Scientists will have increased capacity to forecast future outbreaks to support sustainable fisheries management.

Bitter crab disease is caused by microscopic parasites. Scientists have documented that most infected Tanner crab and snow crab die in laboratory experiments, indicating that the disease can be lethal. Tanner crab and snow crab in the Bering Sea have historically supported two of the largest commercial crab fisheries in the North Pacific. However, both stocks are at unprecedented low levels of abundance in recent years. 

Scientists used sensitive genetic detection methods to examine crab collected during the NOAA Fisheries annual bottom trawl survey in the eastern Bering Sea from 2015–2017. From this, they estimated the annual prevalence of bitter crab disease. It peaked at 42 percent in Tanner crab and 36 percent in snow crab during the 3-year study period.

“That is a nearly four-fold increase in the annual prevalence levels previously detected through standard methods for disease monitoring,” said Erin Fedewa, fisheries biologist, Alaska Fisheries Science Center. “Historically, our standard methods to detect the disease were through visual observation of collected crabs and examination of blood samples under microscopes. These methods produced estimates of less than 10 percent annual prevalence for both species in the survey.”

The big difference with genetic methods is that they enable earlier disease detection. With visual methods, scientists can only detect the disease at advanced stages, when there are visual symptoms present. Symptoms are characterized by the presence of milky-white hemolymph (a bodily fluid equivalent to blood) and by red-pink discoloration or “cooked appearance” of the shell and joint appendages. Infected crabs have a bitter taste hence the name. Thus, visual detection methods substantially underestimate true infection levels. 

Other Study Findings

For smaller snow and Tanner crabs, estimated peak infection probabilities were even more substantial than for adults. Prevalence levels approached or exceeded 50 percent for the smallest size classes of each species. Scientists also found that the probability for infection appears higher in female snow crabs.

If these findings are representative of the entire population, this is the highest prevalence of bitter crab disease detected to date in eastern Bering Sea Tanner and snow crab.

“Given that infections are assumed fatal, a 30–40 percent disease-induced death rate could represent a substantial reduction in young crab (pre-recruits) entering both populations and, ultimately, commercial fisheries, especially if mortality events are compounded across multiple years,” added Fedewa.

Best Available Science Ensures Sustainability and Profitability of U.S. Commercial Fisheries

In 2021, the combined snow and Tanner crab fisheries reached a record high value exceeding  $250 million. The 5-year average value is around $151 million (2017–2021). The snow crab fishery has been the focus of “Deadliest Catch,” a widely viewed reality television show. 

However, both stocks have been at low levels of abundance in recent years. The Tanner crab stock has been at a persistently low level of abundance over recent decades. In this population, recruitment pulses (large groups of young crab born in a given year) often fail to make it to larger sizes that support the fishery. Between 2018 and 2021, snow crab experienced an unprecedented decline, with large declines in immature crab abundance in the eastern Bering Sea. In 2021, allowable catches were reduced by approximately 90 percent. The fishery was closed for the first time in 2022 and the State of Alaska requested that the Secretary of Commerce declare a federal fishery disaster in 2022. Declines in snow crab productivity have been linked to recruitment failures and mortality events.

Information from this study sheds some light on a potential contributing factor in the 2018–2019 mortality event linked to the snow crab population collapse. Scientists documented a 10 percent annual increase in bitter crab disease prevalence from 2015 to 2017 in Eastern Bering Sea snow and Tanner crab populations. This directly preceded the snow crab collapse.

“More importantly, with the help of genetics we can detect the disease at an earlier stage, potentially improving our forecasting capabilities to help fishermen and managers anticipate future disease outbreaks,” said Mike Litzow, Kodiak Laboratory Director and Shellfish Assessment Program Manager for the Alaska Fisheries Science Center.

Fishery managers rely on scientists to identify the factors driving highly variable recruitment and unpredictable population fluctuations to help them effectively manage Tanner and snow crab stocks.

Some next steps for research that scientists recommend:

• Continue bitter crab disease monitoring in the Bering Sea while Tanner and snow crab populations rebuild

• Conduct long-term laboratory exposure experiments to better isolate the effect of environmental variables on bitter crab disease progression and host mortality

• Use estimates of disease prevalence to inform natural mortality estimates in crab stock assessments, or indirect management approaches to provide context for rebuilding timelines or recruitment projections 

NOAA Fisheries

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