What happened to the crabs?

The Alaska snow crab season was canceled for the first time ever because billions of crabs have disappeared from the Bering Sea. This mass population die off is a distressing blow to the Bering Sea ecosystem. The snow crab fishery closure will change sushi menus and seafood options around the world. The reality television show the Deadliest Catch will run spinoff content this season in Norway.


Missing pet style poster with photo of snow crab. Text reads: Missing. 7 Billion Snow Crabs. Last seen in Bering Sea. Call your representatives with leads. Thank you!
Totti | CC-BY-SA-4.0

2018 Snow Crab Population: 8 billion

2021 Snow Crab  Population: 1 billion


So, what happened? 

First off, we’re not finding those crabs alive. The question is what caused their sharp die-off. 

Let’s explore both the clearly culpable parties and some suspects deserving further investigation. 

Climate change and loss of sea ice

It’s clear that a warming climate harms snow crabs. They’ve moved North approximately 40 miles since 1980, chasing cooler water, and even then, that cold water is more scarce. Crabbers caught 92% of their harvest at a latitude north of 58°39’, while in 2011-2022, crabbers caught only 11% above that latitude. This means that the entire fishery has moved significantly North of where it was operating just a few years ago. 

This change in where we can find crabs has a lot to do with how warmer temperatures interact with sea water. When sea water freezes, it sheds a dense layer of briny cold water that sinks to the sea floor. Each summer, that cold briney layer of water and new sea ice melt sinking forms a cold pool on the seafloor. The pool is normally too cold for cod and other predators, so it protects the snow crabs from predation during parts of their lifecycle.

The increasing temperature of the ocean has altered the size of this cold, briny pool. During the ‘80s and ‘90s, the cold pool spanned 30-80% of the Bering Sea floor. In 2018, which saw the lowest level of sea ice in 5,500 years, the cold pool covered only 2% of the Bering Sea floor. 

Areal maximum ice extent in 2012 and 2018. The blue shaded area is 2012 cold pool and red is the 2018 cold pool extent. Note that the cold pool on the northern shelf was beyond the maximum ice extent. That is, it formed as a result of frigid atmospheric conditions, not sea ice.Photo by NOAA Fisheries | Public Domain


As the sea ice and its resulting brine pool disappears, cod can eat more and more crab throughout their life cycles. This problem is compounded by the fact that Pacific cod are also migrating North. In the 1970’s, fishery surveys reported only a few cod in the Bering Sea. In 2010, the northern population accounted for 3% of the fishery. By 2018, more than half of the fishery was in the Northern Bering Sea. 

All that to say, as the waters off Alaska warm, more snow crabs predators are moving North just as those snow crabs are losing their briny hiding spots.

Ocean Acidification

Warmer temperatures are having another impact on the composition of our ocean waters: Climate change is also increasing the acidity of the ocean, and the coast of Alaska has had the fastest increase in acidity in the U.S

A higher acidity makes it harder for crabs to molt and regrow their shells as they gain throughout their life. There are also indications that a higher acidity compromises crab immune systems. That said, estimates assume ocean acidification won’t impact crab populations significantly for a few more years. 


Snow Crabs can be subject to Bitter Crab Syndrome (BCS), a disease caused by Hematodinium, a parasite. The Shellfish Assessment Program has been investigating the prevalence in the Bering Sea since 1988. BCS has consistently been at relatively low levels, but a changing climate will likely increase the prevalence. While crabs infected with BCS won’t hurt humans, it causes crab meat to develop a bitter after taste and a chalky texture that won’t sell at market. The disease is fatal to crabs.


Some good news: fishing in the U.S., and especially in Alaska, is not a free-for-all. Crabbers have to abide by rules that set out how many crabs they can harvest from Alaska’s waters each year. The final harvest numbers for crabs over the last few years are in accordance with sustainability policies. While those policies should probably be a little more conservative, they’re strong relative to the rest of the world and the country

So if the fishery is managed relatively sustainably, can some of the disappearance of snow crabs be attributed to overfishing? 

The short answer: it’s complicated. 

