Ecological Interactions in Lake Superior Sean Cox, Chris Harvey, and Jim Kitchell Center for Limnology University of Wisconsin, Madison.

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