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
PhytoplanktonDetritus
ZooplanktonDiporeia
HerringS. sculpin
L.T.SiscowetBurbot
D. sculpin Chub Whitefish
Mysis
Lake Superiorpelagic food web
(ancestral)
Tro
ph
ic L
evel
2
4
5
3
PhytoplanktonDetritus
ZooplanktonDiporeia
HerringS. sculpin
L.T.SiscowetBurbot
SteelheadCohoChinook
SmeltD. sculpin Chub Whitefish
Mysis
Sea lampreyLake Superior
pelagic food web(modern)
Tro
ph
ic L
evel
2
4
5
3
• Heavy to light isotope ratio in tissues
15N/14N and 13C/12C
• Fractionate predictably up food chain
What are they?
Trophic structure: Stable Isotopes
• Tracers of long-term diet history
• 15N indicates trophic level
• 13C indicates production source
What are they used for?
Trophic structure: Stable Isotopes
15N
(‰
)
Tro
ph
ic level
13C (‰) Production source
0
3
7
-30 -26 -22 -18
10
Phytoplankton
Zooplankton
Forage fish
Top predator
Trophic structure: Stable Isotopes
15N
(‰
)
Tro
ph
ic level
13C (‰) Production source
Coho
Cladocerans
Cycl. Copepods
Diporeia
Kiyi
Cal. Copepods
Bloater
Seston
Dws
LeanBurbotSiscowet
Chinook
Mysis
Smelt
HerringS. Sculpin
0
2
4
6
8
10
12
-32 -30 -28 -26 -24
DeepFood Web
Shallow Food Web
Trophic structure: Western L. Superior
The “real” top predator: Sea lamprey
15N
(‰
)
Tro
ph
ic level
Lamprey body mass (grams)
0
2
4
6
8
10
12
14
16
0 100 200 300 400
Transformers
Parasites
Spawners
Lake herring diet only
Isotopes indicate diet changes
Simulating ecological interactions
Simulating ecological interactions
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
Changes in major species since 1930B
iom
ass /
Bio
mass in
1930
0
1
2
3
4
LakeHerring
Chub Whitefish LakeTrout
Siscowet
1950s 1990s
Deep water food web
0
1
2
3
4
1930 1940 1950 1960 1970 1980 1990 2000
0
1
2
1930 1940 1950 1960 1970 1980 1990 2000
Chub
Siscowet
Bio
mass /
Bio
mass 1
930
Deep water food webB
iom
ass /
Bio
mass 1
930
Shallow water food webB
iom
ass /
Bio
mass 1
930
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
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
Compensatory recruitment
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5
Spawning stock
Ag
e-1
Recru
itm
en
t
Lake herring recruitment, 1929-1970
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5
Spawning stock
Ag
e-1
Recru
itm
en
t
Depensatory recruitment
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5
Spawning stock
Ag
e-1
Recru
itm
en
t
1971-1998