North American lakes with heavy infestations of nuisance macrophytes (e.g.- Eurasian watermilfoil, Myriophyllum spicatum) are often associated with slow growing populations of bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides). Reducing macrophyte densities has often been suggested as one way of improving fish growth in such lakes. We conducted a series of planning and modeling exercises to optimize the design of a multi-lake study designed to test the effects of macrophyte harvesting on growth of bluegill and largemouth bass. From a large group of candidate lakes, we selected thirteen lakes in southern and central Wisconsin for study. These had slow growing bluegill populations and were dominated by watermilfoil and other fine-leafed macrophytes. In 1994, macrophytes were mechanically removed from approximately 20% of the littoral zone in four lakes selected for experimental manipulation. The other nine lakes served as unmanipulated controls. Macrophytes were removed in a series of deep channels spaced evenly around the lake with a macrophyte harvester retrofitted with a deep cutting bar to remove macrophytes at the plant-sediment interface at depths deeper than those reached with a commercial harvester.
We observed significant increases in the growth of some age classes of bluegill and largemouth bass in the harvested lakes in the first full year after manipulation. These increases in growth were observed despite rapid re-growth of macrophytes in the harvested lakes. Fewer than 25% of the channels were visible two years after the manipulation. Our results suggest that removing about 20% of the fine-leafed macrophytes, such as watermilfoil, from the littoral zone of heavily infested lakes may be a valuable tool for improving the growth rates of bluegill and largemouth bass.
