Ecological Interactions in Lake Superior

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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

5

Lake Superior pelagic food web (ancestral)
Siscowet L.T.

4

Burbot

Trophic Level

D. sculpin 3

Chub Whitefish

S. sculpin

Herring

Mysis Diporeia 2 Zooplankton

Detritus

Phytoplankton

5

Sea lamprey

Lake Superior pelagic food web (modern)
Coho

4

Burbot

Siscowet

L.T.

Steelhead

Chinook

Trophic Level

D. sculpin 3

Chub Whitefish

S. sculpin

Smelt Herring

Mysis Diporeia 2 Zooplankton

Detritus

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

Trophic structure: Stable Isotopes
d15N (‰)  Trophic level
10

Top predator

7

Forage fish
Zooplankton Phytoplankton
-30 -26 -22 -18

3

0

d13C (‰) 

Production source

Trophic structure: Western L. Superior
d15N (‰)  Trophic level
12 10 8 6 4 2
Cladocerans Cal. Copepods Cycl. Copepods

Deep Food Web

Siscowet Kiyi Bloater Mysis Smelt Diporeia Seston

Burbot

Lean
S. Sculpin Dws Chinook Herring Coho

Shallow Food Web

0 -32

-30
d13C (‰) 

-28

-26

-24

Production source

The “real” top predator: Sea lamprey

Isotopes indicate diet changes
d15N (‰)  Trophic level
16 14 12 10 8 6

Lake herring diet only

4
2 0

Transformers Parasites Spawners

0

Lamprey body mass (grams)

100

200

300

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

Changes in major species since 1930
Biomass / Biomass in 1930

4 3 2 1 0
Lake H e rrin g

1950s

1990s

Chub

W h ite fis h

Lake T ro u t

S is c o w e t

Deep water food web
4

Chub

Biomass / Biomass 1930

3

2

1

0 1930
2

1940

1950

1960

1970

1980

1990

2000

Siscowet
1

0 1930 1940 1950 1960 1970 1980 1990 2000

Biomass / Biomass 1930

Deep water food web

Shallow water food web
Biomass / Biomass 1930

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
3.0

Age-1 Recruitment

2.5 2.0 1.5 1.0 0.5 0.0

0.0

0.5

1.0

1.5

Spawning stock

Lake herring recruitment, 1929-1970
3 .0

Age-1 Recruitment

2 .5

2 .0

1 .5

1 .0

0 .5

0 .0 0 .0 0 .5 1 .0 1 .5

Spawning stock

Depensatory recruitment
3 .0

Age-1 Recruitment

2 .5

2 .0

1971-1998
1 .5

1 .0

0 .5

0 .0 0 .0 0 .5 1 .0 1 .5

Spawning stock


				
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