The release or escape of specimens into the wild by the industry and the hobbyist aquarium owner has led to introductions. The common goldfish, for example, has become a nuisance species in eastern Washington. Selected non-native species, usually target predators, are intentionally introduced in an effort to control the growth and spread of other introduced species. Achieving the desired effect is difficult. Grass carp introduced to control unwanted aquatic plants in inland lakes resulted in native plant species being decimated.
Ballast water can contain aquatic plants, animals and pathogens.
Marine vessels take on and discharge millions of tons of ballast water daily in ports and harbors around the world. The discharge of ballast water is considered a major pathway for aquatic introductions because of the huge volume of water carried as ballast. Recreational boaters and sea or float planes transport nuisance species in bait buckets or boat wells, often without realizing it. Fouling of vessel hulls by encrusting organisms also provides a mechanism for transfer of species. Aquatic plants, in particular, are easily transported when plant fragments get tangled on boat propellers and fishing gear of recreational boats.