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Ecological Interactions in Lake Superior

Ecological Interactions in Lake Superior. Sean Cox, Chris Harvey, and Jim Kitchell Center for Limnology University of Wisconsin, Madison. Fish community objectives (FCOs). Self-sustaining forage fish populations Maintaining native fish community Self-sustaining lake trout populations

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Ecological Interactions in Lake Superior

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  1. Ecological Interactions in Lake Superior Sean Cox, Chris Harvey, and Jim Kitchell Center for Limnology University of Wisconsin, Madison

  2. Fish community objectives (FCOs) • Self-sustaining forage fish populations • Maintaining native fish community • Self-sustaining lake trout populations • Self-sustaining populations of salmon

  3. Outline • Lake Superior food web structure: Stable Isotopes • Simulating ecological and fishery interactions: 1929-1998 • Recommendations

  4. Siscowet L.T. Burbot 5 D. sculpin Chub Whitefish S. sculpin 4 Herring Mysis 3 Diporeia Zooplankton 2 Detritus Phytoplankton Lake Superior pelagic food web (ancestral) Trophic Level

  5. 5 4 3 2 Lake Superior pelagic food web (modern) Sea lamprey Steelhead Siscowet L.T. Coho Burbot Chinook Smelt D. sculpin Chub Whitefish S. sculpin Trophic Level Herring Mysis Diporeia Zooplankton Detritus Phytoplankton

  6. Trophic structure: Stable Isotopes What are they? • Heavy to light isotope ratio in tissues • 15N/14N and 13C/12C • Fractionate predictably up food chain

  7. Trophic structure: Stable Isotopes What are they used for? • Tracers of long-term diet history • d15N indicates trophic level • d13C indicates production source

  8. Top predator 10 Forage fish 7 Zooplankton 3 Phytoplankton 0 -30 -26 -18 -22 Trophic structure: Stable Isotopes d15N (‰) Trophic level d13C (‰) Production source

  9. Trophic structure: Western L. Superior 12 Deep Food Web Siscowet Burbot Lean 10 Kiyi Chinook S. Sculpin 8 Bloater Herring d15N (‰) Trophic level Coho Dws Mysis 6 Smelt Cal. Copepods Shallow Food Web 4 Diporeia Cycl. Copepods 2 Seston Cladocerans 0 -24 -32 -30 -28 -26 d13C (‰) Production source

  10. The “real” top predator: Sea lamprey

  11. 16 Lake herring diet only 14 12 10 8 6 Transformers 4 Parasites Spawners 2 0 0 100 200 300 400 Isotopes indicate diet changes d15N (‰) Trophic level Lamprey body mass (grams)

  12. Simulating ecological interactions

  13. Fishery Catch - Catch - Effort USGS Trawl Survey - Biomass - Recruitment Exotic Invasion - Smelt - Sea lamprey Food Web Structure - Stable isotopes - Food habits Ecosystem Simulation Model Species Interactions - Competition - Predation Fishery Interactions Simulating ecological interactions Stock Assessment - Biomass - Recruitment - Harvest rates

  14. Changes in major species since 1930 Biomass / Biomass in 1930

  15. Deep water food web Chub Biomass / Biomass 1930 Siscowet

  16. Deep water food web Biomass / Biomass 1930

  17. Shallow water food web Biomass / Biomass 1930

  18. Key Ecological Effects • System continues to respond to invasion and collapses that occurred more than 50yrs ago • Mysis and smelt facilitate energy transfer between deep and shallow food webs • Potentially important interaction between whitefish and herring

  19. Recommendations • Whitefish may be headed for moderate decline as they approach carrying capacity • Siscowet too, but magnitude is uncertain • Develop management plan aimed specifically • at recovery of lake herring

  20. 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 Compensatory recruitment Age-1 Recruitment Spawning stock

  21. Lake herring recruitment, 1929-1970 Age-1 Recruitment Spawning stock

  22. 1971-1998 Depensatory recruitment Age-1 Recruitment Spawning stock

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