In Atlantic Canada, a few millimeters of lobster shell have some people seeing red.

Fishermen in the provinces of Prince Edward Island and New Brunswick are at loggerheads over how big a lobster’s carapace must be for a lobster to be harvested. Fishermen in Prince Edward Island are fighting to keep the minimum carapace at 72 millimeters, or about 2.8 inches, while their counterparts in New Brunswick would like to see fishing restricted to lobsters with larger shell sizesas much as 10 millimeters longer. That is .39 inch.

At stake is tiny Prince Edward Island’s lock on the global market for so-called canner lobsters, younger and smaller lobsters that are prized for their sweeter taste and sought after by major customers, including casinos and cruise ships. The lobster market is worth about 144.2 million Canadian dollars ($130 million) a year to the province, which has a population of just 145,000.

Across the Northumberland Strait in New Brunswick, lobster catchers say tastes have changed and that most customers prefer larger, older lobsters. Those bigger lobsters currently bring in around C$3.75 a poundor 50 Canadian cents more than canners. Processors in their province also like them larger, in part because bigger tails mean bigger profits.

Paul Lucas, head chef at Lobster on the Wharf, a popular restaurant in Charlottetown, Prince Edward Island, says that some customers prefer canner lobsters for having a higher “meat to weight ratio than its bigger brethren. A 1-pound canner lobster usually yields about 4 ounces of lobster meat, while a 2-pound lobster usually gets about 6 ounces. “It’s like eating ribs without the bones, he said. “You get more bang for your buck.

Jim Larkin, who owns the restaurant, says the lobster he buys depends on customer tastes. He hasn’t been selling a lot of canner lobsters lately, as the restaurant’s patrons prefer bigger crustaceans. “The bigger lobsters are easier to eat and they look better on the plate, Mr. Larkin said.

A plunge in the price of lobster in recent years has brought tensions to a boil. New Brunswick catchers even boycotted an industry summit after their request to add carapace sizes to the agenda was rebuffed.

“Prince Edward Island is living in the past, and it doesn’t want to live in the future, said Christian Brun, executive secretary of the Maritime Fishermen’s Union, the group lobbying for bigger lobsters. “People don’t want lobsters that are the size of a large shrimp.

Under pressure from New Brunswick fishermen, the Canadian government last year boosted the minimum harvesting size of lobster carapaces, to 72 millimeters from 71 millimeters. Prince Edward Island fishermen opposed the move, and they are determined to keep the targeted shell size as low as possible amid what they see as their rivals’ multiyear efforts to claw back their market share.

On a sunny Thursday in late August, islander Dean Handrahan sorted the day’s catch aboard the Spring Renegade, armed with a metal gauge hanging from a rope around his neck.

Mr. Handrahan moved with lightning speed despite the choppy waters, taking just seconds to pull a lobster from the trap with a pair of well-used oversize rubber gloves. Deftly avoiding the claws, he placed his gauge between each lobster’s eye socket and the tip of its upper body shell. The lobsters that measured 72 millimeters or more were placed in a live catch box aboard. The ones that fell shortand females carrying eggsgot a reprieve.

At the wheel, Mr. Handrahan’s brother Dwayne offered a friendly wave to passing fishermen from New Brunswick. But afterward, he gritted his teeth and admitted not all his interactions with his neighbors are as friendly. The carapace debate “is very frustrating, he said. “And when you don’t have a good catch, you just keep your head down and get through the day.

Island fishermen say last year’s 1-millimeter size hike reduced their catch by about 15% to 25%. The decrease has had spillover effects in a province that supplies 80% of the world’s canners.

At Spartan Industrial Marine in the capital, Charlottetown, sales of netting, boxes and twine, which are used to create lobster traps, all are way down this season.

“If they’re not making money, they’re not spending it, said store clerk Judy Clowe.

The island’s largest seafood-processor, Royal Star Foods Ltd., says another 1-millimeter hike in the minimum carapace size could cost it about C$4 million in lost revenue.

Royal Star has spent years cultivating relationships with customers in China, Japan and Scandinavia, and the firm’s general manager, Francis Morrissey, said they prefer Prince Edward Island’s canners, which are often packaged whole and then frozen.

“If we all fish the same size of lobster, we’re screwed, he said.

For many on Prince Edward Island, any change is a slippery slope that could put an end to the island’s lucrative canner business.

But for Carl Allen, a fourth-generation fisherman from New Brunswick, who was recently elected president of the Maritime Fishermen’s Union, the industry’s future lies in adaptation.

“Let’s move away from a smaller lobster to suit the market, he said.

He said one solution to the impasse lies in marketing a “canner select, or a lobster with a shell of about 76 millimeters. Or, the warring sides could “put it to a vote, with two-thirds majority ruling, and see what happens, he suggested.

The head of a regional arm of the Prince Edward Island Fishermen’s Association, Lee Knox, threw cold water on that idea.

“As far as I’m concerned, the issue is dead to me, he said.

Lobster politics are snarling efforts to develop a business plan for the resource.

At the industry summit held in March and boycotted by New Brunswick fishermen, those present proposed a new levy of two Canadian cents per pound of lobster to finance the marketing of all the region’s lobster as “Canadian instead of by province.

But Prince Edward Island is focusing on the future. The province’s fisheries minister, Ron MacKinley, said the province has developed a new product that it plans to launch in supermarkets next year: pasteurized lobsters that will keep up to a month after cooking.

“It’s going to be the next big thing, he said.

2014 Dow Jones & Company, Inc.