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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
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Effectiveness of Temporary Carp Barriers for Restoring
Wild Rice Beds in Upper Clam Lake: 2010 to 2013
Burnett County, WI
Prepared for St. Croix Tribal Environmental Services – Natural Resources Department, October 2013
by James A. Johnson – Freshwater Scientific Services, LLC
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Surveys Performed by:
!
!
James A. Johnson (Freshwater Scientific Services, LLC)
Anthony J. Havranek (St. Croix Tribal Environmental Services – Natural Resources Department)
Data Analysis & Reporting:
!
James A. Johnson (Freshwater Scientific Services, LLC)
Cite this report as:
Johnson JA, Havranek AJ. 2013. Effectiveness of temporary carp barriers for restoring wild rice growth in Upper Clam Lake:
2010 to 2013. Report to St. Croix Tribal Environmental Services – Natural Resources Department, Webster (WI). Freshwater
Scientific Services–LLC, Maple Grove (MN). 8 pp.
Report available for download at http://www.freshwatersci.com/fw_projects.html
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Project Context
Upper Clam Lake (Burnett Co., WI) has a documented history of supporting large, dense stands of northern
wild rice (Zizania palustris). However, in 2006 wild rice declined dramatically throughout the lake. Unlike the
periodic, temporary rice declines that occasionally occur in regional lakes (Walker et al. 2006), the decline of
rice in Upper Clam Lake was particularly severe and has persisted for 8 years (2006 to 2013), during which rice
growth in the open lake was limited to extremely sparse growth in very shallow areas immediately along
shore. Previous studies have clearly identified carp as the primary cause of the observed decline of rice stands
in Upper Clam Lake (Johnson and Havranek 2010).
In 2010, the most substantial remnant stands of wild rice were found in a large shallow bay on the southern
end of the lake (~80 acre bay, mean depth <1 m; Fig 1). Rice stands in this bay were very sparse and generally
isolated to extremely shallow areas immediately along shore. As a part of the ongoing wild rice monitoring
and management activities in the lake, staff from St. Croix Tribal Environmental Services installed nets across
the opening to this southern bay to exclude carp during the open water period in 2011, 2012, and 2013. These
nets were left in place from May through September of each year. Freshwater Scientific Services was
contracted to assess the late-summer distribution and density of rice growth in the bay each year from 2010
through 2013. This report summarizes the monitoring methodology and findings from these surveys.
Figure 1. Maps of Upper Clam Lake showing the location of the
southern bay with remnant rice stands (left) and a detailed view
showing the location of the installed carp barriers (below).
Carp
Barriers
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Methods
Installation of Carp Barriers
The installed carp barriers consisted of heavy-duty nets (3/8 in. mesh; height ~2 x max water depth) with
surface floats and bottom weights. After placing the nets and anchoring the ends to shore, staff from St. Croix
Tribal Environmental Services installed fence-posts along each side of the net (roughly every 10 m) to provide
additional support and prevent the nets from shifting position or sagging. These nets were inspected
periodically to remove debris and to document and repair any holes or sagging areas. Carp were not actively
removed from behind the barriers.
Field Surveys
2010 Survey
We conducted an initial assessment of rice stem density (stems/m2) in the southern bay on Sept 9, 2010 to
determine rice abundance in the bay prior to excluding carp. For this survey, we established 10 transects that
ran from the northern edge of the observed area of rice growth to the southern shoreline (roughly
perpendicular to shore, Fig 2). Along each transect, we sampled wild rice stem density after every 10 polepushes (10-ft long ricing pole); or roughly every 100 ft.
At each 10-push distance interval, we recorded the GPS location using a handheld Garmin GPS-76, measured
water depth, visually rated the density of rice growth in the immediate vicinity as sparse, moderate, dense, or
very dense, and counted rice stems emerging from within delineated sample areas (quadrats). The quadrat size
for each location was selected based upon the qualitative rice density ratings (Table 1). We chose to use larger
quadrats in sparse and moderate stands of rice (12.3 m2 quadrat and 3.1 m2 quadrat respectively) to increase
detection of rice stems, as these areas generally had very low stem density with widely-spaced clusters of
stems. These large quadrat areas were delineated using a PVC pole (cut to the appropriate length (3.5 m or
1.75 m) and held parallel to and then perpendicular to the side of the boat. When using these large quadrats,
we collected one sample from each side of the boat (2 samples per location). At near-shore sites with dense or
very dense rice (4 sites total), we used smaller quadrat frames (0.10 m2 and 0.05 m2 quadrats respectively).
