Genetic Testing Uncovers High Infection Rates in Commercially Vital Bering Sea Crabs

A recent study using advanced genetic testing has revealed alarmingly high infection rates of bitter crab disease in Tanner and snow crabs, two commercially important species in the Bering Sea. Scientists estimate that up to 42 percent of Tanner crabs and 36 percent of snow crabs were infected during the study period, considerably higher than previous estimates.

Bitter crab disease, caused by microscopic parasites, has been shown to be lethal in a lab setting. Traditional detection methods which relied on visual observations and blood sample analysis, suggested infection rates of less than 10 percent. However, the more sensitive genetic methods used in this study show a drastically different situation.

“That is a nearly four-fold increase in the annual prevalence levels previously detected,” said Erin Fedewa, a fisheries biologist at NOAA’s Alaska Fisheries Science Center.

Unlike visual methods, which can only identify the disease in its advanced stages, genetic testing allows for earlier detection, even before external symptoms appear. Symptoms include milky-white hemolymph (blood), a “cooked” red-pink shell appearance, and bitter-tasting flesh—hence the disease’s name. However, by the time these signs become visible, the infection is often severe, leading to an underestimation of true prevalence levels.

Findings Raise Concerns for Crab Fisheries

The study also found that infection rates were even higher in smaller crabs, with some size classes showing 50 percent or greater prevalence. Female snow crabs also appeared to be more susceptible to infection.

If these findings are representative of the entire population, this marks the highest prevalence of bitter crab disease ever recorded in Bering Sea Tanner and snow crabs. The disease is believed to be fatal. With a 30–40 percent infection rate, this could have severe consequences for young crabs within the population and, ultimately, the commercial fisheries that depend on them.

“A disease-induced death rate of this magnitude could represent a substantial reduction in young crabs entering both populations,” Fedewa explained. “This could have major implications for fisheries, especially if mortality events occur across multiple years.”

In 2021, the combined Tanner and snow crab fisheries were valued at over $250 million, with a five-year average of $151 million. However, both species have experienced unprecedented declines in recent years.

The Tanner crab population has been struggling for decades, with recruitment failures preventing juvenile crabs from reaching fishable sizes. Snow crabs suffered an unprecedented collapse between 2018 and 2021, leading to a 90 percent reduction in allowable catches in 2021 and the first-ever fishery closure in 2022. The State of Alaska even requested a federal disaster declaration for the snow crab fishery that year.

Potential Clues to the Snow Crab Collapse

This study sheds light on a possible contributing factor to the 2018–2019 mass mortality event that preceded the snow crab collapse. Researchers found that bitter crab disease prevalence increased by 10 percent annually between 2015 and 2017, just before the population crash.

“With the help of genetics, we can detect the disease at an earlier stage, potentially improving our forecasting capabilities,” said Mike Litzow, director of NOAA’s Kodiak Laboratory and Shellfish Assessment Program Manager.

Improved disease monitoring could help fishery managers anticipate future outbreaks and better understand the forces driving population fluctuations.

Next Steps for Research

Scientists recommend several follow-up actions to address the growing concerns over bitter crab disease:

• Continue monitoring in the Bering Sea as Tanner and snow crab stocks attempt to rebuild.
• Conduct long-term laboratory studies to examine how environmental factors influence disease progression and mortality.
• Incorporate disease prevalence data into crab stock assessments to better predict population trends and inform management decisions.

According to the Alaska Department of Fish and Game, there is no danger to humans if a crab caring bitter crab disease is consumed. Many people compare the unpleasant flavor of infected crab to chewing on an aspirin.

As commercial crab fisheries face increasing pressure, genetic testing may become a critical tool for conservation and industry sustainability, providing early warnings and data-driven strategies for protecting these valuable and important stocks.