1. Erik Schoen & Dave Beauchamp
Food web interactions in Lake Chelan: Impacts of predation on salmonids
Erik Schoen & Dave Beauchamp
Washington Cooperative Fish and Wildlife Research Unit
School of Aquatic and Fishery Sciences
University of Washington

2. Lake Chelan 9th deepest lake in the world (453 m) Over 80 km long
Ultraoligotrophic
Important fisheries and
recreation resource

3. Invertebrate Consumers Introduced
Native
Bull trout (extirpated)
Burbot
Westslope cutthroat
trout (collapsed)
Northern pikeminnow
Sculpins
Three-spine
stickleback
Suckers
Top
Predators
Zooplankton &
Invertebrate Consumers
Introduced
Lake trout
Chinook salmon (collapsed)
Smallmouth bass
Rainbow trout
(no longer stocked)
Kokanee
Mysis relicta shrimp
Species composition: native and introduced fishes + Mysis (some rare introduced species omitted)

4. Zooplankton & Invertebrate Consumers Introduced
Native
Burbot
Westslope cutthroat
trout (collapsed)
Northern pikeminnow
Top
Predators
Zooplankton & Invertebrate Consumers
Introduced
Lake trout
Chinook salmon (collapsed)
Smallmouth bass
Kokanee
Mysis relicta shrimp
Focus on red species: restoration of westslope cutts is primary conservation priority. Lake trout and kokanee support popular fisheries. Chinook supported very popular fishery before crash ~ a decade ago, but have recently been showing up again in creel. Secondary focus on other piscivores and Mysis, as they relate to salmonids.

5. A classic management dilemma: predator-prey imbalances in Western lakes
Rapid kokanee collapses
Flathead Lake, MT
Priest Lake, ID
Whitefish Lake, MT
Intensive lake trout suppression efforts
Yellowstone Lake, WY
Lake Pend Oreille, ID
Swan Lake, MT
Spencer et al. 1991

6. Key questions
What are the major predators of salmonids in Lake Chelan?
Especially for kokanee and westslope cutthroat trout
How does predation operate?
Spatial, seasonal, and size-class patterns
Is predation by the lake trout population likely to increase?

7. Basin differences: morphometry
Lake basin differences: morphometry
Lucerne Basin
Wapato Basin

8. Basin differences: habitat
Littoral
Pelagic
Profundal
Deep Lucerne Basin
Lake basin differences: ecology. 1, Wapato is warmer and more productive than deep, cold Lucerne. 2, Lucerne is steep-sided and dominated by pelagic habitat, while Wapato contains moderate slopes, macrophytes, and abundant littoral and profundal habitat.
“Shallow” Wapato Basin

9. Basin differences: piscivore distribution
Lake trout density 7-fold greater
in shallow Wapato Basin
Northern pikeminnow density
similar in both basins
Burbot density 60% greater in
deep Lucerne Basin
Smallmouth bass captured in
Wapato Basin only
Lake basin differences: distribution of piscivores. Gill nets in Lucerne Basin caught mostly native piscivores: NPM and burbot, plus occasional lake trout. Catches in Wapato Basin included large numbers of lake trout and smallmouth bass.

10. Quantifying Predation Impacts
Combine Bioenergetics Modeling &
Directed Field Sampling
Photo: M. Mazur

11. Modeling Process: Simulation day 0 → day t
Growth: W0→Wt
Predator Energy Density (J/g)
Diet proportions by Wt thru time
Prey Energy Density (J/g)
Thermal Experiencethru time
Bioenergetics Model
C = M + W + G
How much food
must be Consumed
to satisfy observed
Growth? or
Consumption Estimate for 1 fish from 1 age class or growth cohort
How much Growth
given Consumption?
Daily time step

12. Temporal Diet Composition Consumption as % of Prey
Modeling Process
Consumer Growth
Predator Energy Density
Temporal Diet Composition
Prey Energy Density
Thermal Experience
Bioenergetics Model
Consumer
Size Structure
& Abundance
Population
Consumption
Consumption Estimate
Consumption as % of Prey
Biomass or Production
Biomass of
Exploitable prey

15. Muscle tissue: Gut contents -Diet Scales & Otoliths: -Stable isotopes
-Contaminants
-Genetics
Gut contents
-Diet
Scales & Otoliths:
-Age & Back-calculate
size-at-age

