SEABROOK — Ever since shellfishing was banned last week in Seabrook-Hampton Harbor because of increasing levels of red tide, state Department of Environmental Services officials say the problem is only getting worse.

On May 9, officials from NHDES and the state Fish and Game Department closed off the harbor and other Atlantic coastal waterways to the harvesting of shellfish until further notice, including mussels, clams and oysters.

This move was in response to elevated levels of paralytic shellfish poisoning, commonly known as "red tide," detected in blue mussels collected in the area last week.

Red tide is a condition in which filter-feeding shellfish accumulate a potent neurotoxin produced by a naturally occurring marine algae, according to NHDES. 

Eating contaminated shellfish is potentially fatal to humans and cooking doesn't make contaminated shellfish safe for consumption, said Chris Nash, shellfish program manager for NHDES.

Nash said the toxicity last week was above the mandatory closure threshold of 80 micrograms of toxin per 100 grams of shellfish tissue. Since then, as testing continues, Nash said he finds the problem is getting worse.

Hampton-Seabrook Harbor is showing a large increase in toxins and, "We're expecting them to be higher this week," he said.

"We don’t know specifically why it’s intensifying. The phytoplankton have good water conditions. ... It could have something to do with all the runoff we had from the spring with all the snow," he said.

Through the department's monitoring program, scientists are collecting blue mussels weekly and testing for the presence of paralytic shellfish poisoning. Weekly samplings will continue until October. This program began in April, said Nash, who noted this is usually the time of year red tide appears.

"We monitor sea plankton that cause this problem," he said. "We could see those numbers were rising for a couple of weeks, plus we knew this was happening up in Maine and the same thing down in Massachusetts."

It is too soon to know how severe the algae bloom will be or how long it might last, Nash said. High levels also appeared in blue mussels found in the Gulf of Maine, near Star Island and the Isles of Shoals.

Other New Hampshire shellfish harvesting areas, including the recreational oyster beds around Nannie Island and Adams Point in Great Bay, and the commercial oyster farms in Little Bay, are not affected by the red tide closure, Nash said.

Although Seabrook is popular for offshore clamming, Nash said there are two small oyster farms in Seabrook waters but they are not affected.

"(Red tide) is not actually affecting them," he said. "They sold all of their market-sized oysters earlier this year. This didn't do anything negative for them. It did set the soft shell harvest short by a few weeks."

The soft shell clam harvest typically lasts through the first Saturday after Labor Day, Nash said. He noted that time frame is a little shorter, from November to May, in Seabrook and Hampton.

Red tide algae typically blooms in the spring, but the levels of toxicity may fluctuate before it returns in the summer or early fall, Nash said.

Seabrook Harbormaster Mike Pike speculated the increase in red tide may be linked to the recent cold spells and rainy weather.

"I’ve been living down the beach for years and every year, it happens at the same time," Pike said. "It might be all the runoff down the mountains down through the streams."

Nash said although there is a species of red tide in warmer climates that affects marine wildlife, there is also cold weather poisonous algae, including amnesic shellfish poisoning, that is appearing in Maine.

Nash said NHDES is dealing with harmful algae blooms year round when in the past, they only appeared seasonally.

NHDES officials will continue to monitor saltwater algae populations and shellfish toxicity levels along the coast and will implement additional closures if needed.

Changes to the open/closed status of shellfish waters will be announced on the Clam Flat Hotline, 1-800-43-CLAMS, and on the state Coastal Atlas www4.des.state.nh.us/CoastalAtlas/Atlas.html

For more information, visit NHDES https://www.des.nh.gov/organization/divisions/water/wmb/shellfish/index.htm.

Staff writer Amanda Getchell covers Newburyport and Seabrook. Follow her on Twitter @ajgetch.