Shellfishing reopened in all areas of the Island at noon on Monday, Oct. 31. According to the Division of Marine Fisheries (DMF), concentrations of the toxin domoic acid, which was associated with a bloom of the diatom Pseudo-nitzchia, had declined to safe levels in local waters. Domoic acid causes amnesic shellfish poisoning (ASP), which can lead to neurological damage.
David Grunden the Oak Bluffs shellfish constable told The Times on Monday that he and his staff have been sending meat samples from quahogs and oysters every four days since the shellfishing was closed on Oct. 9. He sent the most recent sample last week. The DMF also samples toxin levels in the phytoplankton in the water column, but collects those samples from their own boats, according to Mr. Grunden.
Quahogs and oysters are tested mostly because they are easier to collect than steamers and blue mussels, Mr. Grunden said. He is not aware of any significant difference in accumulation of the toxin among the different shellfish species.
This contrasts with the case of paralytic shellfish poisoning (PSP), which is caused by saxitoxin that accumulates in dinoflagellates (particularly the genus Alexandrium), another member of the phytoplankton community. Mr. Grunden said that this toxin accumulates more rapidly in blue mussels, so they are tested preferentially during PSP outbreaks.
As far Mr. Grunden was aware, this was the first ASP outbreak in Massachusetts, although events have occurred farther north. He said that he finds Pseudo-nitzschia in about 70 percent of local samples that he examines, but — until now — only in low concentrations.
He said there were two other algal blooms this summer, one in June and one in August, both involving different species of phytoplankton.
A late-season bloom
Although Mr. Grunden did not recall a previous Pseudo-nitzschia bloom, Kate Hubbard, a diatom specialist with appointments at Woods Hole Center for Oceans and Human Health and the Florida Fish and Wildlife Commission in St. Petersburg, said that she has seen high concentrations of Pseudo-nitzschia in Massachusetts samples before. She is aware of harmful algal blooms (HABs) of this genus up and down the East Coast, but said that it is unusual to see one this late in the fall. Several species in this diatom genus produce a toxic compound called domoic acid (DA).
The combination of environmental conditions that produce a bloom of Pseudo-nitzschia are not well understood, said Ms. Hubbard. “Different species [within the genus] produce varying amounts of DA. You have to look at temperature, salinity, and nutrients. There may be one set of conditions that trigger the bloom, and possibly a second that trigger the increase in the production of DA.”
The recent outbreak was described by Mike Hickey of the Division of Marine Fisheries as the most extensive one seen on the East Coast, but Ms. Hubbard described a precedent event last year on the West Coast. The bloom extended the entire length of the Pacific Coast states, and was detectable up into Alaska. The incident was associated with an El Niño event.
The New England bloom was associated with an unusually dry, warm autumn, but Ms. Hubbard said that cause and effect have not yet been determined.
Ms. Hubbard has seen data collected at the Martha’s Vineyard Coastal Observatory, situated at South Beach, and a mile south of the shoreline in open water. The station automatically collects samples and puts them in front of a microscope and then relays the images to Woods Hole for analysis. Hubbard has observed prior high concentrations of Pseudo-nitzschia in that data record, but never this late in the season.
She was told by colleagues in Maine that both the concentration of DA in the environment and the numbers of diatom cells changed rapidly. The Gulf of Maine samples had lower numbers of cells than Massachusetts samples, but more elevated concentrations of DA because the Maine diatom assemblages were composed of different Pseudo-nitzschia species from those observed in this area.
“This is different from red tide; only one species of Alexandrium [a dinoflagellate species] produces the toxin,” she said, “but there are 14 species of Pseudo-nitzschia in New England, and seven of them produce the toxin.”
The epidemiological history of Pseudo-nitzschia is relatively short, so it is difficult for investigators to say whether the outbreaks are becoming more common. Hubbard said that the first known outbreak was in 1987 in western Canada, where several people became ill and some died. It is known from sediment cores collected in northwestern Europe and from taxonomic records (collections from the field that were reported) in the Gulf of Maine that date back approximately a century. The diatom research said that two species of diatom in the genus Nitzschia and a species of macroalgae (seaweed) in Japan are also known to produce domoic acid.
End of the closure
Shellfishermen did not lose much fishing, Mr. Grunden said, because the state allowed bay scalloping to continue during the closure. The toxin collects in the gut of the shellfish and not in the muscles. Generally only the muscles of scallops and conches are eaten, so their harvest was allowed to continue.
Mr. Grunden said that some people do eat the guts of bay scallops, but he doesn’t recommend it, certainly not during a shellfish closure for ASP.
Shellfishermen have been notified of the end of the ban by the removal of all signs along the shoreline, lowering of the flag at Sengekontacket Pond, and a notice at the town websites, oakbluffsma.gov, aquinnah-ma.gov, chilmarkma.gov, edgartown-ma.us, tisburyma.gov, and westtisbury-ma.gov.