Commercial ships transport oil, iron ore, grain, and other cargo to ports worldwide. Most of these ships have large steel tanks, called ballast tanks, located along the sides and bottoms of their hulls. The ballast tanks contain seawater, or ballast water, which is pumped into or discharged from the ship during cargo transfer, usually in harbors and nearshore waters.
When cargo is unloaded from a ship, the weight of the ship decreases, so seawater from the surrounding waters is pumped into the ballast tanks to compensate for the decreased weight. When cargo is loaded onto a ship, the weight of the ship increases, so ballast water from the ballast tanks is discharged into the surrounding waters to offset the increased weight of the cargo. Modern ships depend on this exchange of seawater to regulate the ship’s stability and operate safely.
The water pumped into a ship’s ballast tanks may contain numerous aquatic organisms, including viruses, bacteria, algae, jellyfish, crabs, mollusks, and fish. If the organisms within a ship’s ballast tanks survive the trip to the next destination, they may be released with the ballast water into waters in which they do not naturally occur. If these nonnative organisms survive and spread throughout their new environment, they may become invasive species. In this way, ballast water can accidentally introduce harmful microalgae and other organisms into the environment.