Martha’s Vineyard has experienced an unusual late-winter population swell since the beginning of March, an analysis of Steamship Authority traffic statistics shows. Publicly available SSA data show that approximately 4,600 people came to the Island from the mainland in the month of March — and stayed.

Between March 1 and March 31, the net difference between car and passenger traffic from the mainland to the Island ballooned over the first few weeks of the month compared with last year, just as the impacts of the coronavirus were beginning to spread. And even as traffic tanked later in the month and into April, the net increase to the Island remained steady.

Using off-season population estimates from the Martha’s Vineyard Commission and the Gazette that put the total year-round population at 16,535, a 4,600-person gain would account for a nearly 30 per cent increase in the Island’s population over the course of March and early April. That number is more than double last year’s population increase in March.

The large discrepancy between travelers coming to the Vineyard versus those leaving happened even as overall traffic declined by about 35 per cent throughout the month.

Although SSA spokesman Sean Driscoll declined to comment on the population change, he confirmed that the interpretation of the numbers was accurate.

As of March 21, boat line data shows that 34,186 passengers made the trip from Woods Hole to Martha’s Vineyard since the beginning of the month. A total of 29,533 passengers left the Island during the same time period, suggesting a net difference of 4,653 people on the Island during the first three weeks of March.

During the same time period in 2019, the net difference in travel to the Island was 2,480 people, approximately half the 2020 difference.

The passenger numbers include both people who walk on the ferry and those traveling as passengers in cars.

The numbers tell a similar story for automobile traffic. By March 21, 1,667 more cars had come to the Island than left. In 2019, the number for the same time period was 855, also about half the difference in 2020.

As of March 31, there was a total net difference of 4,568 passengers traveling from Woods Hole to the Vineyard, compared with the number of people traveling the other way. For automobiles, the net difference between those coming and going over the month was 1,686. Both are approximately twice the net increase in travel to the Island over 2019, suggesting that an influx of about twice as many people and cars had occurred in the first three weeks of March.

Although there are multiple ways for travelers to arrive and depart the Island, including private boats, other charter vessels, commercial airlines and private jet travel, the SSA historically is the main traffic artery to the Island. And while Mr. Driscoll said it is normal for there to be discrepancies between those coming to the Island and those leaving during the spring months, he agreed the overall total for March was unusual.

“At this time of year, it is more than what we would normally see,” he said.

As the coronavirus crisis has unfolded, there is some anecdotal evidence too that the population of the Vineyard is significantly higher than is usual for this time of year, especially considering stay-at-home orders and shuttering of most businesses. Edgartown residents have reported more lights on at night in the ordinarily seasonal downtown neighborhoods, and hiking trails, beach parking lots and other scenic spots are crowded for April.

Steve Bernier, who owns Cronig’s Markets and offers a discount to most year-round residents, said he had no data that showed an overall increase in customers. While he had noticed a few more out-of-state license plates in his parking lot, Mr. Bernier said his dramatic increase in business and sales volume over the past month was due to decreases or closures in the Island’s other two food channels — restaurants and schools. Data from Islander discount versus regular-price customers would not be available until the third week of April, he said.

There are many reports that traffic is steady and even heavy at times in certain hot spots on the Island, including at Menemsha at sunset. Because of that, Chilmark selectmen decided this week to limit parking at the Dutcher Dock to 15 minutes.

That’s enough time to watch the sunset and go home.