other considerations come into play when determining whether a given species will become established (see also Box 3-3). Thus, even when temperatures of discharged ballast water and receiving waters are identical, factors such as salinity, oxygen, light, food resources, competitors, predators, substrate availability, and many others may be inhospitable or limiting.⁵ These biological and ecological factors frequently vary dramatically over seasons and years, with the result that a species that is not successful at one time may later become established (Carlton, 1996).

A critical concept in managing ballast water is that the source regions and release sites of ballast water frequently occur in a complex fashion along the vessel's route. The following hypothetical scenario is an example of how a vessel may have ballast water from multiple sources, with different water in different tanks or mixed in the same tank.

A vessel off-loads its cargo in a discharge port, having arrived with some ballast on board from a prior harbor (site 1). After completing cargo discharge, the vessel ballasts (site 2) in preparation for its transoceanic crossing to pick up cargo for another port. During passage additional water is loaded into storm ballast and cargo/ballast holds because of rough sea conditions (site 3). Some days later, due to fuel and freshwater consumption, more ballast is added mid-ocean for additional stability (site 4). Finally, additional ballast is taken aboard for clearance under a harbor bridge in the arrival port (site 5).

The result of this hypothetical scenario translates biologically into the vessel accumulating organisms from multiple ballastings at several sites. Thus, organisms arriving in ballast water are not necessarily strictly estuarine or coastal in origin, nor do all of them necessarily represent the last port of call. Ballast water may be hours to months old and may contain living organisms for an extended period of time. As noted above, sediment in ballast tanks may reflect an even longer history of ballasting and may include an accumulation of life forms from many ports around the world that have survived in the shipboard environment.

The movement and release patterns of ballast water are such that no coastal site, whether it receives direct shipping or not, is immune to ballast-mediated introductions. It is frequently assumed that only major port systems are at risk from introductions of organisms. However, ships may release ballast water as they pass along coastlines, often sufficiently close to shore that natural onshore advection may carry ballast-discharged organisms into small lagoons, bays, or any other coastal location. Coastal vessel traffic and coastal currents may also disperse introduced species from larger port systems to remote sites all along a coast.