Aliens don't strike everywhere in equal measure. The first comprehensive assessment of the risk posed by ballast water has identified 20 hotspots that are particularly vulnerable to being invaded by species stowed away inside the bowels of cargo ships. San Francisco Bay, New York, Los Angeles, Hong Kong and Durban in South Africa are on the list, which could help focus counter-measures. Empty ships routinely take on seawater as ballast before they set sail, to keep them stable while at sea. They later dump it in the port where they take on cargo. At any one time, an estimated 7 billion tonnes of ballast water is crossing the oceans - almost all of it carrying seeds, spores, eggs, larvae, bacteria and plankton native to wherever the water was loaded. It's probably the top cause of alien invasions worldwide.
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