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Edina’s Arrowhead Lake
GOLDFISH ASSESSMENT - 2024
Report for the City of Edina
Hennepin County, Minnesota
City of Edina, Minnesota
December 31, 2024
Prepared by:
Mary Newman, Sr. Professional Environmental Scientist
Phil Carson, Sr. Professional Environmental Scientist
Tony Havranek, Director of Fisheries
WSB, 178 East 9th Street, Suite 200, St. Paul, MN 55101
Arrowhead Lake Goldfish Population Assessment
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Table of Contents
TABLE OF CONTENTS
SUMMARY .................................................................................................................................................. 1
BACKGROUND AND OBJECTIVES ..................................................................................................... 2
METHOD and RESULTS ......................................................................................................................... 3
Arrowhead Goldfish Abundance Estimate .................................................................................... 3
Goldfish Abundance Estimate - Results ........................................................................................ 4
Arrowhead Goldfish Aging Analysis .............................................................................................. 5
Goldfish Aging Analysis - Results .................................................................................................. 5
DISCUSSION AND RECOMMENDATIONS ......................................................................................... 8
Discussion ............................................................................................................................................. 8
Recommendations ............................................................................................................................... 8
REFERENCES ........................................................................................................................................... 9
Arrowhead Lake Goldfish Population Assessment
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Table of Contents
TABLE OF FIGURES
Figure 1. Arrowhead Lake, Edina, Hennepin County, Minnesota. ..................................................... 3
Figure 2. Photo of a goldfish captured in 2024 sampling event on Arrowhead. ............................... 4
Figure 3. Length histogram of goldfish captured in 2024 boat electrofishing surveys in
Arrowhead Lake. ........................................................................................................................................ 6
Figure 4. Length at age of goldfish sampled at Arrowhead Lake. ...................................................... 7
Figure 5. Relationship of length and weight of sampled goldfish. ...................................................... 7
TABLE OF TABLES
Table 1 - Electrofishing survey results for Arrowhead Lake. ............................................................... 4
Table 2 – 2024 goldfish population summary, Arrowhead Lake. ........................................................ 4
Table 3 – 2024 goldfish aging results. .................................................................................................... 6
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 1
SUMMARY
Arrowhead Lake in Edina, Hennepin County, Minnesota was sampled via boat
electrofishing from August through September 2024. Electrofishing surveys were used
to calculate catch per unit effort (CPUE) and estimate population size and biomass
(kg/ha) of Goldfish (Carassius Auratus). Surveys showed that estimated biomass for
Arrowhead Lake was below the ecological threshold of 100 kg/ha that has been
established for carp (Bajer et al. 2009). The goldfish biomass and population in
Arrowhead Lake was estimated at 14 kg/ha or approximately 215 individuals. An
age/size structure analysis of the goldfish sampled showed a population trend of large,
relatively old adult individuals, as average age was 8.8 years and length 13.4 in.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 2
BACKGROUND AND OBJECTIVES
Goldfish are classified as a regulated invasive species in Minnesota, meaning
introduction into the wild is prohibited. They are closely related to common carp and can
have negative impacts on water quality in much the same way. Feeding behavior of
both species can uproot native aquatic vegetation and suspend sediments into the
water column, leading to a decline in water quality and ecological integrity.
The suspension of sediments and reduction in rooted submergent aquatic vegetation
leads to increased turbidity and increases the internal load of phosphorous, a primary
nutrient in lake ecosystems and one of the primary pollutants in midwestern waters.
Excess phosphorous leads to increased algal production, reduced water clarity, and
inhibits waterbodies from meeting numerical and narrative state water quality standards.
Arrowhead Lake is impaired for excess nutrients, confirmed by water quality studies
completed in 2022 by the Nine Mile Creek Watershed District. The City of Edina and the
watershed district have taken steps to reduce both internal and external nutrients to the
lake. Among those efforts, an aeration unit on Arrowhead that runs year-round is
designed to keep oxygen available in the water column to prevent fish-kill events. An
alum treatment was applied to Arrowhead Lake in early 2024 and has produced a
marked reduction in turbidity to the basin through the growing season in 2024.
