Why Did 11 Billion Alaskan Snow Crabs Suddenly Disappear?

Did nearly 11 billion crabs suddenly vanish from the Bering Sea? The reason for this was announced earlier this month, when Alaska declared that the entire snow crab harvest for the year had been canceled.

Devastated, the region has been left in an abrupt closure. Valued at over $200 million annually, Alaska’s crab fishing is a substantial sector. In a consecutive year, authorities in Alaska have also scrapped the autumn harvest of red king crab in Bristol Bay. Due to its close proximity to the Arctic, this is a dire setback for those who rely on crab harvesting in a region experiencing rapid and abnormal warming. From 2018 to 2021, ninety percent perished, signaling a catastrophic decline in the animal population.

The team led by Dr. Litzow, the manager of the shellfish assessment program at the National Oceanic and Atmospheric Administration’s Center for Science Fisheries in Alaska, conducts annual surveys to help populate models that set fishing quotas for snow crab populations in the Bering Sea. Scientific communities, as well as the fishing industry, were surprised by the mysterious mass die-off of crabs and are still trying to figure out the details of what may have caused this. Experts widely agree that global warming likely played a major role in the disappearance of crabs, but they are still trying to understand the specifics.

This interview has been shortened and modified to enhance clarity.

When did you realize the extent of the decline in population? It seems like this could have been a dramatic thing to unfold.

According to Litzow, the situation was much improved for snow crab in 2018 compared to previous years. Therefore, our estimation in 2018 indicated a population of approximately 11.7 billion snow crab in the southeast Bering Sea, primarily consisting of juvenile individuals that were reaching the size suitable for harvesting. The abundance of snow crab witnessed in 2018 surpassed any previous records, which can be accurately described as remarkable, as stated by Litzow.

Conversely, we were apprehensive but not panicked–simply pondering what was transpiring. Abruptly, the profusion of diminutive creatures was only half as impressive as before, and even though it remained satisfactory, we ventured out and conducted the survey in 2019.

It was not expected and it happened very suddenly. There was a massive collapse in the population. The abundance went from 11.7 billion in 2018 to 940 million in 2021. In 2018, there were 10 billion fewer animals than there are now, and we experienced a decline due to COVID in 2020, of course, which continued into 2021.

The mass mortality event appears to resemble something else, as if it has been relocated to another place. It’s evident that the crabs are truly gone, which confirms the results we obtained from the survey conducted in 2021. We carried out the survey in the current year, 2022.

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What can we be sure of regarding what occurred in this location?

We cannot know that the Bering Sea is warming without human-induced global warming. In fact, in 2016, we observed a significant change. The Bering Sea was much warmer than anything we had seen before.

This is a narrative about climate change, all accessible information strongly indicates that in 2018, we experienced a temperature increase of 3.5 degrees. The mean temperature between 1982 and 2012 was 1.3 degrees when examining the central region of the snow crab habitat in our study.

Have we ever witnessed a die-off similar to this before?

No, the temperatures we have witnessed in the Bering Sea since 2016 have completely no prior occurrence. It has been exceptionally mild.

How precisely does a 2-degree temperature rise cause the demise of billions of crabs?

It is difficult to disentangle, but there are a few leading hypotheses to explain the first thing we know about the snow crab, an arctic animal found only in areas of the Bering Sea that have winter ice cover with temperatures below minus two degrees Celsius, taking you over an ecological threshold.

If you warm up the waters in the subarctic area, there is a hypothesis that it allows predatory ground fish to move into the snow crab range, causing a massive event of predation. It’s like flipping a switch, where if you increase the temperatures by 3.5 to 3 degrees, more subarctic ground fish can come into that area. However, most predatory ground fish, like Pacific cod, cannot tolerate those temperatures and are more abundant further south. It’s an arctic system where the bottom temperatures are colder than 2 degrees Celsius.

Another hypothesis suggests that there were certain diseases that affected the population when the incidence of those diseases increased.

The precise connection between the warming and the mortality occurrence is a blend of those explanations. They’re all believable. It’s challenging to untangle those diverse explanations. Additionally, another theory suggests that due to the substantial abundance of crab [resulting from the 2018 population surge] in a compact region [as they congregated in pockets of frigid lower water], with an exceedingly high metabolic requirement, they exceeded their accessible nourishment, leading to starvation.

