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

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

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

7. Tracers of long-term diet history  15 N indicates trophic level  13 C indicates production source What are they used for? Trophic structure: Stable Isotopes

8.  15 N (‰)  Trophic level  13 C (‰)  Production source 0 3 7 -30-26 -22 -18 10 Phytoplankton Zooplankton Forage fish Top predator Trophic structure: Stable Isotopes

9.  15 N (‰)  Trophic level  13 C (‰)  Production source Coho Cladocerans Cycl. Copepods Diporeia Kiyi Cal. Copepods Bloater Seston Dws Lean BurbotSiscowet Chinook Mysis Smelt Herring S. Sculpin 0 2 4 6 8 10 12 -32-30-28-26 -24 Deep Food Web Shallow Food Web Trophic structure: Western L. Superior

10. The “real” top predator: Sea lamprey

11.  15 N (‰)  Trophic level Lamprey body mass (grams) 0 2 4 6 8 10 12 14 16 0100200300400 Transformers Parasites Spawners Lake herring diet only Isotopes indicate diet changes

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 Stock Assessment - Biomass - Recruitment - Harvest rates Ecosystem Simulation Model Species Interactions - Competition - Predation Fishery Interactions

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

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

16. Deep water food web Biomass / Biomass 1930

17. Shallow water food web Biomass / Biomass 1930

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

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

20. Compensatory recruitment 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.51.01.5 Spawning stock Age-1 Recruitment

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

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