Using these smaller frames greatly reduced the amount of time needed to count stems in areas with denser
rice growth. When using these smaller quadrats, we collected 2 samples from each side of the boat (4 samples
per location).
Table 1. Quadrat frame size selection criteria used during the Sept. 2010 survey (based upon subjective rice density ratings).
Rice Density Rating
Sparse
Quadrat Dimensions
3.5 x 3.5 m
Quadrat Area
12.3 m
2
2
Samples per Location
2
Moderate
1.75 x 1.75 m
3.1 m
Dense
0.32 x 0.32 m
0.10 m
2
4
0.05 m
2
4
Very Dense
0.22 x 0.22 m
2
2011 Survey
We surveyed rice in the bay once again on August 30, 2011 to assess any change in rice abundance after the
carp barriers had been in place for 1 summer. For this survey, we modified the survey methods used in 2010 to
allow for more rapid sampling, reduce damage to rice stands, standardize data collection, and simplify
statistical analyses for future comparisons. These modifications included (1) using a kayak instead of a jon-boat
to minimize destruction of rice plants, (2) using only one quadrat frame size (1.5 x 1.5 m), and (3) establishing
random sample points across the entire bay (94 locations, Fig 3). At each sampled location we recorded water
depth, noted the presence of additional plant taxa, and counted rice stems emerging from within the
delineated quadrat areas. We collected 2 quadrat samples from each side of the kayak at all locations (4
samples per location).
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
2012 and 2013 Surveys
We surveyed rice in the bay again on September 13, 2012 and September 24, 2013 to track rice abundance in
the bay after the carp barriers had been in place for 2 and 3 summers. Due to the high density of rice plants in
2012 and 2013, we again modified the survey methods slightly to allow for more rapid sampling and reduce
damage to rice stands. These modifications included (1) using a canoe and push-pole to navigate through
areas of denser rice growth (could not kayak through dense rice growth) and (2) using a smaller quadrat frame
(0.1 m2) at all sites. In 2012, we used the same set of 94 sample points used in 2011. However, in 2013, the rice
growth was very dense and widespread, making sampling difficult. Consequently, we sampled only the evennumbered points (46 total; randomly located throughout the bay).
Data Analysis
For each survey, we calculated the wild rice stem density (stems/m2) for each collected sample by dividing the
total stem count by the quadrat area. We then calculated the mean stem density and standard error at each
sampled location and across all sampled locations (bay-wide statistics; Table 2). Plotted histograms of 2010
stem density data clearly indicated that the data were not normally distributed (skewed to the right).
Accordingly, we chose to use nonparametric statistics to evaluate between-year changes in stem density (Zar
2010). We used a Wilcoxon rank-sum test to compare between 2010 and 2011 (different points used in each
year) and a Wilcoxon matched-pairs signed-ranks test to compare between 2011 and 2012 (same set of points
in both years), and between 2012 and 2013 (same subset of points, even-numbered points).
The 2010 survey did not include samples from the entire bay, as we only sampled in the southern one-third of
the bay where rice was observed to be growing. Consequently, we were not able to make bay-wide
comparisons of rice density between 2010 and 2011. Instead, we limited our statistical comparison for those
two years to include only the data that were collected from the same area during both surveys. This included
data from points that covered roughly the southern one-half of the bay (Fig 2). We also compared the results
of the two surveys visually by creating wild rice stem density maps using GIS software (ArcView 3.3; ESRI –
Redlands, CA). These maps estimated stem densities between sampled points using IDW interpolation.
Figure 2. Maps showing the locations where wild rice abundance (stems/m2) was surveyed (from 2010 to 2013).
The portion of the bay from which carp were excluded in 2011 and 2012 is shaded in dark-blue. For comparisons
between 2010 and 2011, we only used data from points south of the dashed line; for 2011 and 2012, we used all
of the points shown, and for 2012 and 2013, we used only the even-numbered points.
2010
2010
Freshwater Scientific Services, LLC – October 2013
2011
2011 &
& 2012
2012
2013
2010
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Results & Discussion
Year 1
In 2010, the year before carp were excluded from the bay, we observed almost no rice growth in the northern
half of the bay (only a few individual plants), with the southern half generally supporting only sparse rice in
open-water areas (<5 stems/m2) and moderate density rice growth (>50 stems/m2) in a few isolated shallow
areas along the southern shoreline (Fig 3). In 2011, after one summer of carp exclusion, we found sparse rice
growth (<5 stems/m2) throughout the bay, but the mean density of rice in the bay did not increase relative to
2010 (Table 2, Fig 3). Statistical analyses showed that mean wild rice stem density in the southern portion of
the bay decreased from ~10 stems/m2 in 2010 to ~2 stems/m2 in 2011 (Table 2; p = 0.03, Wilcoxon rank-sum
test). However, much of this decrease appeared to be the result of more intensive sampling immediately
adjacent to shore in 2010 (different set of points used). Four of the near-shore samples assessed in 2010
supported much denser rice growth (50-150 stems/m2) than observed at other locations; roughly 10 to 30
times greater than seen at any of the other of the locations sampled in 2010. These high values heavily
influenced the calculated statistics and the interpolated stem density maps for 2010 (Table 2, Fig 3). When
these four near-shore points were excluded from the analysis, the average stem density in 2010 dropped from
~10 to ~4 stems/m2, and was more similar (p = 0.10) to the average stem density we observed in 2011.
