1. 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.

2. 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

3. 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

4. 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

5. 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)

6. 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.

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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%

16. 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%

17. 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

18. 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

19. 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

20. 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 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