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?

Basin differences: morphometry

Wapato BasinWapato Basin

Lucerne BasinLucerne Basin

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

Stable isotope analysis

pelagic littoral

trop

hic

leve

l

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)

Implications for managers

Photo: Anton Jones

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

Questions?

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)

Bioenergetics Model

Growth

Consumption = + Metabolism + Waste