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