While a population decrease of this size is called “overfished” by definition, NOAA does not attribute this population collapse to overfishing

Even if the overall number of snow crabs harvested and brought to shore for consumption has been within a sustainable range, the fishery might still have had an elevated impact in recent years:  Crabbers discard many of the crabs that come up in their pots, and that could be making a difference in the health of the population. 


Crabbers discard young and female snow crabs because they are legally required to (females aren’t allowed to be harvested because it helps ensure a successful future population to leave them). Crabbers also sometimes discard crabs that lack size or are unsightly in some way and therefore aren’t market grade. 

It is unclear how many crabs that are discarded survive the process, but biologists assume a 30% mortality rate. In 2017, the number of discarded crabs started rising, especially further South. The Alaska Department of Fish and Game estimates that in 2020, the discard rate was almost six times higher than in 2013 with 10 million pounds of crab dying in discard with only 34 million pounds being harvested. 

Those numbers mean that while the harvest was within guidelines- more crabs likely died than sustainability measures intended to permit. 


They [crabbers in the more southern Bering Sea] were pulling pots with 900 crabs in them, but they were only keeping 200 or 300. And they kept doing that. I didn’t partake in that. Mark Casto
Captain of the Pinnacle


Even if they’re not fishing for crabs, the broader fishing industry could still play a role in the decline of the population. 



Bycatch is all the marine life caught through fishing gear that is not the intended prey. In other words, bycatch is the collateral damage . In the Bering Sea, pollack fisheries use trawling gear that is particularly prone to bycatch. Trawling gear drags nets over the ocean floor to catch bottom-dwelling fish. Even when their nets don’t catch crab, they can drag crab along the seafloor, crush them, and rip up their habitat. It’s a destructive fishing method that poorly targets and results in plenty of unintended consequences.

And the problem could be getting worse: The melting sea ice opens up more of the ocean floor to trawlers. Their bycatch can include crabs, which they’re required to toss back in, but most die. Data from Global Fishing Watch shows that fishing boats including many trawlers are moving farther and farther North each spring (January through May) in correlation with less sea ice. 


This screenshot shows the concentration and location of fishing vessels between January 2012 and the end of May 2012 in the Bering Sea. The more vibrant green indicates more instances of fishing vessels occupying that space.

Photo by Global Fishing Watch | Used by permission

This screenshot shows a heat map of fishing vessel hours in the Bering Sea between January 2019 and end of May 2019. This season followed the particularly low sea ice year in 2018. Notice how many hours are further North in 2019 compared to 2012. Most of those hours are trawling boats.

Photo by Global Fishing Watch | Used by permission

This screenshot shows a heat map of fishing vessel hours in the Bering Sea between January 2021 and end of May 2021. Again, notice how many hours are further North in 2021 compared to 2012. Most of those hours are trawling boats.

Photo by Global Fishing Watch | Used by permission

This screenshot shows a heat map of fishing vessel hours in the Bering Sea between January 2022 and end of May 2022. Again, notice how many hours are further North in 2022 compared to 2012. Most of those hours are trawling boats.

Photo by Global Fishing Watch | Used by permission

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What do we do to restore the crab populations?

Obviously, we need to continue tackling climate change as quickly as possible- it underpins everything and is clearly doing tremendous damage. It’s culpable party number 1. However, we likely have opportunities to increase the resiliency of our marine life, including snow crabs, by adjusting some fishing policies and exploring the impacts of other suspects. 

We need to reevaluate discard and bycatch practices, frequency and mortality to ensure what goes into final harvest numbers is compatible with sustainable populations of all our sea life. A rapidly changing climate will require nimble, responsive and conservative practices in order to sustain our fisheries and marine ecosystems for the longterm.

Trawling is one of the most overarchingly destructive fishing practices and responsible for the most bycatch. However, it has continued as low populations have forced the closure of more responsible fisheries. We need to re-evaluate what role, if any, trawling gear can play in a sustainable fishery, and it would be smart to limit trawling in vulnerable areas until that re-evaluation is complete. 


Dyani Chapman

State Director, Alaska Environment Research & Policy Center

Dyani runs campaigns to promote clean air, clean water, and open spaces in Alaska. She lives in Anchorage and loves to hike, ski and cook yummy food.

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