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The town of Nuiqsut, Alaska, sits on the Kuukpik (Colville) River near the Beaufort Sea. Residents harvest seals and ptarmigan, goose eggs and caribou. But nearly one-third of their subsistence diet comes from fish.

The largest catches come during the fall, when Arctic cisco (qaaktaq), broad whitefish (aanaakłiq), and other species move from the ocean into the fresh water to feed and spawn. Local fishers set gill nets in the river or under the ice, hauling in tens of thousands of kilograms of fish annually for the village of about 425 mostly Iñupiat residents. But in recent years, an unusual illness has affected their catch.

In early October 2013, local fishers Eli Nukapigak and Edward Nukapigak Jr. alerted wildlife officials to the discovery of “sick fish” in their nets near Nuiqsut. The aanaakłiq had fuzzy grayish-white patches on their bodies, fins, and heads. Cottony masses almost covered the eyes of some fish. None of the fishers in the community recalled seeing this condition before.

The impact on the community was immediate: some stopped fishing for aanaakłiq, while others refused to eat even uninfected fish that had been caught in the same net with infected fish. Some moved away from traditional fishing locations and others pulled up their nets for the season.

Food security can be precarious in northern communities if more than one target species comes under threat or if the threat appears early in the harvest season. “[A household] might only have 50 pounds [22.7 kilograms] of fish in their freezer, and [that] could mean 350 pounds [159 kilograms] of fish missing,” says Henry Huntington, a researcher from Eagle River, Alaska, who studies subsistence hunting and traditional ecological knowledge. The alternatives to harvested foods in many northern communities are often expensive and unhealthy.

The week after the Nukapigaks’ report, the state fish pathology laboratory identified the growth as a Saprolegnia-like mold. Although the condition is unsightly, the lab considers infected fish safe to eat. “I’m sure nobody ate them,” says Todd Sformo, a wildlife biologist for the North Slope Borough, who investigated the incident and reported the findings in a recent study. “I’m not real squeamish, but once you see that, you don’t want to eat it.”

DNA analysis identified the mold as Saprolegnia parasitica, which is known for causing savage infections in fish eggs and juvenile fish in hatcheries around the world. Often associated with environmental stressors, it kills an estimated 10 percent of hatchery salmon globally.

Although Saprolegnia parasitica is widespread in fresh water in mild and warm climates, it is rarely seen in Alaska. Sformo says a single whitefish caught in 1980 was the first confirmed case of this infection on the North Slope, but it was not in the Nuiqsut area.*

No one knows yet why the infection has suddenly taken hold. “That’s the million-dollar question,” says Paul de la Bastide, a mycologist at the University of Victoria in British Columbia. The river’s water quality—concentrations of metals, diesel, and organics, for example—doesn’t seem to have changed in 20 years, though longer-term data is lacking, says Sformo. There are no water temperature records, but residents say that warmer temperatures and an earlier spring thaw now erode riverbanks and lead to murkier water.

Climate change is having huge effects on Arctic ecosystems. Thawing permafrost allows rivers to carve into once-frozen soil and dump sediment into aquatic ecosystems, which can upset nutrient balances and water chemistry, and stress fish. Warmer air and water temperatures can also bring new diseases to fish, birds, and other wildlife, including some that can spread to humans.

De la Bastide has strong suspicions that human activities have played a role in the Saprolegnia infections near Nuiqsut. “I would bet a lot of dollars that this is a native [organism],” he says. Changes in climate and the environment, such as warmer waters, can make it easier for the mold to attack and trigger an infection.

Six years after the infection’s discovery, Sformo continues to investigate, now in collaboration with de la Bastide through the Fulbright Arctic Initiative.** It has reappeared in aanaakłiq annually—always during the last week of September and early October when the fish are spawning. Fortunately, it hasn’t yet shown up on the more heavily fished Arctic cisco.

Nuiqsut is surrounded by oil development, which limits where the locals can fish. “Maybe in the old days, if you saw a weird fish in one branch of the river, you could go somewhere else,” says Huntington. Industrial development, he says, has made it harder for locals to adapt to fast-changing environmental conditions.