A corner of one fish shack is in Aquinnah, while three-quarters of another lies in Chilmark. The town line is somewhere in the gravel road along Menemsha Creek.

The two small up-Island towns have been grappling for the past year with a plan to straighten out a boundary line that runs along Boathouse Road in Menemsha.

At their annual town meeting Monday night, Chilmark voters approved a change to the town line, but not before an amendment was added on the town meeting floor that will keep Aquinnah, the smallest town on the Island, from losing any more land.

“It is the intention that the area of swapped land remain equal, with no net loss or gain to either town,” executive secretary Tim Carroll told voters.

The boundary line has been the same for 150 years.

The original town line was established between Chilmark and Gay Head in 1855, marked by a granite post near the old post office in Menemsha. One side of the post said “C” and the other “GH.” A new channel was dredged in 1906 and Menemsha basin was created for the harbor. The dredged material was used to build a jetty where the fishermen’s buildings now stand.

The towns have long managed the lots under a gentlemen’s agreement, with Chilmark managing two of the fishing shacks and Aquinnah managing the other. But sometime last year when leases came up for renewal and a dispute arose over ownership of one of the shacks, town leaders decided to take steps to straighten out the boundary line.

Aquinnah is set to take up the question at the annual town meeting May 13. If it is approved by both towns, the line change will be sent to the state legislature for approval. The change then must clear the state highway department, followed by a new survey commissioned by the towns to have bounderies installed properly.

The plan initially called for Aquinnah to lose about 300 square feet. But on the morning of the Chilmark town meeting this week, the selectmen asked Mr. Carroll to propose the amendment on the town meeting floor.

“It was expressed by the Aquinnah selectmen that it wasn’t right the town of Chilmark ended up with more land,” the executive secretary said later. “The reality is we’re taking over the road to pay for and maintain it, instead of trying to have two towns fix it.”

“This will straighten it out so we can deal with it in the future,” he added.

A new map still needs to be drawn, but the engineer was busy on Monday with spring projects.

“As on the Vineyard, the gentleman who does the math for us had taken Monday off to paint the hull of his boat,” Mr. Carroll said.