PORTLAND – Invasive sea squirts are multiplying on the Georges Bank ocean bottom, posing potential threats to lucrative seafood species that live on the productive fishing grounds.

Researchers who inspected the area earlier this month say sea squirt colonies now cover at least 40 square miles, a six-fold increase in the past year. The colonies have created dense slimy mats over gravel and sand on the northern edge of Georges Bank.

The discovery of such a large offshore colony is believed to be unprecedented and is especially troubling because of the history and value of Georges Bank to New England’s seafood industry.

The area is home to haddock, cod and other groundfish, as well as scallops, and it is feared the squirts could push out native marine life and disrupt food chains that support fish.

“It covers everything,” said Page Valentine, a research geologist at the United States Geological Survey. “How much impact it has on fish food such as worms and clams remains to be seen. But it’s definitely changing things, and it’s getting to the point where you can’t eradicate it, and it has no known predators, so the only option right now is don’t spread it.”

The sea squirts are a type of tunicate, filter-feeding animals with a flexible but firm outer covering. The animals are known to live and multiply in nutrient-rich inshore waters, not deeper offshore areas such as Georges Bank.

The same research team came back from its trip to Georges Bank last fall with reports of the sea squirt covering more than 6 miles of the gravel bottom. The small animals are not native and may have dropped off a passing ship.

This year, the team searched a larger area but had to come home after examining about 40 square miles. “We were surprised at how big the area is,” Valentine said.

Valentine believes the conditions there just happen to be right for the tunicates. Georges Bank, which lies about 200 miles southeast of Portland, is a relatively shallow plateau rich with nutrients. It also has areas of gravel bottom, which allows the sea squirts to glue themselves to the sea floor.

Some scientists have suggested making the area off limits to fishing vessels that drag nets across the ocean floor. But fishery regulators are quick to point out that it’s unknown whether fishing or other factors, including storms, are helping to spread the colonies.

Barbara Stevenson, owner of offshore draggers based in Portland, said the area has been avoided in recent years because of regulations and the economic struggles of the industry.

She said she believes fishing could be more of a solution than a problem. “It got a toehold because there was no fishing,” Stevenson said.

The federal agency that manages ocean resources and fisheries is participating in the sea squirt research.

But there are too many unknowns to make any decisions about what, if anything, should be done, said Teri Frady, chief of research communications for the National Marine Fisheries Service in the Northeast.

“We know they’re out there, but what difference is it making?” Frady said. “We need to find out more about it.”