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 vessels 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, although these included many taxa already found in the Great Lakes, Four freshwater-tolerant zooplankton species that were identified as living specimens in ballast water have apparently not been recorded from the Great Lakes. Voluntary ballast water controls reduced but did not eliminate the risk of species invasion, since some ships did not comply with the guidelines, and even ships that did exchange ballast water could introduce viable freshwater-tolerant organisms into the Great Lakes. About half of the ballast water carried into the Seaway by ocean-going vessels and lakers each year originates from the St. Lawrence River, portions of which are not yet protected by any ballast controls.