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

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

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)

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.

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

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 Lake basin differences: morphometry Lucerne Basin Wapato Basin

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

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.

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

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

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

Muscle tissue: Gut contents -Diet Scales & Otoliths: -Stable isotopes -Contaminants -Genetics Gut contents -Diet 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 Kokanee Mysids Cyprinids

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 patterns Overall, 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 Lake trout 1,057 6 kokanee per lake trout per 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 Lake trout trophic level Kokanee Northern pikeminnow Simplified N vs. C plot to explain concepts Zooplankton Crayfish pelagic littoral

Stable isotope analysis trophic level pelagic littoral

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

Northern pikeminnow 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 salmonids Currently no juvenile cutthroat trout present In 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 Basin 20 40 60 80 Depth (m) Shallow Wapato Basin 20 40 60 Depth (m)

Growth Consumption = + Metabolism + Waste Bioenergetics Model