The Massachusetts Division of Marine Fisheries (DMF) has prohibited all quahaugging in the west end of Lagoon Pond, after repeated water testing found high coliform bacteria counts.

The area behind Prime Marine, used by local quahauggers, already had been governed by an annual state-imposed closure in August and September. It is now closed year-round, as is the adjoining pond as far as Hines Point, a press release from the town said.

The source of contamination isn’t clear, said Tisbury shellfish constable Danielle Ewart, who was notified Monday that the closure would begin at sunrise Tuesday.

Ms. Ewart posted warning signs along the west end of the pond Monday night, and has mass-emailed Tisbury residents with shellfishing permits to advise them of the closure, she said.

“This is a health issue, and my role is to enforce the state’s closure,” said Ms. Ewart, who has been shellfish constable since 2010.

“It’s going to take a lot of tests,” showing lower levels of contamination over a sustained period of time, before the state will lift the ban, Ms. Ewart said.

The closure does not affect the harvest of bay scallops. (Unlike clams, with scallops only the muscle is eaten.)

Meanwhile, Ms. Ewart said she is working with the Lagoon Pond Association and state, local and regional water quality experts to investigate the source of the bacteria.

The state considers an area unsafe for shellfishing if testing finds more than 31 colony forming units (CFU) of coliform in 100 milliliters of water.

The western end of Lagoon Pond tested above that limit more than 20 per cent of the time over a series of 15 tests, according to Monday’s letter to the town from the marine fisheries department.

While heavy rains and high tides can contribute to elevated bacteria counts in ponds, Ms. Ewart said it’s not the only factor in the pond’s contamination, which has been high in dryer, warmer weather as well.

“We’ve had (coliform) counts of 80, which is as high as the test reads,” she said.