San Francisco, New York and Los Angeles are on the list created by researchers at the University of Oldenburg in Germany who studied the threat level of invasive species faced by 1,400 ports across the globe, NewScientist.com reported Tuesday.
Empty ships routinely take on seawater as ballast before beginning a voyage to keep them stable at sea, then dump it in the port where they take on cargo, along with invasive species native to wherever the water was taken on.
Ballast water is probably the leading cause of alien species invasions worldwide, the researchers said.
Invasions are more likely to occur if the water at both ports is of similar temperature and salinity, they said, and are more likely with warmer waters, which usually contain more species.
The findings could help port officials in their efforts to control invasive species, Oldenburg researcher Hanno Seebens said.
"Usually port authorities cannot check all ships," he said. "Our model can help inspectors identify the ships with the highest probability of carrying potentially invasive species."