Exchanging ballast water does not kill all the hitchhiking organisms, so the ultimate answer may be treating with heat or chemicals or redesigning ships. But ballast exchange is the most effective response for now, said Ms. Swanson of the bay commission. Maryland and Virginia, however, are legally powerless to require it because the U.S. Constitution does not allow states to regulate international or even interstate ship traffic. Only the federal government has that authority.
Congress has directed the Coast Guard to study the problem of shipborne aquatic invasions, but the only crackdown on ballast water has been in the Great Lakes.
Ms. Swanson said that state-by-state or even regional measures would be ineffective anyway. If ballast exchange is required for the Chesapeake but not other ports, organisms released by vessels in places like New York or Delaware Bay eventually would find their way here, she said.
The bay states need not rely solely on the federal government to combat the threat, Ms. Swanson said. The panel recommends that Maryland and Virginia launch "an aggressive, multilingual education campaign" to inform captains and companies of the need for precautions in taking on and discharging ballast water.
The commission's likely call for federal and international action has the support of Maryland shipping lines, which fear that imposition of new rules solely for the Chesapeake would cost them business.
Ships unable to exchange their ballast while in transit might be required to delay their arrival at port by a day or two, adding tens of thousands of dollars to the cost of the shipment, said Rupert A. Denney, president of the Maryland Maritime Association.
"If it's done only on the Chesapeake," Mr. Denney said, "a [ship's agent or master] is going to say, 'To heck with it,' and go to another port."
Mr. Denney also said that Maryland shipping agents favor regulations instead of a voluntary effort because some ships, to gain an edge on competitors, would ignore nonmandatory guidelines. "The industry has to realize that there is a threat here, and it's in our longer-term interest to protect the bay and any other body of water," he said.