Three years ago, the Nantucket bay scallop harvest suddenly more than doubled in size, from around 15,000 bushels to more than 32,000. It was the year the industry ate its future.

The following season the harvest crashed. The total catch in 2005-06 was one-sixth as large — just 5,500 bushels. It was even worse last season, when fewer than 4,000 bushels were hauled up, the lowest tally since they began keeping records 30 years earlier.

There is no mystery about why it happened, according to Keith Conant, the Nantucket town biologist with the Nantucket marine and coastal resources department.

It happened because, in that record year of 2004-05, the fishery abandoned its prohibition on the taking of what are called nub scallops — late-set shellfish which have not spawned by the time the fishing season comes around.

All this occurred on Nantucket, but what makes it relevant to the Vineyard is that the dispute about the taking of nub scallops now has come to Edgartown.

To fully understand the reason, you have to know a little about the biology of the species, beginning with the fact that most bay scallops live for only a little over two years, and the course of their life cycle is heavily dependent on water temperature.

Adults spawn when the water reaches 68 to 70 degrees, which may be any time between early June and late July. The larvae drift around for a while and then set, or settle to the bottom and begin growing. The majority will be ready to spawn again the following summer, when the water again reaches the right temperature.

But those which set late, in September or October, often do not get around to spawning before the water temperature drops again. These are the nubs, and they usually can be distinguished externally from normal scallops because the annual growth ring on their shells is poorly defined and close to the hinge of the shell. Inside, they are distinguishable because their reproductive organs are underdeveloped.

Left in the water, they will become late-life parents the following year, but because they are relatively old, many — maybe about half of them or more — will not live until the fishing season.

“But,” said Mr. Conant, “they will spawn early in the season and create the [next generation of] large-ringed classic scallop.”

But the fact that they have not spawned makes many of these nubs very marketable.

“That’s because the gametes [the sperm and eggs] get reabsorbed and that energy, that fat storage gets bottled up into the meat and makes that meat real plump and fat,” he said, adding: “This what makes them commercially attractive.”

The law says before a scallop can be taken it must have a growth ring. This protects juveniles.

The trouble is, said Edgartown shellfish constable Paul Bagnall, “a nub is essentially a ringless adult.

“The scallop is large, but has no annual growth ring, or if it has a ring, it is way down near the hinge, sometimes only three–eighths of an inch from the hinge.”

Some fishermen think they should be entitled to take the nubs. And technically, they may be right. The law says there should be a growth ring, but it is a matter of interpretation where it should be.

Other fishermen, convinced of the nubs’ importance to the future of the fishery, will not take them, and think no one should.

Given the amounts of money to be made, it is all bound to cause conflict. And it has, in Edgartown recently, with some fishermen complaining others are taking scallops they should not. Complaints and confrontations have taken place, and there was argument at the most recent meeting of the shellfish committee.

Mr. Bagnall, in the middle, has some sympathy for both sides.

“The people that have taken them don’t want to be labeled as seed thieves,” he said.

“I feel for the people who want to take them, as a harvestable product, because that’s what they are is right now.

“In the old days when we were catching 10,000 or 20,000 bushels, it was a relatively minor problem, because there were plenty of scallops with rings. Now, with a reduced population, it’s a much bigger problem.

“It makes sense to be conservative, that’s the point I make. People need to make money, but you need spawn, and they’re going to spawn in the spring.”