After a ship unloads its cargo, it routinely sucks water into its hold as ballast to steady the vessel for the return voyage, usually releasing the water when it reaches its destination. Ports in the United States receive about 79 million tons of ballast water from overseas annually. Worries have grown over the years that ballast water is transporting creatures around the world and depositing them in areas where they have no natural predators. The zebra mussel has caused billions of dollars of damage since it arrived in the Great Lakes in the 1980s. In the Bay, researchers at the Virginia Institute of Marine Science recently discovered the rapa whelk, a species native to the Sea of Japan.
Scientists believe the whelk was brought here in ballast water.
The new study suggests those more visible examples of invasions via ballast water may be only the tip of the iceberg. Ruiz said that while there is no reported evidence of outbreaks of human diseases from nonindigenous microbes in ballast water, the findings indicate the need for a much greater emphasis than has been shown. Most of the microorganisms counted by the scientists were not identified, although Dobbs said that effort is continuing. They found one type of cholera in the ballast of all 15 ships examined, and two types in 14 ships. When a strain of cholera not native to the United States was introduced to Mobile Bay several years ago, it resulted in the closure of infected oyster reefs.