University of Michigan researchers said such water-filled tanks have enabled at least 185 non-native aquatic species such as zebra and quagga mussels to enter the U.S. Great Lakes from overseas.

The University of Michigan researchers, led by Professor Michael Parsons, said their ballast-free ship concept resolves the problem in an economical manner.

Ships take on ballast water to gain stability when they're not carrying cargo, the researchers said. The ballast is discharged when they load freight, expelling tons of water -- as well as pathogenic microbes, mollusks and fish -- that are in the tanks.

The researchers said a ballast-free ship would create a constant flow of local water through a network of large pipes, called trunks, that runs from the bow to the stern, below the waterline.

"You're continuously sweeping water through the ship and out," said Parsons. "So you're always filled with local sea water, not hauling water from one part of the world to the other."

A report on the project appears in the April issue of the journal Transactions of the Society of Naval Architects and Marine Engineers.