Water taken aboard for stability can carry unwanted passengers. Ballast water is seawater pumped into a ship’s tanks to maintain trim and stability; when discharged elsewhere it may release non-native organisms that disrupt local ecosystems.

Zebra mussels, toxic algae, and various plankton have spread worldwide through ballast water. The International Maritime Organization’s Ballast Water Management Convention requires ships to treat or exchange ballast so that discharge meets strict organism limits. Treatment systems use filtration, ultraviolet light, chemicals, or combinations of methods.

Crews must keep detailed ballast water record books and operate treatment equipment correctly. Port state inspectors check compliance. Managing ballast water has become a routine but non-negotiable part of modern ship operation.

• Seawater carried for stability that can transfer species
• Regulated by international convention and national rules
• Treated or exchanged before discharge in many areas
• Documented in mandatory record books