Potential Effect of Grass Carp Herbivory on Waterfowl Foods

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Reservoirs in the southern United States provide important habitats for migrating and wintering waterfowl. Use of grass carp (Ctenopharyngodon idella) to control nuisance aquatic vegetation may reduce habitat quality for waterfowl because foods of grass carp and waterfowl overlap. In 1992-93 we used exclosures (1 m 2 ) to investigate effects of herbivory on introduced Eurasian watermilfoil (Myriophyllum spicatum; hereafter referred to as milfoil) and several other aquatic plant taxa in Guntersville Reservoir, Alabama. We compared exclosure and open plot treatments in milfoil and native plant areas in a large embayment. Milfoil within monospecific stands was not affected (P = 0.566) by herbivory in either year, but wet mass of milfoil at the edge of monospecific stands at 1 of 4 sites in 1993 was greater (P < 0.001) in exclosures than in open plots. Wet mass of native vegetation was greater (P < 0.001) in exclosures than in open plots in 1992-93. Taxa in native plant sites included muskgrass (Chara spp.), milfoil, southern naiad (Najas guadalupensis), slender spikerush (Eleocharis acicularis), pondweed (Potamogeton spp.), and spinyleaf naiad (N. minor; introduced exotic). In both years, milfoil and southern naiad dry mass was greater (P < 0.001 and P = 0.001, respectively) inside exclosures, while slender spikerush dry mass was greater (P = 0.043) outside exclosures. Dry mass of muskgrass and pondweed was greater (P < 0.001 and P = 0.002, respectively) inside exclosures than outside in 1992, but not different (P = 0.590 and P = 0.315, respectively) between inside and outside exclosures in 1993. Herbivory, predominately by grass carp, reduced native aquatic vegetation in some areas while milfoil remained unaffected. We discourage use of grass carp to control aquatic vegetation in areas dominated by plant species that are poorly consumed (i.e., milfoil) by grass carp without consideration of the potential effect on more palatable species.