These are the 10 of the most unwanted species
introduced to ocean environment after ballast water (http://globallast.imo.org/)

Ballast water (BW) is water carried by ships to ensure stability, trim and structural integrity. When it loads cargo, the ballast water is discharged. Shipping moves over 80% of the word’s commodities and transfers approximately 3 to 5 billion tonnes of ballast water internationally annually. When a ship is empty of cargo, it fills its tanks with ballast water. Ballast water is plenty of micro’ organisms phytoplankton, zooplankton, bacteria and when introduced to new marine environment they begun a treat to the local marine ecological system. Invasive aquatic species are one of the four greatest threats to the world's oceans, the other three are: land-based sources of marine pollution, over-exploitation of living marine resources and physical alteration and destruction of coastal and marine habitat.

Most species carried in ships’ ballast water do not survive the voyage. Most of those that do, do not survive when discharged into the new environment. Under certain circumstances some species do survive to form viable populations, and may become serious pests.