An empty ship is usually very unstable. If a ship becomes very unstable it can roll over and sink. Empty ships are also higher in the water and the propeller and the rudder become less effective if they are above the water line.
The quickest and easiest way to increase the weight of a ship is to pump water in. This is called ballasting. Water is pumped into tanks in the bottom and sides of the ship. As an empty ship is loaded, the water can be pumped out. This water in the ship is called ballast water. The ballast water will have many tiny living marine animals in it. Most sea creatures produce eggs and have tiny young, so the ballast water can have very young, but very small marine animals that in time will grow much larger.
If the ship only travels in Australian waters, the ballast water will only have Australian marine animals.
However ships from other countries will probably have some marine animals in their ballast water that don't live in Australia's waters. In this way many marine creatures have lifted a ride to our ports. Some of these animals can become major marine pests.
To prevent this happening, ships that could be holding pest species in their ballast water must exchange their ballast water while traveling over the ocean. This may not be possible if the weather is bad. Ships must keep permanent written records about the movement of water in and out of their ballast tanks.
For more information on problems with marine pests in ships' ballast water see 'Marine Pests and Threats' on the AUSMEPA website www.ausmepa.org.au