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" " We're fearful of other snails , little mitten crabs , little other crustaceans that have the potential to survive in the wild and be able to really wreak havoc with the natural fish and animal communities , " said George Madison , the fisheries supervisor at the Michigan Department of Natural Resources for the Western Upper Peninsula in Baraga , Mich. " Oftentimes people think , ' Well , gee , if I just dumped in one fish , that 's not going to make a difference , ' " Schofield said . This parasitic flatworm requires three different hosts to complete its life cycle : freshwater snails , mitten crabs , and mammals . The United States bans certain animals as injurious ( including live snakehead fish , walking catfish , mitten crabs , brown tree snakes , and brush-tail possums ) because they crowd out native species or cause environmental damage . Our overall goal is to assess the importance of coast-wise transport of invasive species established in West Coast ports and bays , specifically : ( I ) To quantify the coast-wise voyages of commercial ships along the West Coast ; ( ii ) To characterize the seasonal availability of key invasive species with potential to be in domestic ballast water in a highly invaded estuary ( San Francisco Bay ) ; ( iii ) To quantify temporal ( seasonal , annual ) and spatial variation in prevalence and density of particular invasive species that are established in West Coast ports and found in tanker ballast water ; ( iv ) To collect and analyze additional ballast water samples taken at crucial seasons and locations for key established invaders in San Francisco Bay , ( v ) To collect / analyze additional ballast water samples using light traps and large folding nets for highly mobile species ( especially crab larvae , mysids and other pericarideans , and fish ) that are important invaders poised to spread via domestic ballast water ; and ( vi ) To deted analysis , combining use of a key array of existing samples and additional field work , to assess the risk of ' invasion by alien species in domestic ballast water including such species as crabs ( especially green and mitten crabs , copepods .