The overall diversity of biota found in ballast and the complexity of ballasting patterns and operations mean that predicting the presence of a particular unwanted species in any one vessel is a scientific challenge. Similarly, the great diversity of the larval or juvenile stages of marine invertebrates and fish in ballast water, many of which are not identifiable, means that certifying a vessel as free or safe from all unwanted species is not possible. A further complication is that many of the most prominent introductions of recent years were not recognized as problem species in their donor regions. Thus the Asian clam, Potamocorbula amurensis, which has led to fundamental changes in the energy flow and dynamics of the San Francisco Bay estuary since its appearance in 1986 (Carlton et al., 1990; Alpine and Cloern, 1992), was not signalled as a species of concern in Western Pacific estuaries, nor would its larvae have been identifiable if ballast sampling programs had been in place in California prior to 1986.
In terms of managing ballast water, the multiplicity of factors influencing the establishment of nonindigenous organisms means that species (particularly those from comparable environments) found in ballast water samples that have not previously or subsequently been found in the receiving port cannot be assumed to be safe species that could be put on a ''clean" list. Possible explanations have been postulated for the observation that a species may eventually become extraordinarily successful in waters where it may have been inoculated for many decades. The zebra mussel, Dreissena, in the North American Great Lakes (Carlton, 1996) is one example. Important factors include changes in the donor region, new donor regions, changes in the recipient region—including improved water quality—the opening of introduction windows (the proper combination of environmental conditions), stochastic inoculation events, and changes in the dispersal vector. As long as there are transport mechanisms, such as ballast water, there may be new introductions.
The objective of developing strategies for managing ballast water is to reduce the likelihood of new invasions to an acceptable level without in any way compromising ship safety. This requirement is complicate by the diversity of potential invaders and by the many environmental factors—often overlapping and synergistic—determining the fate of organisms discharged with ballast water.
A ship is a biological island (see Box 1-1). Besides the potential complement of synanthropic hitchhikers (e.g., mice, rats, flies, and cockroaches) and the rich bouillabaisse of ballast water and sediment, living marine and estuarine organisms can occur elsewhere in and on the vessel. In this regard, it is necessary to distinguish the unique role of ballast water an sediment in transporting non-indigenous species from other ways in which a vessel may transport organisms.