Years 2 and 3
Unlike the 2010 survey, the 2011 and 2012 surveys sampled rice growth over the entire bay. In 2012, after 2
summers of carp exclusion, we observed a dramatic increase in the mean density of rice growth throughout
the bay (Fig 3), increasing from 1.2 ±0.4 stems/m2 in 2011 to 53 ±10 stems/m2 in 2012 (p <0.001; mean ±2SE).
This trend continued between 2012 and 2013, with mean stem density increasing further to 85 ±14 stems/m2
in 2013 (p = 0.006). Furthermore, the standard error of stem density in 2013 (±7) was similar to that seen in
2012 (±5) despite the reduced number of samples collected in 2013, suggesting that the reduction in the
number of samples was justified.
Table 2. Comparison of wild rice stem density (stems/m2; mean ±2SE) in the southern bay of Upper Clam Lake in
each year from 2010 to 2013. P-values (p) associated with each year reflect the significance of changes in mean
stem density from the previous year; calculated using Wilcoxon rank-sum test for comparison of 2010 to 2011
(different points used in 2010 and 2011), and Wilcoxon matched-pair signed-ranks test for comparison 2011 to 2012
(same points used in both years) and 2012 to 2013 (same subset of even points compared).
Whole Bay
Samples
Stems/m2 (±2SE)
2010
57
2011
93
Survey
2012
2013
86
b
46
c
Southern Portion Only
p
Samples
Stems/m2 (±2SE)
p
~5 a
–
57
9.9 ±6.8
–
1.2 ±0.4
–
37
2.3 ±0.8
0.03
53 ±10
<0.001
32
87 ±13
<0.001
75 ±21
<0.001
85 ±14
0.006
17
c
a
In 2010, samples were collected only in roughly the southern half of the bay. However, we observed almost no rice growth in the
northern half of the bay. We have estimated the bay-wide mean stem density as one-half that observed in the southern portion
of the bay in 2010.
b
In 2012, 7 points immediately adjacent to the southern shoreline were not accessible due to very shallow water and dense rice.
C
In 2013, only even-numbered locations were sampled (subset of same points used in 2011 and 2012)
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Figure 3. Maps of wild rice stem density in Sept of 2010, 2011, 2012, and 2013 (IDW interpolation of stem density at sampled points)
2010 Survey
2011 Survey
2
<1
0 / No Data
2012 Survey
2013 Survey
Figure 4. Aerial images of the southern bay of Upper Clam Lake (carp excluded) taken in Aug 2012 (left) and Aug 2013 (right) showing
the extent and density of rice growth in each year. Shoreline traced in yellow.
N
N
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Figure 5. Sparse, patchy rice growth (<5 stems/m2)
typical of most areas of the bay that supported rice in
2010 and 2011, and in northern portions of the bay in
2012.
Figure 6. Moderate density rice growth (~10 to 20 stems/m2;
foreground) as found immediately along shore in 2010 and
2011, in the north-central portion of the bay in 2012, and
along the southern shoreline in 2013.
Figure 7. Dense rice growth (~100 to 150 stems/m2)
as found in the southern one-third of the bay in 2012
and throughout most of the bay in 2013.
Freshwater Scientific Services, LLC – October 2013
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Evidence of Reduced Carp Activity
We did not directly measure carp abundance in the southern bay after the net barriers were installed, but the
nets appeared to reduce carp activity in the bay. We observed a small amount of carp activity in the bay in
both 2011 and 2012 after the nets were installed (bubble trails and v-waves), suggesting that some carp had
moved into the bay before the net barriers were installed or had been able to get past the nets during the
summer. However, the amount of carp activity in the bay appeared to be much lower than observed in the
open lake where we documented very high carp activity in similar shallow areas (video documentation). In
2013, we did not observe any carp activity in the bay. Furthermore, we observed a dramatic increase in native
aquatic plants in the area behind the nets in 2011 and sustained diversity of native plants in 2012 and 2013.
