Presentation on theme: "Erik Schoen & Dave Beauchamp"— Presentation transcript:
1 Erik Schoen & Dave Beauchamp Food web interactions in Lake Chelan: Impacts of predation on salmonidsErik Schoen & Dave BeauchampWashington Cooperative Fish and Wildlife Research UnitSchool of Aquatic and Fishery SciencesUniversity of Washington
2 Lake Chelan 9th deepest lake in the world (453 m) Over 80 km long UltraoligotrophicImportant fisheries andrecreation resource
4 Zooplankton & Invertebrate Consumers Introduced NativeBurbotWestslope cutthroattrout (collapsed)Northern pikeminnowTopPredatorsZooplankton & Invertebrate ConsumersIntroducedLake troutChinook salmon (collapsed)Smallmouth bassKokaneeMysis relicta shrimpFocus 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 collapsesFlathead Lake, MTPriest Lake, IDWhitefish Lake, MTIntensive lake trout suppression effortsYellowstone Lake, WYLake Pend Oreille, IDSwan Lake, MTSpencer et al. 1991
6 Key questionsWhat are the major predators of salmonids in Lake Chelan?Especially for kokanee and westslope cutthroat troutHow does predation operate?Spatial, seasonal, and size-class patternsIs predation by the lake trout population likely to increase?
7 Basin differences: morphometry Lake basin differences: morphometryLucerne BasinWapato Basin
8 Basin differences: habitat LittoralPelagicProfundalDeep Lucerne BasinLake 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 greaterin shallow Wapato BasinNorthern pikeminnow densitysimilar in both basinsBurbot density 60% greater indeep Lucerne BasinSmallmouth bass captured inWapato Basin onlyLake 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 SamplingPhoto: M. Mazur
11 Modeling Process: Simulation day 0 → day t Growth: W0→WtPredator Energy Density (J/g)Diet proportions by Wt thru timePrey Energy Density (J/g)Thermal Experiencethru timeBioenergetics ModelC = M + W + GHow much foodmust be Consumedto satisfy observedGrowth? orConsumption Estimate for 1 fish from 1 age class or growth cohortHow much Growthgiven Consumption?Daily time step
12 Temporal Diet Composition Consumption as % of Prey Modeling ProcessConsumer GrowthPredator Energy DensityTemporal Diet CompositionPrey Energy DensityThermal ExperienceBioenergetics ModelConsumerSize Structure& AbundancePopulationConsumptionConsumption EstimateConsumption as % of PreyBiomass or ProductionBiomass ofExploitable prey
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 dataGrowth 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 troutLake trout and Chinook salmon were minor preyKokaneeMysidsCyprinids
18 Lake trout prey consumption: Size patterns Smallest size class consumed most total preyLargest size class consumed most salmonid preyLargest size class: yr old, 2+ kgTL > 24”
19 Lake trout prey consumption: seasonal patterns Overall, more prey consumed during stratified July-Dec periodPredation on kokanee shifted seasonally between basinsMost lake trout cannibalism during summerx7
20 Annual prey consumption per 1000 lake trout LucerneWapatoBiomass consumed (kg)Total prey2,5273,277Fish1,5491,011Salmonids477217Number of prey consumedKokanee4,7641,198Chinook138Lake trout1,0576 kokaneeper lake troutper year
21 Key lake trout results Lake trout density ~ 7x greater in Wapato Basin Lake trout eat 4x more kokanee per capita in Lucerne BasinLake trout > 550 mm fork length are key predators, especially in Wapato BasinManagement actions may be slow to affect lake trout predation: key size class is > 9 years oldNo cutthroat trout found in lake trout diets (n = 219 non-empty stomachs)
22 Stable isotope analysis Lake trouttrophic levelKokaneeNorthern pikeminnowSimplified N vs. C plot to explain conceptsZooplanktonCrayfishpelagic littoral
24 Chinook salmon diet Few stomach samples Quantified diet by stable isotope mixing model (n = 6 Chinook, mm FL)Diet dominated by MysisKokanee made up ~5% of dietConsistent with diet data from salmon derbies in 1990s
25 Northern pikeminnow diet Smaller pikeminnow ate mostly invertebratesOnly largest pikeminnow ate kokanee and unidentified salmonids, and only in Wapato Basin
26 Burbot dietLarge burbot in Wapato Basin ate mostly fish, including unidentified salmonidsBurbot in Lucerne Basin ate mostly invertebratesSmall sample sizes
27 Smallmouth bass dietCyprinids, suckers, and crayfish comprised most of dietsSample size small, mostly from summerBass captured in Wapato Basin onlySummerSeasonal segregation from salmonidsCurrently no juvenile cutthroat trout presentIn Wapato Basin to attract predation
28 Key results: other piscivores Large northern pikeminnow and burbot consumed kokanee and unidentified salmonids, but only in Wapato BasinGood news for kokanee, which spend most of year in Lucerne BasinNo cutthroat trout identified in diet of any species (n = 1296 total stomachs, 896 non-empty)
30 AcknowledgementsFunding: USGS, Chelan County PUD No. 1, UW School of Aquatic and Fishery Sciences, Lake Chelan Sportsmen’s AssociationNathanael Overman, Anna Buettner, Chris Sergeant, Martin Grassley, Brittany Long, Cara Menard, Cathy Ekblad, Mike Shepard, Erin LoweryAnton and Sandy Jones, Frank and Patricia Clark, and Joe HeinlenPhil Archibald, Mallory Lenz, Robert Sheehan, and US Forest ServiceArt Viola, Matt Polacek and WDFWReed Glesne, Vicki Gempko, and NPSJeff Osborn, Steve Hays and Chelan PUDLake Chelan Fish Hatchery
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