Located between the Haida Gwaii archipelago on the north coast of British Columbia and the southern tip of Alaska's Panhandle, Dixon Entrance’s nutrient-rich waters, which attract orcas, albatross and five species of salmon, surge towards the rocky shores and green forests of Prince of Wales Island and the mainland. At some point as we sailed through the waves, we left Canadian waters and entered the US. But really, the only way we knew we’d travelled from one country to the next is that our electronics jumped back an hour to Alaska Standard Time after we passed a Canadian Fisheries patrol boat on the lookout for border violators.

In fact, the actual line where we crossed from one country to the next has long been under dispute. Even before European contact with the nearby indigenous peoples, the Haida, Tlingit and Tsimshian occasionally warred over the land and sea boundaries in this abundant territory. These days, this boundary disagreement continues between new adversaries and the treasure at the heart of this dispute has evolved from furs and gold to salmon.

Though the US and Canada have the longest undefended border in the world, Dixon Entrance is one of four long-running border disputes between the friendly neighbours. The roots of the quarrel date back to the 18th Century; a time when the colonising stakeholders in the Alaskan Panhandle region (the narrow strip of mountains, fjords and channel islands bordering modern British Columbia) were England and Russia, followed by the US.

You may also be interested in:

• A US town only reached through Canada

• Canada’s tiny disputed island

• The US land lost in Canada

When the Russian fleet arrived in Alaska in 1732, they discovered that the area was rich with sea otters and set up a fur trade with the local indigenous people. By the 1760s, Captain James Cook had arrived and began mapping and surveying the North Pacific for the English. Then in the early 1800s, American explorers Meriweather Lewis and William Clark found their way to the Pacific Northwest.

Dixon Entrance is one of four long-running border disputes between the friendly neighbours

This set the stage for a territorial dispute. The local indigenous population was soon overpowered by illness and wars, and throughout the period of Russian colonisation, the southern and eastern borders of the Alaskan Panhandle were never firmly established. The 1825 Treaty of Saint Petersburg between England and Russia set the southern coastal border of the Panhandle near modern Prince Rupert, British Columbia – but the region was so mountainous that much of it remained unsurveyed. In 1867, the United States bought Alaska from Russia. A few years later, British Columbia joined Canada. Ottawa suggested to Washington, DC, that it was time for an official survey of the Panhandle so the two countries could agree on the border, but the US considered the effort too costly for such a remote piece of land.

But then gold was discovered; first in British Columbia and then in 1897 in the Klondike in Canada’s north-western Yukon territory. An estimated 100,000 prospectors migrated to the area, and it turned out that one of the easiest ways to reach the gold fields was to travel by sea through Dixon Entrance, into the fjords and then inland across the Panhandle. Canada wanted unimpeded travel to their territory, but the US wasn’t prepared to give up any of the land they’d recently considered too insignificant to map. The countries tried to come to a resolution, but by 1899, they’d reached a stalemate.

An international tribunal was formed in 1903 to solve the Alaska Boundary Dispute. Made up of six impartial jurists from the US, Canada and England, the group ended up setting Alaska’s eastern boundary 56km east of where the ocean touched the mainland coast. The southern boundary, known as the A-B line, extends from Cape Muzon, the southern-most point of Alaska’s Dall Island, straight east through Dixon Entrance to Portland Channel, where it wobbles around a few islands before heading up Portland Canal and into the Coast Mountains.

Canada was outraged by aspects of the decision, but another issue soon arose. As far as Canada was concerned, the A-B line was the dividing line between the nations; on land and at sea. But the US had a different opinion; they declared that the decision only pertained to the land border, and that according to maritime law, the sea boundary was actually 20km south of the line, halfway across Dixon Entrance. This disagreement continues today.

These days, about 1.5 million people sail across the contested border each year. Comfortably taking in the region’s soaring mountains and ruggedly picturesque islands from the decks of cruise ships, ferries and sailboats, most people are unaware of the border’s disputed status.

It may seem odd that two close allies still can’t compromise over the ownership of this narrow, 80km-wide and 50km-long passageway for the sake of easier international relations. But there’s a good reason: the Pacific salmon run.