Overall, the abundance and diversity of aquatic plants appeared to be substantially greater in the bay than in
the areas immediately outside of the carp barrier nets, and was also much greater than seen in the bay in
previous years. In 2011, we observed dense growth of native aquatic plants over roughly 80% of the bay, with
many areas supporting a fairly diverse assemblage of plant species (Table 3). In 2012 and 2013, we observed a
similar diversity of native aquatic plants in the bay, but overall abundance appeared to be lower in areas with
dense rice growth. These dramatic changes in native plant abundance and diversity were very similar to what
we observed during the carp exclosure plot experiment conducted in 2010 (Johnson and Havranek 2010). In
that study, we observed much denser and more diverse growth of native aquatic plants inside our carp
exclosures (no carp), with substantially lower plant abundance and lower diversity immediately outside of the
exclosures where carp were present (Figure 8).
Table 3. List of aquatic plant taxa observed growing in the southern bay of Upper Clam Lake
in Aug 2011 after carp were excluded.
Common Name
Taxonomic Name
Frequency
Bushy Pondweed
Canadian Waterweed
Coontail
Narrowleaf Pondweed
Common Bladderwort
Flat-stem Pondweed
Floating-leaf Pondweed
Illinois Pondweed
Muskgrass
Northern Watermilfoil
River Bulrush
Spatterdock
Water Marigold
White Watercrowfoot
White Waterlily
Arrowhead
Najas flexilis
Elodea canadensis
Ceratophyllum demersum
Potamogeton spp.
Utricularia macrorhiza
Potamogeton zosteriformis
Potamogeton natans
Potamogeton illinoensis
Chara spp.
Myriophyllum sibiricum
Schoenoplectus fluviatilis
Nuphar variegata
Bidens beckii
Ranunculus longirostris
Nymphaea odorata
Sagittaria spp.
Common
Common
Common
Common
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Occasional
Rare
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Effectiveness of Temporary Carp Barriers for Restoring Wild Rice Beds in Upper Clam Lake (Burnett Co., WI)
Figure 8. Images showing the dramatic difference in the abundance and diversity of native aquatic plants inside and
immediately outside plots where carp had been excluded for one full summer (Johnson and Havranek 2010).
Carp
Carp
No
No Carp
Carp
No
No Carp
Carp
Carp
Carp
Wild Rice Management Implications
The 2010 carp exclosure plot experiment clearly indicated that exclusion of carp alone did not result in rice
growth; seeding of rice was also necessary (Johnson and Havranek 2010). In that study, it appeared that the
wild rice seed bank in lake sediments had been severely depleted by carp. This suggests that similar seed
depletion may have occurred in the southern bay over the past 10 years, and likely explains why we did not
see a recovery of dense rice growth in the bay after only one season of carp exclusion.
The dramatic recovery of dense rice growth in the southern bay after 2 to 3 years of carp exclusion without
supplemental manual seeding strongly suggests that rice beds may naturally recover in the rest of the lake if
carp abundance can be reduced. In fact, a few areas in the open portion of the lake (particularly the shallow
southeastern portion of the lake) clearly supported low to moderate density rice growth in 2013 after several
years of carp removal. This suggests that the carp population may have been suppressed sufficiently by recent
nettings to allow natural recovery of rice in the lake. However, the abundance of rice seeds in most of the lake
is likely extremely low (Johnson and Havranek 2010). Consequently, in the first few years of recovery, rice
growth will likely be sparse and very susceptible to grazing by the remaining carp. Accordingly, the recovery of
dense rice beds in the open lake would almost certainly be enhanced by manual seeding with seed collected
from the fully recovered southern bay. Furthermore, the carp-barrier nets should continue to be installed at
the same locations each spring to maintain the current “nursury stock” of rice seed in the southern bay until
large-scale rice bed recovery is achieved in the rest of the lake.
References
Johnson JA, Havranek AJ. 2010. Effects of carp on the survival and growth of wild rice in Upper Clam Lake –
Burnett County, WI. Final report to St. Croix Tribal Environmental Services – Natural Resources Department,
Webster (WI). Freshwater Scientific Services, LLC; Maple Grove (MN). 18 pp.
http://www.freshwatersci.com/fw_projects.html
Walker RD, Pastor J, and Dewey BW. 2006. Effects of wild rice (Zizania palustris) straw on biomass and seed
production in northern Minnesota. Can J Bot. 84: 1019-1024.
Zar JH. 2010. Biostatistical analysis, 5th ed. Upper Saddle River (NJ). Pearson Prentice Hall.
Freshwater Scientific Services, LLC – October 2013
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