Ballast water in ships is a principal way in which alien species are introduced into new aquatic habitats. Commercial trading ships are, therefore, required to treat their ballast water to meet discharge standards and regulation. The International Maritime Organisation (IMO) has approved a range of methods for ballast water treatment, such as filtration, ultraviolet radiation and chlorination. A recent study used DNA metabarcoding-based analyses to explore the efficacy of the most widely adopted approach — chlorination — finding that it affects zooplankton organisms unequally and may potentially even increase the chances of introduced populations becoming established in new habitats.