-structured unit of 100 bull trout and population expansion Sufficient numbers ≠ sufficient access Refinements to baseline food web interactions still needed Seasonal diet for NPM and Burbot (FALL) Distribution Age & growth

Baseline Bioenergetics Model Next Steps Modeling effects of alternative pumping strategies on thermal structure (Scott Wells, Portland State) adf adf Consumption = Metabolism + Waste + Growth Annual growth in weight Predator & prey energy density (J/g) Size and age-structure M Gill netting Baseline Bioenergetics Model Seasonal diet composition C SONAR Midwater trawling After finalizing these baseline interactions, what’s the next step? W G Seasonal thermal experience Matt Polacek, LLRT Electrofishing Questions? Consumption Estimate (g prey/d by age class) Dependent on temperature and body size

Simulated effect of drawdown on prey density Prey fish density (# per 1,000 m3) Little change w/n region of operation Prey density Reservoir capacity Buffered by steep bathymetry & depth 50% reduction in volume Not really changing the game Daylight Night **Access to spawning tribs, thermal structure, downstream impacts** Dusk 1850

Annual consumption of fish by size-structured population unit of 100 bull trout Available to age 6-7 bull trout Assumptions: Acoustics Annual growth: averages for 400-700 mm bull trout from Bumping & Rimrock 22,000 age-1 (22%) 550 age-2 (2%) Available to age-3 bull trout Annual survival and size-age structure: Lake Billy Chinook Ages 3-7 dominate reservoir population Consumption by bull trout Diet 100% fish (kokanee) Feeding rate: ~50% Cmax