From the book Stemming The Tide

   It has been estimated that in the 1990s ballast water transported over 3,000 species of animals and plants a day around the world and there is evidence that the number of ballast introductions is growing. More than 40 species have appeared in the Great Lakes since 1960; more than 50 have appeared in San Francisco Bay since 1970. Virtually all ballast water samples taken to date in Canadian, Australian, German, and U.S. studies contained living micro-organisms, indicating that ballast water, is a viable habitat for a wide variety of freshwater, brackish water, and saltwater organisms. Virtually all organisms less than 1 cm in size that are adjacent to a vessel could be ballasted into a vessel. These include viruses, bacteria, plants and animals. Ballast water has a clearly identified role in directly and consistently releasing large numbers of organisms in every major port of the world every day.