Download 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
• 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