MECHANICAL REMOVAL OF HYDRILLA IN THE POTOMAC RIVER, MARYLAND - LOCALIMPACTS ON VEGETATION AND ASSOCIATED FISHES

Tidal freshwater fish assemblages were sampled quantitatively from bed s of Hydrilla verticillata in the Potomac River, Maryland, to assess l ocal impacts of mechanical plant harvesting. Seasonal and diel estimat es of fish density, biomass and species richness were compared at an u ndisturbed site and an adjacent mechanically harvested site. Harvestin g had a ''pruning'' effect on Hydrilla: plant biomass levels at the ha rvested site exceeded those at the undisturbed site after periods > 23 d. Fish species composition at the two sites was very similar and spe cies richness differences were minor. Significantly lower (P<0.05) mea n fish density and biomass values were found at the harvested versus t he undisturbed site < 23 d after plant removal. However, 43 d after pl ant removal, fish density was significantly higher (P<0.05) at the har vested site, while biomass differences were minor. Species-specific di fferences suggested that harvesting improved habitat for pelagic speci es (e.g., Menidia beryllina) but negatively affected cover-oriented sp ecies (e.g., Fundulus diaphanus). Results suggest impacts of mechanica l harvesting on the fish assemblages investigated were short-term and minor at the local, community level. However, ten species were killed in the mechanical harvesting process. We estimated that this immediate loss represented 11-22% of fish numbers and 4-23% of biomass. Mechani cal harvesting, when macrophyte beds are in short supply, may not be p rudent.