How precisely did you determine that all those crabs vanished? How did this investigation function?

It gives us a comparison of how populations change over time. It also gives us an estimate of the abundance of crabs and fish. We collect different types of biological data, measure and identify all the crabs and fish, and determine their sex. Every year, we go out on a commercial fishing boat and set a bottom trawl net for half an hour. We then sample these 375 spots and collect data. The Bering Sea, which is located southeast across from the productive fisheries, has 375 standardized stations, making it a sort of breadbasket for all of these incredibly productive fisheries.

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To comprehend the situation, it is most effective to utilize a mathematical model of the population that encompasses all of these diverse datasets. Each dataset provides a unique narrative, and it is necessary to consider all of them. This includes data obtained from the capture of snow crab in other fisheries, data from the fishery, as well as survey data. Therefore, we incorporate the survey data, fishery data, and data from the capture of snow crab in other fisheries.

How do you know the crabs didn’t just relocate outside the scope of your stations?

When considering the available habitat in deeper waters, it becomes apparent that there is insufficient space for 10 billion crabs. However, upon closer examination, it is possible that the crabs relocated due to a rise in temperature, as suggested by the survey conducted in the northern Bering Sea. As expected, our survey in the northern Bering Sea confirms that the crabs did not migrate north. Additionally, there was speculation about the crabs moving deeper into the Bering Sea beyond the area we sampled, especially considering the extreme outcome of the 2021 survey.

The concept is not truly backed by the accessible data from the Russian perspective. That theory cannot be evaluated without information from the Russian side. The curiosity lies in whether the stock relocated to Russian waters.

Are snow crabs exceptionally susceptible? Are there other creatures we should anticipate to witness perish as well?

The snow crab population is declining in their arctic habitat, and some species have been observed moving north in large numbers. It is not surprising that these species would experience negative consequences from a warming event, and we should not expect immediate negative effects on them. It is worth noting that the term “subarctic” refers to the slightly warmer region just south of the arctic, where economically valuable species like red salmon, king crab, red crab, pollock, and cod are found. Among these species, snow crab is the most economically valuable in the arctic region, making them unique.

Did we know the degree to which snow crabs were susceptible to warming temperatures? Why did this die-off take people by surprise?

In the upcoming year or this current year, what truly matters is the well-being of a population like snow crab. However, it is exceedingly challenging to forecast the temperature for the following year or the subsequent year, considering the presence of various natural climate fluctuations. Nevertheless, it is certain that in 50 years, the temperature will be significantly higher compared to the present. Hence, we have been aware of the impending significant changes in a general sense, and the Arctic region is experiencing a more rapid warming compared to the entire planet. For several decades, we have been aware that if the sea ice in the Bering Sea diminishes, it will have a profound impact on snow crab, making it one of the most affected stocks. This has been a widely discussed topic for many years.

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The community was expecting that the Bering Sea, which has been covered in ice during winter for our lifetimes, would continue to be ice-covered. However, in 2018 and 2019, something unexpected happened. The ice in the northern Bering Sea disappeared much more quickly than anticipated. Oceanographers have recently informed us that this rapid and abrupt ice loss is due to strong and rapid warming. In some cases, we have observed a lot more warming in the Bering Sea than we had previously expected.

The response definitely predicts that we will have the ability to know that the temperatures in the upcoming winter will be much warmer than normal. We know this because we see very low sea ice cover and warm conditions in the preceding winter. So, it should be easier to make predictions about what we will see next time. But now that we have this observation of the Bering Sea in a really warm state, it’s hard to predict the exact consequences. We’ve never seen the Bering Sea this warm before, so it’s difficult to predict.

So what information do we have regarding the future of the snow crab population?

There may be some hope on the horizon for the fishery, as over the next four to five years, if we experience colder temperatures and the sea temperatures in the Bering Sea revert back to average conditions similar to the last 30 years, we might be able to commercially harvest smaller snow crabs. Additionally, we are currently observing a significant number of small snow crabs in this year’s survey.

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  • Unfortunately, the fishery does not look promising for the next four or five years. This year’s survey revealed that the small crabs, which would take five or four years to grow big enough and live long enough to support the fishery, are not sufficient in number. It is a difficult situation, and there are real hardships involved. However, this does not mean that we are giving up. We are hopeful that in the colder years ahead, we can revitalize the population of smaller crabs.

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