Food web interactions in Lake Chelan: Impacts of predation on salmonids

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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
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Lake Chelan

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

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• Native
• Bull trout (extirpated)
• Burbot
• Westslope cutthroat
• trout (collapsed)
• Northern pikeminnow
• Sculpins
• Three-spine
• stickleback
• Suckers

Introduced Lake trout Chinook salmon
(collapsed) Smallmouth bass Rainbow trout (no
longer stocked) Kokanee Mysis relicta shrimp
Top Predators Zooplankton
Invertebrate Consumers
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• Native
• Burbot
• Westslope cutthroat
• trout (collapsed)
• Northern pikeminnow

Introduced Lake trout Chinook salmon
(collapsed) Smallmouth bass Kokanee Mysis
relicta shrimp
Top Predators Zooplankton
Invertebrate Consumers
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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
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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?

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Basin differences morphometry
Lucerne Basin
Wapato Basin
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Basin differences habitat
Deep Lucerne Basin
Shallow Wapato Basin
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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

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Quantifying Predation Impacts
Combine Bioenergetics Modeling Directed Field
Sampling
Photo M. Mazur
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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
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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
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Muscle tissue -Stable isotopes
-Contaminants -Genetics
Gut contents -Diet
Scales Otoliths -Age Back-calculate
size-at-age
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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 (L8 greater
  in Lucerne Basin)
• Mortality estimated from catch curves (Z
  0.34 annual S71)

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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
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Lake trout prey consumptionSize patterns

• Smallest size class consumed most total prey
• Largest size class consumed most salmonid prey
• Largest size class 9 yr old, 2 kg

TL gt 24
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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
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Annual prey consumption per 1000 lake trout
6 kokanee per lake trout per year
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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 gt 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 gt 9 years old
• No cutthroat trout found in lake trout diets (n
  219 non-empty stomachs)

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Stable isotope analysis
Lake trout
trophic level
Northern pikeminnow
Kokanee
Zooplankton
Crayfish

• pelagic
  littoral

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Stable isotope analysis
trophic level

• pelagic
  littoral

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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

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Northernpikeminnow diet

• Smaller pikeminnow ate mostly invertebrates
• Only largest pikeminnow ate kokanee and
  unidentified salmonids, and only in Wapato Basin

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Burbot diet

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

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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
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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)

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Implications for managers
Photo Anton Jones
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Acknowledgements
Funding USGS, Chelan County PUD No. 1, UW School
of Aquatic and Fishery Sciences, Lake Chelan
Sportsmens 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
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Questions?
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Lake Chelan kokanee thrive after lake trout and
Mysis become established
Sources WDFW stocking records, Chelan PUD 2005,
Brown 1984, DES 2000
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Increased Mysis aggregation at shallower sites
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Growth Consumption
Metabolism Waste