FALMOUTH, Mass. – Colonies of sea squirts living on an 88-square-mile section of ocean floor in Georges Bank grew denser during the last year, but the species that could threaten the fishing industry hasn’t spread eastward into Canadian waters, according to new research.

The annual survey of the sea squirts was conducted in August and September by the National Oceanic and Atmospheric Administration ship Delaware II. Preliminary data show gravel in some areas 50 to 75 percent covered by the sea squirts.

Page Valentine, who tracks the species for the U.S. Geological Survey, said the area covered by the colonies doubled since last year at 75 percent of sites observed in 2006.

Sea squirts are tunicates, a filter-feeding creature named for its flexible outer covering called a tunic. The Didemnum type of sea squirt found at Georges Bank off the Massachusetts coast tends to grow rapidly.

Scientists say the creatures could threaten the fishing industry in the region because their colonies could prevent fish from feeding on worms and crustaceans that live in and on the gravel floor, reduce shelter for animals, and crowd out the larvae of sea scallops and other species.

“We haven’t seen any dramatic changes yet, but as the percentage of the area covered by the tunicate gets higher and higher, it’s going to seal off the sea floor,” University of Rhode Island biologist Jeremy Collie said. “That’s when we expect to see significant effects.”

Scientists don’t know how to stop the sea squirts’ proliferation.