7.4 FACTORS DETERMINING WHETHER VESSELS DISCHARGE BALLAST INTO VICTORIAN PORTS

Not all ships discharge ballast water upon arrival in a Victorian port. Vessels which may never discharge ballast include a number of Australian coastal vessels carrying fresh water ballast, which, rather than being discharged, is shifted from one tank to another through an internal pumping system, and small roll on, roll off vessels which carry permanent solid ballast³⁶⁰

Whether a ship discharges ballast or not is determined operationally by the loading activity of the vessel. A ship entering port full of cargo is unlikely to be carrying significant quantities of ballast and will not need to discharge ballast. Conversely, a vessel empty of cargo will usually carry ballast and is likely to discharge this as cargo is loaded. For ships which both load and unload cargo during a single visit, there will be no major changes in displacement and ballast may not need to be discharged.

The Committee notes, that it is therefore important to consider port trading patterns when making assumptions about the fact of and quantities of ballast discharge. The contrast in loading activity between Victorian ports is shown by Figure 5.

FIGURE 5
Percentage of vessels which loaded or unloaded cargo, or both, in Victorian ports, August 1994 to July 1995

Source: Walters, S. (1996), Ballast Water, Hull Fouling and Exotic Marine Organism Introductions via Ships - A Victorian Study, Publication 494, Environment Protection Authority (Victoria).