Paul Bagnall has seen many cuts, the trenches of sand dug to connect pond and ocean, but they’re all a little different.

As shellfish constable Mr. Bagnall oversees the opening of Edgartown Great Pond between three and five times a year. The opening resalinates the pond, purges nutrients and allows shellfish to thrive. It also fills the pond with herring and striped bass, much to the delight of local fishermen.

Conditions have to be just so. As a baseline there needs to be a head of water between three and four feet in the pond. Then a low tide is preferable, a calm sea essential, and a north wind handy to help coax the water from the pond. All it takes is the whisper of a change in the tide or chop to ruin a good pond opening.

Mr. Bagnall has watched a cut made with the old Wampanoag tribe method of using a team of oxen and a scoop blade, seen it done by bulldozer (witnessing firsthand the pitfalls of this method when a White Brothers construction machine plunged into Oyster Pond in the early 1990s) and, back before the herniated discs, he has even dug out a cut with some shovels and a team of fishermen.

Today the cutting is the job of the aptly named Steve Handy, of Handy Trucking and Bobcat Service.

“It’s a game of inches,” advises Mr. Bagnall as he watches Mr. Handy scooping sand by the ton with an excavator into neat mounds.

Mr. Handy uses the excavator to build a platform out ten feet or so into the pond to dig out a pool there. Then leaving a break at the shoreline, he works backwards from the sea, sculpting a path between the shores. He will dig a deeper trench at the other end of the cut to halt back flow from the surf.

Mr. Bagnall flashes a worried look at the tide, which should be heading out by the time Mr. Handy completes the opening.

Given the scale of the job the tools of operation seem rudimentary. On the south shore of the Vineyard, the Atlantic Ocean averages 35 parts per thousand of salt while today, before the opening, the Edgartown Great Pond, which stretches across 800 acres, is around 10 parts per thousand.

The plan is to get it up to 22 parts per thousand. To accomplish this Mr. Bagnall aims to drop three feet, or a cubic yard, from the top of the pond.

That’s a lot of water ­­­— the five to seven million gallons which flow daily into the pond during spring from the groundwater table, equal to roughly one eighth of an inch.

“To do this job artificially, you would need to set up something three times the size of the Edgartown water plant,” says Mr. Bagnall.

He and Mr. Handy have conferred daily for the past week on whether to cut. Shortly after 5 a.m, following a dawn run down to the beach to check the tide and a survey of the Woods Hole Oceanographic Institution Web site for swell levels, somewhere below two feet today, Mr. Bagnall made the call.

A few minutes after Mr. Bagnall arrives at the beach at around 8:30 a.m., George Halkiotis, his son James, 12, and friend Josh Johnston pull up in a van and set up their fishing poles.

The two boys are playing hooky from school up in New Hampshire to get in a long weekend of fishing. Mr. Halkiotis spotted the digger from down the other side of the beach and knew the cutting was in progress.

A pond opening is always news in the fishing community. The anadromous herring will brave the current of the cut to make it into the pond. In hot pursuit will be the striped bass, in search of a meal of herring.

Followed closely by the fishermen.

Mr. Halkiotis been fishing this stretch of beach at this time for over 30 years. He has to drive across private property to get to their spot so he takes care to keep the news of the opening as quiet as possible.

“I don’t shout, I don’t let off fireworks, otherwise Letterman’s guards will be out,” he says, referring to the late night talk show host and nearby property owner who posts guards on his private beach.

But by mid-morning father, son and friend are flanked by the SUVs of fishermen. Somehow words spreads.

Shortly before 1 p.m, Mr. Handy digs out a connecting bank of sand and water cascades through the narrow channel. The job has taken him a little over five hours.

Taking care not to stray too far from his excavator parked close to the shore — he is aware how fast the shape of the trench can change — Mr. Handy peers over the bank and observes his, if you will, handiwork.

“It’s ripping out, it looks pretty good,” says the digger.

“Don’t get too cocky,” shoots back the shellfish constable.

If the tide keeps rising, he warns, it could be enough to reverse the flow and cause a sand bank and block the opening.

Regardless, there’s little left to do now but to watch and wait — a surprisingly hypnotic exercise. Chunks of sand in the trench subside constantly in the current, creating constant mini avalanches. The bricks immediately break up and are swept out to sea. The deep pool at the mouth of the cut churns up a little surf where waves of sand lighten the water.

Fisherman Rusty Hoxie ambles up to inspect the trench. He’ll be back at around sunset to drop a line in.

“There’ll be 20 Jeeps out here tonight,” says Mr. Hoxie. “It’s like ringing a dinner bell.”

Not everyone gets excited about pond openings, specifically those who like freshwater fish. Despite a current moratorium on the fishing of herring, a hard-core faction of herring boosters persists, but with nothing like the vigor of the old days.

Mr. Bagnall has heard stories of fist fights on South Beach between herring and shellfishermen during the 1930s.

“A group would come down to open up the pond and the herring guys would be waiting,” he says.

But beyond the occasional threat of violence, fishermen were much freer to salinate the pond to their liking, says the constable. Today, for one thing, approval for the pond opening has to come from selectmen.

“Back then you didn’t need attorneys to dig a trench,” he says.

Mr. Bagnall expects that even if this opening goes well it will be closed within 10 days. The openings don’t last as long these days because the ponds are smaller, he explains. Big weather events have pushed back the shoreline — during Hurricane Bob alone the pond lost a tenth of an acre — and sea levels are rising at a rapid clip, a fact Mr. Bagnall attributes to global climate change.

“I’m the first to spot rising sea because with this the margins are so small,” he says.

As a result Mr. Bagnall measures the minutia on an almost daily basis, putting him on the front lines of climate change.

He predicts the Vineyard shoreline will rise up to half a foot over the next 50 years.

“We’ll see extreme erosion,” he says.

Next morning he barrels west over the South Beach sand in the shellfish department van toward a cluster of SUVs huddled around the opening.

Mr. Bagnall inspects the trench. It has grown more than 50 feet wide and water is now running at at least five knots. The car-sized mounds of sand deposited along the trench by Mr. Handy’s excavator have simply disappeared, consumed by the millions of gallons of pond water rushing out to sea.

“It’s stepping and fetching now,” he says.

Each one’s a little different.