BOSTON – His family has been digging clams for five generations, but John Grundstrom avoided the shoreline this summer after a toxic red tide closed shellfish beds from Maine to Massachusetts. It was just too frustrating.

“It was like a guy that loves golf and you’ve got an 18-hole professional golf course across the street and you can’t go play,” he said.

Grundstrom’s been back working Ipswich Bay for about a month, and said he’s not dwelling on the thousands he lost or predictions the red tide algae bloom could recur for years. But the red tide made a mark.

“It’s the worst I’ve seen in all the years I’ve been digging,” said Grundstrom, 50, of Rowley.

It was bad enough to drive some fishermen away from the business, and leave a lingering leeriness among some consumers.

But the red tide also provided a wealth of data for researchers, who hope they’ll be able better predict the movements of future algae blooms.

The toxic Alexandrium algae cells are absorbed by shellfish, making them unsafe and even fatal to eat. Starting in mid-May, that led authorities to shut down 1.2 million acres of Massachusetts shellfish beds as the algae spread in astonishing levels of toxicity from Cape Ann, around Cape Cod, to Martha’s Vineyard before dispersing last month.

The state estimated the red tide cost shellfishermen as much as $7 million in lost income, though officials had no final number Thursday.

Seafood shacks were forced to import clams from out of state and ease concerns of customers. Jim Limberakis, owner of the Clam Shack of Falmouth, said Thursday that clam sales remain down about 20 percent.

“[Customers] don’t ask the questions, but seem to be a little leery,” he said.

Initial demand for native clams after the flats reopened was quickly filled, and Grundstrom said the price per pound is about $1.90 now, below the usual $2 to $2.25.

Wellfleet shellfish constable Andrew Koch said most shellfishermen are back to work, but others have had more trouble absorbing the hit. A shellfishermen can make as much as $50,000 annually, but the down time during prime summer months could mean as much as $15,000 in lost income, he said.

“I’ve heard of a few people just throwing in the towel, saying ‘I don’t want to deal with this anymore,”‘ Koch said.

Don Anderson, a red tide expert from the Woods Hole Oceanographic Institution, said this year’s red tide gave researchers unprecedented amounts of data, and more than a few surprises.

Scientists were stunned the tide could travel around the arm of Cape Cod to Martha’s Vineyard, as well as up to 40 to 50 miles offshore, and remain highly toxic.

They’d assumed the tide would diffuse before getting that far, he said.

They also learned water pathways could conceivably carry the tide as far south as Rhode Island and Connecticut, though this year’s bloom did not get that far.

Anderson said scientists need to understand exactly where the tide might have deposited the cysts that could revive it in the future.

Scientists also learned the importance of at-sea monitoring, and the case for installing ocean systems to observe how red tide got stronger, Anderson said.

The wealth of information about how the algae bloom spread and responded in different weather can be used to fine tune computer models, he said.

“You start to be much closer to the predictions everyone seems to be interested in,” he said.