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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 • Self-sustaining populations of salmon Outline • Lake Superior food web structure: Stable Isotopes • Simulating ecological and fishery interactions: 1929-1998 • Recommendations Lake Superior pelagic food web (ancestral) 5 Trophic Level 4 Burbot D. sculpin 3 Siscowet L.T. Chub Whitefish S. sculpin Herring Mysis Diporeia Zooplankton 2 Detritus Phytoplankton 5 Trophic Level 4 Sea lamprey Burbot D. sculpin 3 Lake Superior pelagic food web (modern) Siscowet L.T. Chub Whitefish Steelhead Chinook S. sculpin Smelt Herring Mysis Diporeia Zooplankton 2 Detritus Coho Phytoplankton Trophic structure: Stable Isotopes What are they? • Heavy to light isotope ratio in tissues 15N/14N and 13C/12C • Fractionate predictably up food chain Trophic structure: Stable Isotopes What are they used for? • Tracers of long-term diet history • d15N indicates trophic level • d13C indicates production source d15N (‰) Trophic level Trophic structure: Stable Isotopes Top predator 10 Forage fish 7 Zooplankton 3 Phytoplankton 0 -30 d13C (‰) -26 -22 Production source -18 d15N (‰) Trophic level Trophic structure: Western L. Superior 12 Deep Food Web 10 Siscowet Burbot Lean Kiyi S. Sculpin 8 Bloater 6 4 Dws Mysis Smelt Cal. Copepods 2 Cladocerans 0 -32 Diporeia Cycl. Copepods -30 d13C (‰) -28 Chinook Herring Coho Shallow Food Web Seston -26 Production source -24 The “real” top predator: Sea lamprey d15N (‰) Trophic level Isotopes indicate diet changes 16 Lake herring diet only 14 12 10 8 6 Transformers 4 Parasites Spawners 2 0 0 100 200 300 Lamprey body mass (grams) 400 Simulating ecological interactions Simulating ecological interactions Fishery Catch - Catch - Effort USGS Trawl Survey - Biomass - Recruitment Stock Assessment - Biomass - Recruitment - Harvest rates Exotic Invasion - Smelt - Sea lamprey Food Web Structure - Stable isotopes - Food habits Ecosystem Simulation Model Species Interactions - Competition - Predation Fishery Interactions Biomass / Biomass in 1930 Changes in major species since 1930 4 1950s 1990s 3 2 1 0 Lake Herring Chub Whitefish Lake Trout Siscowet Deep water food web Biomass / Biomass 1930 4 Chub 3 2 1 0 1930 1940 1950 1960 1970 1980 1960 1970 1980 1990 2000 2 Siscowet 1 0 1930 1940 1950 1990 2000 Biomass / Biomass 1930 Deep water food web Biomass / Biomass 1930 Shallow water food web 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 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 Compensatory recruitment Age-1 Recruitment 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 Spawning stock 1.5 Lake herring recruitment, 1929-1970 Age-1 Recruitment 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 Spawning stock 1.5 Depensatory recruitment Age-1 Recruitment 3.0 2.5 2.0 1971-1998 1.5 1.0 0.5 0.0 0.0 0.5 1.0 Spawning stock 1.5