Table 1. Effectiveness of grass carp for control of aquatic weeds in ponds.
Species Usually Controlled1 Species Sometimes Controlled2 Species Not Usually Controlled
Naiad Duckweeds4 Lotus
Fanwort Watermeal4 Rushes
Hydrilla Aquatic grasses Cattails
Coontail Water pennywort Bulrushes
Pondweed Eurasian watermilfoil3 Smartweed
Bladderwort Waterfern (Salvinia) Maidencane
Watermilfoil3 Mosquito fern (Azolla) Waterlilies
Widgeongrass Spikerushes
Parrotfeather Torpedograss
Creeping rush Alligatorweed
American elodea Waterhyacinths
Brazilian elodea Filamentous algae4
Muskgrass (Chara) Reeds (Phragmites)
Proliferating spikerush Stonewort (Nitella)
Eelgrass (Vallisneria)
Watershield (Brasenia)
1 All of these species are submersed plants.
2 All of these species are floating, floating-leaved, or emergent plants, except Eurasian watermilfoil, stonewort, and filamentous algae. Most of these plants have rather tough, woody stems which can not be eaten.
3 The watermilfoils, particularly Eurasian watermilfoil, are less preferred than many of the other submersed plants and often are not readily eaten until more preferred species have been consumed first.
4 There is some confusion as to how effective grass carp are on these weeds. Small fish stocked in very high numbers (50 to 75 per acre) often are effective. The gill rakers of older fish are spaced too far apart to effectively strain out these plants from the water and thus the fish usually look for more suitable vegetation. Consequently, the grass carp normally stocked for weed control rarely are effective on these plants. Submersed aquatic macrophytes frequently are heavily encrusted with algae and often resemble mats filamentous algae. The fish readily consume these algae-covered plants. This probably has led to the conclusion that the fish are effective on filamentous algae.