The city also pursued fish surveys completed in 2022 that revealed the presence of
goldfish in the basin. The City of Edina and the Nine Mile Creek Watershed District,
wanted to know if this species was abundant enough in Arrowhead Lake to negatively
impact water quality. This 2024 study is the first step to invasive carp and goldfish
management and aims to develop an abundance estimate of goldfish in the basin.
Abundance information can be used to determine management actions, if any, and to
track progress towards management goals. In this project period, an aging analysis is
also pursued. The aging analysis is coupled with a histogram of lengths collected in this
project period to begin describing the age classes present in the lake. Understanding
age classes can help to determine how often recruitment of young goldfish occurs and
can be used to guide management actions.
This project had two (2) primary objectives. These included:
1) Develop Goldfish abundance estimate for Arrowhead Lake, Edina, MN.
2) Age a subsample of goldfish collected in sampling events.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 3
*LAKE NAME DOW ID Total Acreage Littoral Acres
Arrowhead 27004500 22.03 22.03
Figure 1. Arrowhead Lake, Edina, Hennepin County, Minnesota.
METHOD and RESULTS
Arrowhead Goldfish Abundance Estimate
To develop an understanding of the abundance of invasive goldfish in Arrowhead Lake,
a common carp catch per unit effort (CPUE) abundance estimate is applied (Bajer and
Sorenson, 2012). To collect data to fit to this model, Arrowhead Lake was sampled on
three dates August 22, August 28, and September 3, 2024 using a boat electrofishing
unit. Field sampling on each date consisted of three 15-minute boat electrofishing
transects in the littoral zone and the transects covered 85-95% of the shoreline. The
lake was visited in the late summer – fall to fit within the model parameters.
For each survey date, goldfish captured were enumerated, measured, weighed, and
kept for the collection of aging-structure (otolith). Counts were used to calculate mean
CPUE and to generate population density and biomass estimates. Lengths and weights
were used to generate size-structure plots.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 4
Figure 2. Photo of a goldfish captured in 2024 sampling event on Arrowhead.
Goldfish Abundance Estimate - Results
Goldfish were sampled in the August 22 and September 3 electrofishing surveys but not
on August 28. Catch rate of goldfish overall was low and the calculated estimate of 14.2
kg/ha is below the ecological threshold of 100 kg/ha.
Table 1 - Electrofishing survey results for Arrowhead Lake.
DATE ELECTROFISHING
TIME (MINUTES) GOLDFISH CAUGHT
8/22/2024 15 1
8/22/2024 15 0
8/22/2024 15 3
8/28/2024 15 0
8/28/2024 15 0
8/28/2024 15 0
9/3/2024 15 4
9/3/2024 15 1
9/3/2024 15 1
Table 2 – 2024 goldfish population summary, Arrowhead Lake.
Mean
Length (in)
Mean
Weight
Lake
Area (ha)
Population
Estimate SE
Biomass
(kg/ha) SE
13.4
0.59 kg
(1.3 lbs) 8.9 215 150 14.2 10
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 5
Arrowhead Goldfish Aging Analysis
Goldfish that were collected during the electrofishing sampling events were euthanized
and brought to WSB laboratory to process for otolith extraction. Otoliths are ear bones
found within the skull cavity and are commonly used as a fish aging structure. Growth
rings (annuli) are visible under a microscope when otoliths have been processed and
mounted on a slide. These annuli are similar to growth rings that can be found within a
tree trunk.
Otoliths were sectioned and annuli counted under a microscope by two independent
readers in order to reduce bias. Any otoliths for which there was a discrepancy in age
between the two readers were analyzed a third time in a “concert read.” This consisted
of the two readers ageing the otolith together and coming to agreement on the number
of annuli present. Agreed upon age from the “concert read” was considered the final
age estimate and was used in analysis.
Figure 3. Image of a sectioned goldfish otolith.
