Effect of Triploid Grass Carp on the Aquatic Macrophyte Community of Devils Lake, Oregon

Abstract Two years after stocking 274-hectare Devils Lake, Oregon, with 27,090 triploid grass carp Ctenopharyngodon idella (180 fish/vegetated hectare, 6.1 fish/tonne wet vegetation), the total volume of vegetation declined by 30%, whereas the total vegetation biomass increased, primarily from the expansion of short, compact communities of Brazilian water-weed Egeria densa. Comparison of the open lake with areas protected by net barriers, and the results of grass carp feeding preference experiments, suggested that the increased dominance of E. densa was caused by factors other than selective feeding. Furthermore, as the grass carp grew, they seemed to slow the invasion of E. densa and to help maintain species diversity in the plant community. Feeding experiments suggested that food preference was related to grass carp size, which should be considered when evaluating control of target plant species. Grass carp can provide economical and effective control of plants in cool northern waters, although stocking...