16. Lake trout growth and mortality
Lake trout aged with opercles (Sharp & Bernard 1988)
Break-and-burn technique with otoliths did not yield usable age data
Growth curves differed between basins (L∞ greater in Lucerne Basin)
Mortality estimated from catch curves (Z = 0.34; annual S=71%)

17. Lake trout diet Mysis and cyprinids were major prey in Wapato Basin
In Lucerne Basin, kokanee was major prey of large lake trout
Lake trout and Chinook salmon were minor prey
Kokanee
Mysids
Cyprinids

18. Lake trout prey consumption: Size patterns
Smallest size class consumed most total prey
Largest size class consumed most salmonid prey
Largest size class: yr old, 2+ kg
TL > 24”

19. Lake trout prey consumption: seasonal patterns
Overall, more prey consumed during stratified July-Dec period
Predation on kokanee shifted seasonally between basins
Most lake trout cannibalism during summer x7

20. Annual prey consumption per 1000 lake trout
Lucerne
Wapato
Biomass consumed (kg)
Total prey
2,527
3,277
Fish
1,549
1,011
Salmonids
477
217
Number of prey consumed
Kokanee
4,764
1,198
Chinook
138
Lake trout
1,057
6 kokanee
per lake trout
per year

21. Key lake trout results Lake trout density ~ 7x greater in Wapato Basin
Lake trout eat 4x more kokanee per capita in Lucerne Basin
Lake trout > 550 mm fork length are key predators, especially in Wapato Basin
Management actions may be slow to affect lake trout predation: key size class is > 9 years old
No cutthroat trout found in lake trout diets (n = 219 non-empty stomachs)

22. Stable isotope analysis
Lake trout
trophic level
Kokanee
Northern pikeminnow
Simplified N vs. C plot to explain concepts
Zooplankton
Crayfish
pelagic littoral

23. Stable isotope analysis
trophic level
pelagic littoral

24. Chinook salmon diet Few stomach samples
Quantified diet by stable isotope mixing model (n = 6 Chinook, mm FL)
Diet dominated by Mysis
Kokanee made up ~5% of diet
Consistent with diet data from salmon derbies in 1990s

25. Northern pikeminnow diet
Smaller pikeminnow ate mostly invertebrates
Only largest pikeminnow ate kokanee and unidentified salmonids, and only in Wapato Basin

26. Burbot diet
Large burbot in Wapato Basin ate mostly fish, including unidentified salmonids
Burbot in Lucerne Basin ate mostly invertebrates
Small sample sizes

27. Smallmouth bass diet
Cyprinids, suckers, and crayfish comprised most of diets
Sample size small, mostly from summer
Bass captured in Wapato Basin only
Summer
Seasonal segregation from salmonids
Currently no juvenile cutthroat trout present
In Wapato Basin to attract predation

28. Key results: other piscivores
Large northern pikeminnow and burbot consumed kokanee and unidentified salmonids, but only in Wapato Basin
Good news for kokanee, which spend most of year in Lucerne Basin
No cutthroat trout identified in diet of any species (n = 1296 total stomachs, 896 non-empty)

29. Implications for managers
Photo: Anton Jones

30. Acknowledgements
Funding: USGS, Chelan County PUD No. 1, UW School of Aquatic and Fishery Sciences, Lake Chelan Sportsmen’s Association
Nathanael Overman, Anna Buettner, Chris Sergeant, Martin Grassley, Brittany Long, Cara Menard, Cathy Ekblad, Mike Shepard, Erin Lowery
Anton and Sandy Jones, Frank and Patricia Clark, and Joe Heinlen
Phil Archibald, Mallory Lenz, Robert Sheehan, and US Forest Service
Art Viola, Matt Polacek and WDFW
Reed Glesne, Vicki Gempko, and NPS
Jeff Osborn, Steve Hays and Chelan PUD
Lake Chelan Fish Hatchery

31. Questions?

32. Lake Chelan kokanee thrive after lake trout and Mysis become established
Sources: WDFW stocking records, Chelan PUD 2005, Brown 1984, DES 2000

33. Increased Mysis aggregation at shallower sites
Deep Lucerne Basin 20 40 60 80
Depth (m)
Shallow Wapato Basin 20 40 60
Depth (m)

34. Growth
Consumption = + Metabolism
+ Waste
Bioenergetics Model