Food web interactions in Lake Chelan: Impacts of predation on salmonids
Erik Schoen & Dave BeauchampWashington Cooperative Fish and Wildlife Research Unit
School of Aquatic and Fishery Sciences
University of Washington
Lake Chelan9th deepest lake in the world (453 m)
Over 80 km longUltraoligotrophic
Important fisheries and recreation resource
NativeBull trout (extirpated)
Burbot Westslope cutthroat
trout (collapsed)Northern pikeminnow
Sculpins Three-spine
sticklebackSuckers
IntroducedLake trout Chinook salmon
(collapsed)Smallmouth bass
Rainbow trout(no longer stocked)
KokaneeMysis relicta shrimp
Top
Predators
Zooplankton &
Invertebrate Consumers
Native
Burbot Westslope cutthroat
trout (collapsed)Northern pikeminnow
IntroducedLake trout Chinook salmon
(collapsed)Smallmouth bass
KokaneeMysis relicta shrimp
Top
Predators
Zooplankton & Invertebrate Consumers
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
A classic management dilemma: predator-prey imbalances in Western
lakes
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?
Deep Lucerne Basin
Pelagic
PelagicLittoral
Littoral
Profundal
Profundal
Basin differences: habitat
“Shallow” Wapato Basin
• 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
Basin differences: piscivore distribution
Quantifying Predation Impacts
Photo: M. Mazur
Combine Bioenergetics Modeling & Directed Field Sampling
Modeling Process: Modeling Process: Simulation day 0 Simulation day 0 → day t
Thermal Experiencethru time
Diet proportions by Wt thru time
Prey Energy Density (J/g)
Growth: W0→Wt
Predator Energy Density (J/g)
Bioenergetics Model
C = M + W + G
Consumption Estimate for 1 fish from 1 age
class or growth cohort
How much foodmust be Consumedto satisfy observedGrowth? or
Temperature
0 5 10 15 20 25
Rat
es g
/(g
*d)
0.00
0.05
0.10
0.15
0.20
0.25
Cmax
Cmax- Waste
Resp. + SDA
GrowthHow much Growth given Consumption?
Daily time step
Body mass (g)
0 50 100 150 200 250 300
Cm
ax
& M
g F
ood e
quiv
ale
nt
/ (g
Body
mass *
d)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Cmax
Metabolism (Basal + Active)
Modeling ProcessModeling Process
Thermal Experience
Temporal Diet Composition
Prey Energy Density
Consumer Growth
Predator Energy Density
Bioenergetics Model
Consumption Estimate
ConsumerSize Structure& Abundance
PopulationConsumption
Biomass ofExploitable prey
Consumption as % of PreyBiomass or Production
Fork length (mm)
200 300 400 500 600
Len
gth
fre
qu
en
cy
0
20
40
60
80
100
120
Gut contents-Diet
Muscle tissue: -Stable isotopes -Contaminants -Genetics
Scales & Otoliths: -Age & Back-calculate size-at-age
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%)
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
MysidsCyprinids
Kokanee
Lake trout prey consumption:Size patterns
Smallest size class consumed most total prey
Largest size class consumed most salmonid prey
Largest size class: 9+ yr old, 2+ kg
TL > 24”
Lake trout prey consumption:
seasonal patternsOverall, more prey
consumed during stratified July-Dec period
Predation on kokanee shifted seasonally between basins
Most lake trout cannibalism during summer
x7
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 0
Lake trout 0 1,057
6 kokaneeper lake troutper year
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)
Stable isotope analysis
pelagic littoral
trop
hic
leve
l
Zooplankton Crayfish
Northern pikeminnowKokanee
Lake trout
Chinook salmon diet
• Few stomach samples
• Quantified diet by stable isotope mixing model (n = 6 Chinook, 411-785 mm FL)
• Diet dominated by Mysis
• Kokanee made up ~5% of diet
• Consistent with diet data from salmon derbies in 1990s
Northernpikeminnow
diet• Smaller
pikeminnow ate mostly invertebrates
• Only largest pikeminnow ate kokanee and unidentified salmonids, and only in Wapato Basin
Burbot diet
• Large burbot in Wapato Basin ate mostly fish, including unidentified salmonids
• Burbot in Lucerne Basin ate mostly invertebrates
• Small sample sizes
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 salmonidsCurrently no juvenile cutthroat trout presentIn Wapato Basin to attract predation
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)
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
Lake Chelan kokanee thrive after lake trout and Mysis become established
Sources: WDFW stocking records, Chelan PUD 2005, Brown 1984, DES 2000
Increased Mysis aggregation at shallower sites
Deep Lucerne Basin0
20
40
60
80
Dep
th (
m)
Shallow Wapato Basin0
20
40
60
Dep
th (
m)