Snow Crab in Warming Waters

October 21, 2020

Recent dramatic shifts in snow crab population structure highlight the importance of monitoring change to keep valuable Alaska fisheries productive and sustainable.

Photo of a pair of Bering Sea snow crabs on a lab table. Bering Sea snow crab support a valuable commercial fishery. Photo: NOAA Fisheries.

In 2019, masses of large Alaska snow crab appeared in the northern Bering Sea, where they had not been observed during past surveys. At the same time, the number of small snow crab plummeted. Across all sizes, snow crab range shrank. 

These shifts occurred during a time of unprecedented warming and loss of sea ice in the Bering Sea.

Those were some of the findings from the first study examining snow crab responses to recent rapid climate shifts. The study compared recent bottom trawl survey data from the northern Bering Sea with data collected over 30 years by the Alaska Fisheries Science Center in the southeastern Bering Sea. It provided new insight to help manage a valuable Alaska fishery in changing waters.   

“Our motivation was to better understand how recent anomalous conditions are affecting the Bering Sea ecosystem,” said Erin Fedewa, NOAA Fisheries biologist at the Alaska Fisheries Science Center, who led the study. “Snow crab were an obvious species to study to look at potential effects of warming.”

Snow Crab and the Cold Pool

Unsurprisingly, snow crab like cold water. Snow crab support valuable fisheries in the frigid waters of the eastern Bering Sea and North Atlantic.

But how cold snow crab need their habitat to be changes over the course of a crab’s life. Immature snow crab live in colder waters; as they mature, they migrate to slightly warmer habitat.  

Although snow crab dominate the bottom-dwelling biomass of the northern Bering Sea (NBS), there has been no commercial fishery for them there. Historically, only immature and small crab have been found in these northern waters. Too cold temperatures have likely prevented snow crab from growing to commercial size in the NBS.  

The eastern Bering Sea (EBS) is the southern extent of snow crab range in the North Pacific. It supports one of the largest crab fisheries in the world. 

The large male crab targeted by the EBS fishery have historically been associated with the “cold pool.” The cold pool is a body of less-than 2 ℃ bottom water left behind by melting sea ice. 

Rising Temperatures and Receding Sea Ice

The years 2017–2019 brought unprecedented warming to the Bering Sea. Sea ice extent was the lowest on record in 2017–2018. In 2018, for the first time, the cold pool was virtually nonexistent.

”The changes we are seeing are worrisome for Arctic species like snow crab. Temperatures suitable for them could begin to disappear in the EBS,” said Fedewa. “There are two possible ways snow crab could respond: acclimate or move.”

Gif heat map image showing shifts in Bering Sea bottom temperatures across multiple years.
Bering Sea bottom temperatures during a cold year (2010), and recent warm years (2017-2019). In 2018 the <2 ℃cold pool did not form for the first time on record. Figure: NOAA Fisheries.

Long-Term Monitoring Reveals Patterns of Change

To understand how snow crab are responding to climate change, the team needed to look at population structure across Bering Sea regions and over time.

Annual Alaska Fisheries Science Center bottom trawl surveys provided the time series data that made that possible.

“The NOAA Fisheries annual bottom trawl survey offered the means to examine snow crab distribution patterns during both cold and warm periods,” said Fedewa.  

Eastern Bering Sea survey data used for the analyses span 1988–2019. The NBS was surveyed in 2010 and 2017–2019.

Map showing standard northern and eastern Bering Sea NOAA Fisheries survey areas and sampling stations.
Standard survey area for the NOAA Fisheries eastern Bering Sea (1988–2019) and northern Bering Sea (2010, 2017–2019) summer bottom trawl surveys. “X” denotes standard station centers, while “♦” denotes northern Bering Sea (NBS) station centers in 2018. The dashed line labeled EBS-NBS divide is the boundary between the two survey areas. Figure: NOAA Fisheries.

 

The scientists compared snow crab abundance and size structure across years. They looked for evidence of range contraction into the NBS. They evaluated temperature, cold pool extent, and cold pool latitudinal center as drivers of snow crab distribution.

Photo of Erin Fedewa sorting snow crab on a boat deck.
NOAA Fisheries biologist Erin Fedewa determines the size, sex, and abundance of snow crab sampled during the 2019 Bering Sea bottom trawl survey. Photo: NOAA Fisheries.

Dramatic Shifts During Recent Climate Changes

The study revealed major changes in snow crab size structure and abundance  under recent anomalous warm conditions. 

