CHATHAM – Having received encouraging test results from shellfish harvested there last week, town and state officials lifted the red tide shellfish ban for Chatham Harbor on Friday. The closure remains in effect for Pleasant Bay and its connected waterways, including Bassing Harbor, Crow’s Pond, Ryder’s Cove and Muddy Creek.

The reopening is key because Chatham Harbor has a commercially important set of mussels and is home to the flats where there is a set of softshell clams. By comparison, the closed areas of Pleasant Bay are less critical to commercial shellfishermen.

“It’s not a real frequently harvested area,” Shellfish Constable Renee Gagne said.

On April 2, routine testing of blue mussels showed elevated concentrations of the toxin that can cause red tide. Those and subsequent tests prompted officials to ban the harvesting of all shellfish species from all east-side waters, from Pleasant Bay to the southern reaches of Chatham Harbor. The red tide outbreak is believed to be the first in Chatham since around 2006. Shellfishing in the productive grounds west of Monomoy Island and elsewhere on the south side of town was not affected.

On Tuesday, Gagne said the news is generally encouraging. Concentrations of paralytic shellfish poisoning (PSP) toxin remained below the legal limit in both the north and south sectors of Chatham Harbor, allowing the red tide ban to be lifted Friday at sunrise. There was also good news In Pleasant Bay, with shellfish harvested near Jackknife Beach.

“They actually were under the threshold last week,” she said. Additional samples were sent this week and the results were pending. If those and a third week’s tests continue to show a downward trend in PSP concentrations, officials can consider lifting the ban.

For now, shellfishing for all species except carnivorous snails is prohibited north and west of a line from Minister’s Point to the southern point of Strong Island, and from Strong Island to the Orleans town line. Chatham’s portion of North (Nauset) Beach is open to shellfishing, Gagne said.

What remains to be seen is whether Chatham’s outbreak is a rarity, or whether the spores created by the red tide algae will remain in the area to cause future outbreaks, as they have done in parts of the Nauset Estuary in Eastham.

“We’re hoping this is just a one-year event,” Gagne said.

So-named because severe outbreaks can cause a reddish tinge to the water, red tide involves naturally occurring microscopic algae that create the neurotoxins that cause paralytic shellfish poisoning. The algae creates cysts that can lie dormant in bottom sediment until environmental conditions are right, triggering an outbreak. As filter feeders, shellfish can develop a concentration of toxins in their tissue, which can be dangerous or fatal if eaten.

During the closure, restaurants and fish markets have carried shellfish safely harvested from open areas.