There is a huge illegal immigrant problem in this country and it doesn't involve human beings. Marine organisms brought to our shores by ships from overseas are wreaking havoc with our inland freshwaters.
While some attach themselves to the hulls, the majority travel in the ships' ballast tanks, which carry large quantities of water for stability when not loaded with cargo. On arrival the water is pumped overboard, often releasing tens of thousands of opportunistic organisms from overseas.
Transport of foreign plant and animal species by ships is nothing new. As long as vessels have crossed the seas bioinvaders have traveled as unwelcome passengers. Since the 1950s international shipping trade has greatly increased such that today 97 percent of world trade is carried by some 33,000 large cargo ships. Opportunities for these illegal aliens are better than ever.