No Evidence of Northward Migration

Extreme temperatures in 2018–2019 exceeded the cold-water temperature preferences of juvenile snow crab. Nevertheless, study findings suggest that snow crab in the EBS did not redistribute to colder habitats.

Shrinking Snow Crab Range

Though there was no evidence of a northward population shift, higher temperatures and a reduced cold pool resulted in a smaller area occupied by snow crab.

Plummeting Abundance of Small Crab Across Range

The number of juvenile snow crab dropped substantially across their range in both the EBS and NBS. The decline coincided with extreme warming in 2019. 

“We are not able to identify whether the decline is a direct effect of warming on survival of juveniles that require cold water habitat,” Fedewa said. “But it is a red flag. We need continued monitoring and targeted studies to better understand snow crab vulnerabilities to climate change.”

A Vanishing Refuge from Predators  

In addition to potential direct effects of temperature on juvenile survival, climate change may influence snow crab survival indirectly through predation.  

While there was no evidence of a northward population shift by snow crab, many other species have begun to move north. Large-scale northward shifts of commercially important fish like Alaska pollock and Pacific cod have been reported during recent warming.

Historically, snow crab in the EBS concentrated in the cold pool. Major crab predators like Pacific cod were restricted to warmer waters. The cold pool acted as a thermal barrier preventing predators from moving north. It provided a refuge from predation for snow crab. 

In the last decade, that has changed.

“Climate changes are opening avenues for increased predation pressure on snow crab. In the NBS we have a new predator—Pacific cod—that has never been there before,” said Fedewa. “Pacific cod predation on snow crab in the NBS may explain continued declines in juvenile snow crab abundance since 2017.”

Graph plot showing x axis snow crab-pacific cod spatial overlap percentage and y axis mean bottom temperature.
Shifts in species distributions with warming temperatures have increased snow crab interaction with predators like Pacific cod. Figure: NOAA Fisheries.

Soaring Numbers of Large Crab in the North

The most striking finding was a dramatic shift in snow crab size structure in the NBS.

Median size of both male and female populations rose substantially in 2019 due to an increase in abundance of large snow crab. Abundance of the largest size class of male snow crab shot up by more than 2000 percent from 2018 to 2019. 

“That was the biggest surprise. We’ve always seen huge catches of snow crab in the NBS, but the crab were typically small juveniles. Until 2019. I saw it first hand, on the back deck during the surveys—these large male snow crab that we’ve never seen in the NBS. Soon it became very apparent that there were substantially more than we’ve seen in past years,” said Fedewa.

The origin of these large males in the NBS remains unclear. There was no evidence that the entire snow crab population shifted north to escape warming waters. However, it is possible that the adult male segment of the population did. Tagging studies have documented migrations of more than 100 kilometers by mature male snow crabs.  

An alternate explanation is that warmer NBS waters made it possible for crabs to grow faster and reach a larger mature size. That could mean the development of a self-sustaining mature snow crab population in the NBS. 

These findings have important implications for management and industry. 

“Our finding that large, legal-sized male snow crab exist outside of the EBS survey area highlights the need to incorporate NBS survey data into the snow crab stock assessment,” Fedewa said. “If these large males move south into the EBS  during the winter fishery, they need to be accounted for to set sustainable harvest quotas.”

Photo of staff sizing up a catch of small and immature snow crab on a boat deck.
Until 2019, small and immature snow crab dominated in the northern Bering Sea. Photo: NOAA Fisheries.

Shifting Into the Future

The possible development of a self-sustaining mature snow crab population suggests a potential future NBS fishery. But cascading effects of continued warming, increased predation, and declining numbers of juveniles may negatively affect Bering Sea snow crab production in years to come. 

“The challenge of managing a stock prone to climate-driven shifts emphasizes the importance of assessment,” said Fedewa. “Our results underscore the importance of and need for continued surveys, especially in the NBS.”

The research also points to implications for the Bering Sea ecosystem beyond snow crab. 

“Our findings highlight the underlying importance of cold water habitat,” Fedewa said. “As we see more and more stocks respond to change, the emphasis will need to be on more concerted efforts to understand what is happening in the NBS.” 

 

This research was a collaborative effort between NOAA Fisheries Alaska Fisheries Science Center (E.J. Fedewa, J.I. Richar, J.L. Gardner, M.A. Litzow) and the Alaska Department of Fish and Game (T.M. Jackson).