Ballast water may be taken onboard by ships forstability. It can contain thousands of aquatic or marine microbes, plants and animals, which are then carried across the globe. These organisms may be released when the ballast water is discharged. The International Convention for the Control and Management of Ships’ Ballast Water and Sediments (BWM Convention) was adopted in 2004. It entered into force in 2017 to introduce global regulations to control the transfer of potentially invasive species. As a temporary solution, pending the installation of an on-board ballast water treatment system, ships should exchange ballast water mid-ocean.
If regulations are complied with, untreated ballast water should not be discharged near wind farms. The convention requires ships to discharge ballast water at a considerable distance from the coast (see text box and diagram below) to prevent the spread of non-indigenous species.