FISHERIES MANAGEMENT Managing Macrophytes to Improve Fish Growth: A Multi-lake Experiment By Mark H. Olson, Stephen R. Carpenter, Paul Cunningham, Sarig Gafny, Brian R. Herwig, Nathan P. Nibbelink, Tom Pellett, Christine Storlie, Anett S. Trebitz, and Karen A. Wilson ABSTRACT Macrophyte harvesting often has been suggested as a way to improve fish growth and size struc- ture in lakes with high densities of submergent macrophytes and stunted fish populations. How- ever, previous experimental tests have provided no clear consensus on whether the technique works for management. We conducted a series of whole-lake manipulations to test the effects of macrophyte removal on growth of bluegill and largemouth bass. We selected four lakes in south- ern and central Wisconsin for experimental manipulation and nine others for controls. In August 1994, we removed macrophytes from approximately 20% of the littoral zone by cutting a series of evenly spaced, deep channels throughout each treatment lake. In the first year after manipulation, we observed substantially increased growth rates of some age classes of both bluegill and large- mouth bass in treatment lakes relative to controls. Growth rates of other age classes were less responsive to manipulation. We observed increased bluegill and largemouth bass growth despite rapid regrowth of macrophytes in our treatment lakes. By May 1996, fewer than 25% of the chan- nels remained. Our results suggest that harvesting macrophytes in a series of deep channels may be a valuable tool for integrated management of fish and macrophytes. any lakes and reservoirs across North America suf- fer from high densities of submergent macrophytes. The thick beds of vegetation that cover the littoral zone are perennial problems in some lakes and recent developments in others (Colle and Shireman 1980; Cooke et al. 1993). Excessive macrophyte growth often can be traced to the invasion of exotic macrophytes such as Eurasian milfoil (Myriophyllum spicatum) (Aiken et al. 1979) and hydrilla (Hydrilla verticil- lata) (Haller 1979). As these invaders spread, they displace the diverse community of native species and create a near-monoculture of dense macrovhvtes (Haller and Sutton * 1975; Nichols 1994). Consequently, the aesthetic quality and recreational value of a lake can decline severely. High densities of macrophytes also can harm the quality of a fish- ery (Wiley et al. 1984; Bettoli et al. 1992). Dense macrophytes can cause panfish and game fish to be- come stunted via two pathways. First, feeding rates generally are reduced in lakes with dense macro- phytes (Crowder and Cooper 1982). Although prey abundance (either insect or fish) may increase as macrophyte density increases, macrophytes also reduce predator foraging efficiency by providing a refuge for prey. Therefore, feeding rate (determined by the combina- tion of prey abundance and forag- ing efficiency) is maximized at intermediate macrophyte densities and reduced as densities increase beyond that point (Heck and Crow- der 1991; Savino et al. 1992). Sec- ond, dense vegetation generally reduces foraging efficiency of pisci- vores and lowers predator-induced mortality rates of small fishes (Savino and Stein 1982; Gotceitas and Colgan 1989). This reduction in mortality leads to a greater popula- tion density and stronger competi- tive interactions among small fishes (Mittelbach 1988). Macrophyte Harvesting as a Management Strategy The potential for improving growth and size-structure of fishes by reducing macrophyte densities Mark H. Olson is a senior research associate at the Cornell Biological Field Station, 900 Shackelton Point Road, Bridge- port, NY 13030; mho2@cornell.edu; FAX 315/633-2358. Stephen R. Carpenter is a Halverson professor of limnology at the Centerfor Limnology, University of Wisconsin. Paul Cunningham is a fisheries ecologist at the Bureau of Fish Manage- ment, Wisconsin Department of Natural Resources. Sarig Gafny is a research scientist at the Institute for Nature Conserva- tion Research, Tel Aviv University, Israel. Brian R. Herwig is a research technician at the Center for Limnology, University of Wisconsin. Nathan P. Nibbelink is a research assistant at the Department of Zoology and Physiology, University of Wyoming. Tom Pellett is an aquatic ecosystems ecologist for the Bureau of Integrated Science Services, Wisconsin Depart- ment of Natural Resources. Christine Storlie is a fisheries ecologist at the Bureau of Integrated Science Services, Wisconsin Department of Natural Resources. Anett S. Trebitz is a research ecologist at the Environmental Protection Agency in Min- nesota. Karen A. Wilson is a research assistant at the Centerfor Limnology, University of Wisconsin. Vol. 23, No. 2 6 Fisheries