When it comes to keeping invasive species out of the Great Lakes, ballast water has long been seen as the bad guy.

Ballast is the water that oceangoing ships carry into the lakes, and scientists say it carries a host of unwanted organisms that can foul Michigan waters and choke out natural plants and fish.

But new studies suggest it’s not the only way potentially dangerous stowaways can hitch a ride.

Researchers are now turning their microscopes on another area of those ships — the hull — as a possible entry point for invasive species into the lakes.

The mass of plants, algae and organisms that latch on to ships beneath the water line — called biofouling — may contain life-forms that could be harmful to Great Lakes. It’s a potential problem that has received little attention from legislators and conservationists, who have focused on ballast water bills now before Congress.

Biofouling for years has been considered a second-tier concern in the fight against invasive species because organisms coming into the Great Lakes from the Atlantic Ocean are thought to thrive only in saltwater. The presumption was that exposure to the lakes’ freshwater would kill them off.

But in one of the first examinations of biofouling on an oceanic/Great Lakes vessel, samples from a single ship produced more life-forms than expected. Those samples, taken by two California-based researchers and analyzed over the past several years, found eight species that previously had not been seen in the Great Lakes.

“Overall invasion risk from biofouling may be comparable or exceed that of ballast water discharge,” concluded researchers John Drake and David Lodge, of the Santa Barbara, Ca.-based Environmental National Center for Ecological Analysis and Synthesis.

“This finding implies that even if policies aimed at ballast water and sediment discharge are successful, future invasions may continue at a high rate unless biofouling is also addressed.”

Hull fouling has been identified as “the next big issue on the national scale now that (fixes for) ballast water have been created,” said Dave Reid, a senior physical scientist with the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory in Ann Arbor.

“In the Great Lakes right now, it’s a question of how big an issue it should be. There is little known about it.”

Similar studies are under way in the Northwest. Ian Davidson, a researcher with Portland State University and the Smithsonian Environmental Research Center, has taken hull samples off ships that cruise the Columbia River in Oregon — a waterway that, like the Great Lakes system, can be accessed from the ocean.

“It is much harder to do sampling for hull fouling than for ballast water,” Davidson said, because of difficulties researchers face in getting access to ships and docks. “But we need to determine how serious a threat fouling is as a vector for invasive species.”