BALLAST WATER EXCHANGE AS A MEANS OF CONTROLLING DISPERSAL OF FRESH-WATER ORGANISMS BY SHIPS

During May-December 1990 and March-May 1991, 546 foreign ocean-going v essels entered the Laurentian Great Lakes and upper St. Lawrence River , areas protected by the Great Lakes Bal last Water Control Guidelines . Between 88 and 94% of the vessels exchanged their ballast water with seawater as required by the guidelines. Living representatives of 11 invertebrate phyla were sampled from ballast tanks. Between 14 and 33% of ships that exchanged freshwater ballast in midocean carried living freshwater-tolerant zooplankton at the time of entry to the Seaway, a lthough these included many taxa already found in the Great Lakes, Fou r freshwater-tolerant zooplankton species that were identified as livi ng specimens in ballast water have apparently not been recorded from t he Great Lakes. Voluntary ballast water controls reduced but did not e liminate the risk of species invasion, since some ships did not comply with the guidelines, and even ships that did exchange ballast water c ould introduce viable freshwater-tolerant organisms into the Great Lak es. About half of the ballast water carried into the Seaway by ocean-g oing vessels and lakers each year originates from the St. Lawrence Riv er, portions of which are not yet protected by any ballast controls.