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Benton Lake in Cologne has high levels of a nutrient called phosphorus which causes algae blooms in hot summer months. A study done by the Carver County Water Management Organization determined the high phosphorus levels in Benton Lake were primarily coming from within the lake, when carp and other rough fish disrupt lake sediments.
Common carp are an invasive rough fish that can severely impact water quality in lakes and rivers. Carp are bottom feeders, and while rooting around lake beds for bugs and other food they muddy the water and re-suspend phosphorus back into the water column. Their feeding also uproots native aquatic plants which then have a harder time re-establishing in the now muddy water. Without plants, lakes become more turbid (cloudy looking), have more frequent algae blooms and have less or sometimes no habitat for native game fish. Rough fish such as carp and bullheads make up almost all of the fish species found in Benton Lake.
CCWMO staff explored a number of options to remove carp from Benton Lake in order to improve water quality. The first option was a lake drawdown. This option was not favorable as it proved too expensive. The second option was to do a rotenone treatment. Rotenone is a piscicide: it targets fish. A licensed contractor is needed to do the treatment. Staff put together requests for proposals twice but no contractors bid on the project citing concerns about mitigating downstream impacts as a main reason. After discussions with staff from the Minnesota Aquatic Invasive Species Research Center, a third option was developed: a multistep method to control carp populations similar to that done on Starring Lake in Eden Prairie. Research has shown this method can be as effective as a rotenone treatment. The project will consist of 1) Electrofishing and tagging carp to estimate populations and track movements 2) removing carp through traps and seine nets, 3) stocking the lake with bluegill which eat carp eggs, and 4) potentially installing an aerator to help keep bluegill populations alive during the winter. CCWMO has contracted with a consultant, Carp Solutions, to carry out the project.
Tasks and Timeline
The project started fall of 2017. CCWMO expects to continue this contract with Carp Solutions and repeat these steps as necessary for the next 2-3 years.
In September 2017, data on the number of, size and age of carp and other fish (bullheads, fatheads, etc.) in the lake was gathered. To do this staff from CCWMO and Carp solutions used electrofishing to capture fish. All fish were counted and a representative sample was measured for length. Captured fish were fin-clipped and released to collect further data.
Radio-telemetry is used to locate where carp group/aggregate seasonally, to determine migrations and under ice mortality, and places where carp find refuge under the ice. 10 carp caught during the electrofishing had radio transmitters implanted in them.
Show below on left, staff checking bait and net. One right the fishing boat and staff electrofishing.
The two techniques to remove carp that are being tested are 1) baited box net and 2) and water seines. The baited box net method uses 30 ft. by 60 ft. mesh box nets that are placed in the lake and baited with corn. Carp are trained to aggregate in the trap and when they do it is raised quickly capturing the carp inside. This stationary is very selective and does not harm native fish. The second method is water seining. This best is effective during late fall and winter when carp tend to form tight groups.
In October 2017, staff from CCWMO and Carp Solutions removed 4400 carp from the lake through the baited box net method. Staff and consultants are estimating that this first capture is probably 25% of what is in the lake.
In March 2018, box nets were installed at four locations around the perimeter of the lake. The nets were baited with cracked corn all summer to establish a consistent feeding pattern with the carp. The nets were pulled up on June 5th, July 13th, August 10th and Sept. 11th. These three pulls removed about 2,500 carp. One more pull is scheduled for October and we are hoping for the large removal numbers observed last fall. Continued removal efforts are planned for 2019 and beyond depending on grant funding.
In May 2018, CCWMO staff stocked Benton Lake with over 5,000 Blue Gill Sunfish and some Large Mouth Bass. Bluegill, especially, have been shown to eat carp eggs and larvae, thus reducing the numbers of young carp. Staff monitoring shows a good number of bluegills and bass have survived the summer. To help sustain the bass and bluegills, two aerators were installed in June and have been operating since early July. The aerators combat the low oxygen levels that occur during winter.
Once carp populations have been reduced, it is the goal that the water will become clearer which will allow for aquatic plants to grow. The lake has a healthy seed bank for aquatic plants. The aquatic plants with further root the lake sediments in place and keep both phosphorus and sediment from re-suspending into the water column. The stocked bluegills should keep the carp population in check and two carp barriers will prevent new carp from getting into Benton Lake or Meuwissen Lake.
More Information on Research
The Minnesota Aquatic Invasive Species Research Center has completed a number of research projects on behaviors, impacts and removal methods for carp. This includes nearby projects such as Starring Lake in Eden Prairie and a new project on 6 Mile creek and lakes included in its watershed.