Shallow fertile lakes characteristically are turbid with dense algae and few submersed macrophytes when benthivorous fish such as carp are overabundant. These lake conditions often represent a “stable state” until benthivorous fish densities are reduced either by natural die-offs (e.g., winterkill) or through management efforts. Many shallow lakes have been “restored” to a clear-water, aquatic macrophyte state by whole-lake carp eradications, but such chemical treatments are not always effective due to lake size or interconnecting waters. And in urban settings, chemically eradicating fish is sometimes not even possible due to public opposition.
Lake Wingra, an NTL-LTER study site since 1996, is a popular urban lake in Madison, Wisconsin. While the 140-ha lake is turbid with poor water clarity, a diverse community of aquatic macrophytes does exist even though most of the community is dominated by the invasive Eurasian water milfoil (Myriophyllum spicatum). Native macrophytes have been able to survive because the lake has had little aquatic macrophyte management (herbicide treatments or mechanical cutting/harvesting) to control milfoil given most of the lake’s shoreline is natural habitat and in public ownership.
For years, local citizen groups and governmental agencies have been trying to increase water clarity by watershed management practices that target reductions in phosphorus and sediment inputs to the lake. Realizing that real water clarity improvements in the lake would require addressing an overabundant carp problem, local lake managers and scientists in conjunction with NTL-LTER initiated two studies in late summer 2005. One study was a carp exclosure experiment to demonstrate how water clarity would respond to reduced nutrient recycling and sediment resuspension while also testing the response of milfoil and native macrophytes to clearer water. The second study used radio-telemetry to determine when and where carp might be vulnerable to targeted removals by large seines.
Carp exclosure demonstration:
The 1-ha exclosure experiment was conducted for 3 years at a scale that allowed lake managers/researchers and the general public to evaluate whether a whole-lake restoration project centered on reducing carp densities would be worth pursuing (picture at right). Water clarity increased rapidly once the exclosure was installed, but the contrast between the lake and the exclosure was most dramatic during the summer when blue-green algae growth was most abundant (picture at right). As expected, the density and depth distribution of aquatic macrophytes increased in response to the much clearer water, but most of the increased growth was due to Eurasian water milfoil. Native plants expanded their depth distribution only a little after 3 years of clear water and no carp browsing because dispersal in most native plants is mostly through seeds, tubers, and/or rhizomes rather than vegetative cuttings like milfoil. However, in spite of the prospect of increased milfoil growth, the exclosure’s demonstration of how much clearer the lake water could become galvanized public support for removing carp.
Carp radio-telemetry study:
In the fall of 2005, local fish managers implanted radio-transmitters in 14 carp captured from Lake Wingra; following fish recovery NTL-LTER conducted the radio-tracking for 2 years. Results of that study indicated that carp spend most of the open water season in relatively shallow water around the perimeter of the lake with many carp exhibiting fidelity to the same location. One important finding, however, was that in mid-November immediately prior to the lake freezing over, all carp migrated to the deeper, center region of the lake (~3.0-3.5 m) where they remained during most of the winter. This was the perfect location for winter commercial seining to reduce carp densities.
Water clarity response to carp reduction:
In March 2008, a commercial fisher removed 23,600 kg of carp followed by 1,500 kg removed in March 2009. Based on mark recapture estimates, carp densities in the lake declined by 51% (from 351 to 172 kg ha-1). NTL-LTER scientists are closely following the full set of physical, chemical and biological responses in the lake. One sign the lake restoration is working is that water clarity as measured by Secchi disc readings increased soon after the carp removals. In both 2008 and 2009, Secchi readings have been greater than the average seasonal reading for the previous 12 years of record (graph at left). In fact, many seasonal readings in 2008 and especially 2009 have been greater than the maximum seasonal reading observed during the previous 12 years, a condition that is particularly pronounced in the summer months when blue-green algal blooms have been dense.
Carp removal poster at Friends of Lake Winga