The salmon’s abundant numbers and annual journey have made fishing a key industry in the Pacific Northwest’s economy. From the 1880s to 1950s more than 100 canneries and fishing villages sprang up throughout British Columbia, and in recent years, wild salmon from the province has been exported to 53 different countries. In this aquatic gold rush, Dixon Entrance is the jackpot: Through it flow five species; sockeye, coho, chinook, chum and pink, each returning from the ocean intent on reaching their specific home river in Alaska, British Columbia, Washington or Oregon where they spawn and die.

There to meet the salmon each year on their journeys home are two sets of boats following two sets of laws. With no mutually agreed-upon border, fishermen from both countries try to catch their share of the salmon from the disputed waters. The ongoing clash results in a slow, simmering battle that occasionally boils over – such as during the so-called “salmon wars” that pit Canadian and Alaskan fishermen against one another during the 1990s.

Ever since the 1930s, the US and Canada have tried to determine who “owns” which salmon. This is both an exact and opaque concept: because each salmon is returning not just to a specific river, but to a precise place on the river, unregulated fishing has caused some salmon runs to become locally extinct; in some cases destroying the indigenous food fishery.

It took 15 years, but the Pacific Salmon Treaty, whose goal was to ensure that fishermen from both countries had access to a fair share of spawning salmon in designated rivers along the Pacific Coast, was finally signed in 1985. Yet, when portions of the treaty expired in 1992, a six-year-long international incident began with both Canada and the US occasionally arresting each others' commercial fishing boats. Tensions escalated when a few hundred Canadian fishermen eventually blockaded an Alaska State ferry in Prince Rupert for three days in 1997 and effectively took its passengers hostage.

A new 10-year agreement that increased scientific management and indigenous fishing rights was signed in 1999 and eased the conflict, but it didn’t solve the border dispute. Instead, the stalemate quietly continued. Normally, Canada and the US each police their own fishermen in the disputed waters, but occasionally minor skirmishes arise when Canadian fishermen encounter Alaskan boats fishing in the disputed waters and call a 24-hour hotline that’s specifically set up for salmon-related violations. By passing along the GPS coordinates and photos of the violating boats, they’re able to file a complaint with Canada’s Department of Fisheries and Oceans. Since more violations tend to occur by American vessels than Canadian boats, the US hasn’t needed to set up such a hotline yet.

Meanwhile, in Canada, a movement highlighting the rights and needs of indigenous people has made the indigenous food fishery a priority. So, when part of the Pacific Salmon Treaty expires on 31 December 2019, the expected diminished salmon stocks may again disrupt the uneasy border truce.

Standing at the edge of the Adam’s River last November, almost 1,500km south-east (as the fish swims) from Dixon Entrance, Tanner Francois, of Secwepemc Nation was singing a salmon song, thanking the fish for making the long perilous journey back to their spawning grounds. In local lore, there are stories of rivers running so thick and red with sockeye salmon, that you could walk across their backs and reach the other side with dry feet. But on this chilly morning, just a few fishermen were left on the river casting for the last of the season’s run.

Without salmon we would not be

The annual migration through Dixon Entrance that sends salmon into rivers flowing through much of British Columbia has earned the salmon a central place in the minds of the First Nations’ people, and in the psyche of many British Columbians. Making a roundtrip journey of up to 4,000km from rivers and headwaters, out the sea and then back, the salmon feeds not just the economy, but orcas, bears and eagles that rely on the rivers. And they, in turn, feed the forest.

“Without salmon we would not be,” Francois explains.

So people on the coast clean and clear creeks and rivers to protect the salmon grounds. School kids get lessons at their local salmon hatcheries and set the newly hatched fry free in the rivers each spring. The indigenous people sing the salmon home through rivers and across mountain ranges. And the fishermen of Dixon Entrance guard their section of the ocean carefully, always on the lookout for border violations, ready to call a 1-800 phone number to settle a diplomatic dispute.

Meanwhile, the salmon swim under it all – oblivious to the politics of borders and boundaries. Above the waves, the cruise ship and ferry passengers are just largely oblivious as well. They marvel at the landscape and life; the snow-capped peaks, the green velvet mountains that reach for the sky, the waterfalls, sea otters, orcas, seabirds and bears – unaware that it’s all beautifully and perfectly connected by the salmon which swim deep below.

Join more than three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Worklife and Travel, delivered to your inbox every Friday.