Lake Superior Benthic Fish Community Structure

ByMichael H. Hoff

U.S. Geological SurveyGreat Lakes Science Center

Lake Superior Fish Community Objectivesand

Joint Strategic Plan for Management of Great Lakes Fisheries

“The Parties must…meet the biological, chemical, and physical needs of desired fish communities.”

Lake Whitefish

Background

•Great Lakes fishery managers agreed to change from fish species to community management approaches

in 1987.

•The Lake Superior Committee developed the Fish Community Objectives to manage Lake Superior fish communities

Lake herring

Background

•A community is an ecological unit that is structured with unique, quantifiable attributes.

•Statistical analysis has been used in community ecology to; Discover community data structureGraph community data Test for differences in communities

Rainbow smelt

•Dynamics of fish communities can be used to evaluate effects of management changes and ecosystem perturbations (e.g. invasions)

Background

Spoonheadsculpin

Deepwater sculpin

Slimysculpin

•Using statistical analyses, only two fish communities have been described from Lake Superior-

both reside in Chequamegon Bay.

•Fish communities have not been described from Lake Superior’s main basin.

Background

Burbot

Report Objective

To quantitatively describe the summer, benthic fish community structure and habitat associations in Lake Superior by examining the on-contour, bottom-trawl data set.

On-contour trawl tows Cross-contour trawls

Lake Superior

Ship

Trawl

Fishing with a bottom trawl.

Mouth of bottom trawl – side view.

Methods

Sampling

•22 locations sampled during mid-June to mid-September

•937 bottom-trawl tows were taken

•Fish densities (no./hectare) computed from catches and the area swept by trawls

N Ontario

Michigan

Wisconsin

Minnesota

Locations of on-contour bottom-trawl stations sampled during 1972-1995.

Lake Superior

Methods

Data Analysis

•4 statistical procedures 3 multivariate (many variables used)1 univariate (one variable used)

Kiyi

Why all of those statistics?

• No single test exists that examines communitystructure

• An integrated set of procedures was needed for a thorough exploration, examination, and portrayal of the

existence and structure of communities.

Round whitefish

Results

•937 on-contour bottom trawl tows at 22 stations captured 396,390 fish from

26 taxa23 species and stocked lake trout, wild lake trout, siscowet lake troutsplake

Siscowet lake trout

Lake trout

•Only four or less shortjaw ciscoes, splake, pink salmon, white suckers, spottail shiners, and yellow perch were caught in all tows.

•15 taxa were represented by at least 300 specimens.

Shortjaw cisco

5-9.

9

10-1

9.9

20-2

9.9

30-3

9.9

40-4

9.9

50-5

9.9

60-6

9.9

70-7

9.9

80-8

9.9

90-9

9.9

100-

109.

9

110-

119.

9

120-

141

0

50

100

150

200

250

300

Depth (meters)

No.

of

tow

sSample sizes of bottom-trawl tows, by depth group.

•Analysis of densities of 26 taxa accounted for only 30% of the data variation,

•Analysis of densities of 9 taxaaccounted for 65% of the variation.

•Densities of 9 taxa used in further statistical analyses

Ninespine stickleback

Trawl tows classified to depth groups

•Highest overall correct classification rate (80%) was to 5.0-39.9 meters (82% correct) 40.0-79.9 meters (77% correct) > 80.0 meters (56% correct)

•All depth groups classified at rates greater than were possible by chance

Bloater

Differences of 9 taxa densities across depths

•Multivariate analysis showed that densities of the 9 taxa were different across

shallow (5.0-39.9 meters), intermediate (40.0-79.9 meters), and deep (80.0-141 meters) trawl depths.

•Therefore, 3 communities existedand habitats were their depth ranges.

