Lake Superior Benthic Fish Community Structure
By
Michael H. Hoff
U.S. Geological Survey
Great Lakes Science Center
Lake Superior Fish Community Objectives
and
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 structure
Graph community data
Test for differences in communities
Rainbow
smelt
Background
•Dynamics of fish communities can be used to
evaluate effects of management changes and
ecosystem perturbations (e.g. invasions)
Deepwater sculpin
Spoonhead
sculpin
Slimy
sculpin
Background
•Using statistical analyses, only two fish communities
have been described from Lake Superiorboth reside in Chequamegon Bay.
•Fish communities have not been described from
Lake Superior’s main basin.
Burbot
Report Objective
To quantitatively describe the summer, benthic fish
community structure and habitat associations in
Lake Superior by examining the on-contour, bottomtrawl data set.
On-contour trawl tows
Cross-contour trawls
Lake Superior
Fishing with a bottom trawl.
Ship
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
Minnesota
Ontario
Lak
eS
upe
rior
Wisconsin
Michigan
Locations of on-contour bottom-trawl stations sampled
during 1972-1995.
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 community
structure
• 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 taxa
23 species and
stocked lake trout,
wild lake trout,
siscowet lake trout
splake
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
Sample sizes of bottom-trawl tows, by depth group.
300
No. of tows
250
200
150
100
50
0
Depth (meters)
•Analysis of densities of 26 taxa accounted for
only 30% of the data variation,
•Analysis of densities of 9 taxa
accounted 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 existed
and habitats were their depth ranges.
Pygmy
whitefish
Comparison of densities of 9 fish taxa across depths.
Significance test
rainbow smelt
shallow > intermediate > deep
shallow > intermediate, deep
trout-perch
shallow, intermediate > deep
shallow, intermediate > deep
intermediate > deep
deep > shallow, intermediate
deep > intermediate > shallow
deep > shallow, intermediate
deep > intermediate > shallow
ninesp. stickleback
slimy sculpin
lake whitefish
siscowet trout
bloater
kiyi
deepw. sculpin
0
Shallow (5.0-39.9 m)
10
20
No./hectare
Intermediate (40.0-79.9)
30
40
Deep (80.0-141 m)
50
Significant Depth Associations of Other 17 Taxa
None found – 10 taxa
alewife
splake
lake chub
spottail shiner
longnose sucker burbot
spoonhead sculpin
pink salmon
white sucker
yellow perch
Sample sizes small (< 216) of all but
alewife and spoonhead sculpin
Densities, by depth, of 16 taxa in Lake Superior.
Density (no./hectare)
Depth (meters)
5.0-39.9
6
5
4
3
2
1
0
Shallow
Shallow & Intermediate
Intermed.
Deep
40.0-79.9
80.0-141.0
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)
Trout-perch
Deep (80.0-141 meters)
Lake Superior bays, and main basin depth zones.
Bays
< 80 m
> 80 m
Bays
•Although a gradient of densities occurred across
all depths for some taxa,
densities changed abruptly with depth for:
wild lake trout
round whitefish
bloater
slimy sculpin
trout-perch
Johnny darter
siscowet lake trout
pygmy whitefish
kiyi
deepwater sculpin
ninespine stickleback
johnny darter
Recommendations
1. Lake Superior fish populations have changed
since much of the data were collected
A study of the present structure of benthic
fish communities is needed.
Recommendations
2. Agencies need to consider fish communities when
implementing management strategies to achieve
Fish Community Objectives.
Recommendations
3. Fish community structures should be documented in;
bays
harbors
estuaries
tributaries
Areas 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 management
to 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 AOC
fish community and habitat objectives.
Recommendations
Areas of Concern (continued)
•Monitoring structures of communities in AOCs will
track progress in rehabilitating their degraded
populations, communities, and habitats.
Acknowledgements
Thanks to G. Cholwek, M. Burnham-Curtis, and
J. Lyons for some of the photographs.
Densities, by depth group, of 16 taxa in Lake Superior.
Density (no./hectare)
Depth (meters)
5.0-39.9
6
5
4
3
2
1
0
Shallow
Shallow & Intermediate
Intermed.
Deep
40.0-79.9
80.0-141.0