Anderson Lake (Oconto County) Late-Summer 2017 EWM Survey Results Report
Earlier this week (September 25), we had two crews on Anderson Lake conducting the late-season EWM mapping survey. The conditions were perfect for the survey, with full sun and almost no wind. We were thankful of the good conditions, as the plethora of northern watermilfoil present within the system required a high level of attention to discern the difference. Our crews meandered the area in which EWM could inhabit in Anderson Lake, out to approximately 15 ft. During the survey, EWM was located between 1 and 8 feet of water, with the average water depth being 5 feet deep. All EWM incidences were mapped with a sub-meter GPS using either points or polygons. Almost all EWM occurrences were marked with point-based methods, where a single GPS point represented the EWM at that location as being either a single or few plants, clump of plants, or small plant colony . We did mark 3 areas that contained an EWM colony that we were able to delineate the extents and attribute the density using a 5-tiered scale (Highly Scattered < Scattered < Dominant < Highly Dominant < Surface Matting). In the north part of the lake where you alerted our attention, we found adominant colony surrounded by scattered EWM (about 1/3 of an acre in total). In the eastern part of the lake, there is a small highly dominant colony (less than a 10th of an acre). In the southeastern part of the lake, there is a larger highly dominant colony with scattered EWM surrounding it (less than a 1/4 of an acre)
Unfortunately, we found more EWM that you likely suspected there to be in the lake. At this point, we see three options moving forward of which we would be supportive of fleshing out a strategy for either option 1 or option 2.
Option 1: Periodically monitor the EWM population. In some lakes, EWM populations reach an equilibrium level that is not exerting an overly negative stress on the function of the ecosystem and doesn’t interfere with peoples use of the lake. If the EWM population of Anderson Lake stayed at its current levels, there would be no need to do any active management. It may be a good idea to monitor the population at roughly this time next year to see if it gets any worse before jumping into an active management program. We could design a project where the costs of the survey(s) could be shared by the WDNR through an AIS-Early Detection and Response Grant.
Option 2: Conduct hand-harvesting effort. At this early stage in a new population, conducting management can alter the trajectory of the population greatly. Considering the small size and relatively low density of EWM in Anderson Lake, the population may be able to be tackled by hiring divers to hand-remove the EWM. Some professional hand-harvesting firms use basic snorkeling or scuba divers, whereas others might employ the use of a Diver Assisted Suction Harvest (DASH) which involves divers removing plants and feeding them into a suctioned hose for delivery to the deck of the harvesting vessel. The DASH methodology is considered a form of mechanical harvesting and thus requires a WDNR approved permit. DASH is thought to be more efficient in removing target plants than divers alone and is believed to limit fragmentation during the harvesting process. While every situation is different, a budget of $4K may allow divers to impact the three colonies of EWM in the lake. Because of the plethora of native plants in these areas, we would likely need to have the hand-harvesting take place earlier in the growing season when the native plants are smaller. These costs would be applicable to an AIS-Early Detection and Response Grant, as well as the pre/post monitoring and assessment.
Option 3: Conduct spatially targeted herbicide spot treatments. Some lake groups wish to take an aggressive approach to a newly identified EWM population and target the area with an herbicide treatment. As a hardy perennial, EWM is most effectively treated with a systemic herbicide like 2,4-D. This herbicide needs a relatively long exposure period. Ongoing studies are indicating that in small spot treatments (working definition is less than 5 acres) the herbicide dissipates too rapidly to cause EWM mortality if systemic herbicides like 2,4-D are used. Even in some cases where larger treatment areas can be constructed, their narrow shape or exposed location within a lake may result in insufficient herbicide concentrations and exposure times to kill the plants. No areas of EWM in Anderson Lake are large enough to be effectively targeted with systemic herbicides. And with the abundance of northern watermilfoil and water marigold in these areas, the impacts of an herbicide treatment to the native plant community may be more harmful to the lake than the current level of EWM. Therefore we do not recommend an herbicide treatment strategy at this time. But if the EWM population increases in the future, these concepts may require revisiting for discussion.
As mentioned above, we think you should share this report with your team and consider options 1 and 2. We would be glad to host a teleconference if you and a few folks would like to discuss these options further and dive into the details.
Eddie J. Heath
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2014 Fall meeting / Guest speaker: Linda Williams / WDNR
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