Pygmy whitefish

rainbow smelttrout-perch

ninesp. stickleback

slimy sculpinlake whitefish

siscowet troutbloater

kiyideepw. sculpin

0 10 20 30 40 50

Shallow (5.0-39.9 m) Intermediate (40.0-79.9) Deep (80.0-141 m)

shallow > intermediate > deepshallow > intermediate, deepshallow, intermediate > deepshallow, intermediate > deep

intermediate > deep

deep > shallow, intermediate

deep > intermediate > shallowdeep > shallow, intermediatedeep > intermediate > shallow

Significance testComparison of densities of 9 fish taxa across depths.

No./hectare

Significant Depth Associations of Other 17 Taxa

None found – 10 taxaalewife splake pink salmon lake chub spottail shiner white suckerlongnose sucker burbot yellow perchspoonhead sculpin

Sample sizes small (< 216) of all but alewife and spoonhead sculpin

Rainbo

w smelt

Trout

-per

ch

Round

whit

efish

Pygm

y whit

efish

John

ny d

arte

r

Ninesp

. stic

kleba

ck

Slimy s

culpi

n

Wild

lake

trou

t

Stock

ed la

ke tr

out

Lake

her

ring

Lake

whit

efish

Siscow

et la

ke tr

out

Bloate

rKiyi

Deepw

ater

sculp

in

Shortj

aw ci

sco

0123456

Density (no./hectare)5.0-39.9 40.0-79.9 80.0-141.0

Densities, by depth, of 16 taxa in Lake Superior.

Depth (meters)

Shallow Shallow &Intermed.

Intermediate Deep

Conclusions

•Lake Superior contained 3 summer, benthic fish communities.

•Their habitats were described by depth ranges Shallow (5.0-39.9 meters)Intermediate (40.0-79.9 meters)

Deep (80.0-141 meters) Trout-perch

Lake Superior bays, and main basin depth zones.

Bays

Bays

< 80 m > 80 m

•Although a gradient of densities occurred across all depths for some taxa,

densities changed abruptly with depth for:

wild lake trout siscowet lake trout round whitefish pygmy whitefish

bloater kiyi slimy sculpin deepwater sculpintrout-perch ninespine stickleback

johnny darter

Johnny darter

Recommendations

1. Lake Superior fish populations have changedsince much of the data were collected

A study of the present structure of benthic fish communities is needed.

2. Agencies need to consider fish communities when implementing management strategies to achieve Fish Community Objectives.

Recommendations

Recommendations

3. Fish community structures should be documented in;bays harbors estuaries tributariesAreas of Concern, and pelagia (above bottom) of main basin.

Except pelagia, all those habitats are highly vulnerable to pollution and habitat degradation.

Recommendations

4. Need to construct models that relate fish population & community data to habitat measures

Model outputs will better enable managementto rehabilitate damaged ecosystems(Remember: Ecosystem = community + its habitat).

Recommendations

Areas of Concern (AOC)•7 of the 8 have degraded fish populations and habitats.

•Comparisons of community structures in AOCs, with similar areas not damaged, will help managers refine AOCfish community and habitat objectives.

Areas of Concern (continued)

•Monitoring structures of communities in AOCs will track progress in rehabilitating their degraded

populations, communities, and habitats.

Recommendations

Acknowledgements

Thanks to G. Cholwek, M. Burnham-Curtis, and J. Lyons for some of the photographs.

Rainbo

w smelt

Trout

-per

ch

Round

whit

efish

Pygm

y whit

efish

John

ny d

arte

r

Ninesp

. stic

kleba

ck

Slimy s

culpi

n

Wild

lake

trou

t

Stock

ed la

ke tr

out

Lake

her

ring

Lake

whit

efish

Siscow

et la

ke tr

out

Bloate

rKiyi

Deepw

ater

sculp

in

Shortj

aw ci

sco

0123456

Density (no./hectare)5.0-39.9 40.0-79.9 80.0-141.0

Densities, by depth group, of 16 taxa in Lake Superior.

Depth (meters)

Shallow Shallow &Intermed.

Intermediate Deep