Goldfish Aging Analysis - Results
Goldfish lengths ranged from 12.7 in to 15.4 in with a mean length of 13.4 in and mean
weight of 1.4 lbs (Figure 3; Figure 5). Fish ranged in age from 7 to 11 years old with
there being a slight correlation between age and length (Figure 4).
The trends described here are based off a small sample size of only nine fish (Table 3).
If more fish were collected during the surveys the correlation between age and length
would have likely been stronger, showing the usual trend of smaller, younger fish and
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 6
larger, older fish. Secondly, the slowing of growth may be at play at around age-7 for
Arrowhead Lake goldfish leading to homogenized size structure despite age class.
Finally, while total length (measuring anterior tip of head to caudal fin tip) is a standard
practice for inland fisheries management, variation in tail length between individual
goldfish may make fork length (measuring anterior tip of head to base of caudal fin) a
more appropriate measurement.
Table 3 – 2024 goldfish aging results.
Sample
# Length (in.) Age
1 12.1 7
2 12.5 8
3 12.5 9
4 13 11
5 13.1 8
6 13.1 8
7 14 7
8 14.5 10
9 15.8 11
Figure 4. Length histogram of goldfish captured in 2024 boat electrofishing surveys in Arrowhead Lake.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 7
Figure 5. Length at age of goldfish sampled at Arrowhead Lake.
Figure 6. Relationship of length and weight of sampled goldfish.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 8
DISCUSSION AND RECOMMENDATIONS
Discussion
While goldfish biomass appears low in Arrowhead Lake, continued monitoring is
recommended as booms in recruitment and rapid growth are not uncommon. Abiotic
factors leading to fish-kill events can drastically alter fish community structure and lead
to reduced populations of fish species that predate upon goldfish at various stages of
their life history (i.e. bluegill sunfish, largemouth bass) contributing to large year classes
of goldfish.
Bluegill sunfish and largemouth bass populations appeared to be in good health and
within the MN DNR normal range based on surveys that were completed by WSB in
2021 and 2022. High numbers of bluegill of various size classes at least partly explain
the low numbers of goldfish in Arrowhead Lake. Largemouth bass in the 12-15 in range
were also abundant across electrofishing transects in 2024. Largemouth bass predation
on young goldfish, likely helps prevent the vast majority of young of year goldfish from
recruiting to adult size classes.
Arrowhead managers have gained information on recruitment variability through aging
analysis in 2024. It appears from the data collected that the population of goldfish in
Arrowhead Lake are larger and older and recruitment events may be infrequent.
However, the small sample size may not fully distinguish the recruitment frequency of
goldfish in Arrowhead Lake.
Recommendations
1.) Regular sampling every 3-5 years to detect changes in the population of goldfish
in Arrowhead.
2.) Increased sampling of goldfish and native fish assemblage in fall-time following
confirmed or suspected large scale winterkill event in Arrowhead Lake.
3.) Stocking Arrowhead Lake with bluegill and largemouth bass if sampling after a
winterkill event shows a decline in population and/or if goldfish young of the year
are detected.
Arrowhead Goldfish Assessment 2024
City of Edina, Hennepin County, MN
WSB Project No. 024715-000 Page 9
REFERENCES
Bajer, P.G. and P.W. Sorensen. 2012. Using boat electrofishing to estimate the
abundance of invasive common carp in small midwestern lakes. North American
Journal of Fisheries Management 32(5): 817-822.
Bajer, P.G., G. Sullivan, and P.W. Sorensen. 2009. Effects of a rapidly increasing
population of common carp on vegetative cover and waterfowl in a recently
restored Midwestern shallow lake. Hydrobiologia 632(1): 235-245.
Silbernagel, J.J. and P.W. Sorensen. 2013. Direct field and laboratory evidence that a
combination of egg and larval predation controls recruitment of invasive common
carp in many lakes of the Upper Mississippi River Basin. Transactions of the
American Fisheries Society 142(4): 1134-1140.