The science of sustainable seafood, explained

Trying to find answers for the mysterious Bering Sea crab collapse

On October 8th, the Alaska Department of Fish & Game (ADFG) announced more bad news for the state’s commercial crab fishermen. The snow crab quota for 2021-2022 will be 5.6 million pounds, down 88% from the 45 million pounds allocated in 2020. This decision came one month after the Bristol Bay red king crab fishery was completely closed for the winter 2021-2022 season. Altogether, these fisheries are expecting to lose around $200 million.

Jamie Goen, executive director of Alaska Bering Sea Crabbers told Alaska Journal of Commerce, “It is simply catastrophic. We have boats that are not going to be able to make their payments, vessel repairs that will be delayed, and long-time skippers and crew that are losing their jobs, not to mention all the downstream effects to processors, communities, supply chains, and support businesses.”

The Bristol Bay red king crab fishery closure was not completely unexpected. In 2019, the last year of survey data, managers reduced the total allowable catch (TAC) by 12%, down to 3.8 million pounds, marking a 6-year decline in male king crab numbers. In 2020, the average weight for male king crabs in the Bering Sea was 7.1 pounds with a low catch per unit of effort. This meant fewer, smaller male crabs in the population, suggesting future weak age classes due to poor recruitment.

Snow crab populations in the Bering Sea have a history of boom-and-bust years. In 1998-1999 the fishery was closed and declared overfished, however, the drastic quota slash for 2021-2022 was alarming. “We really do think that … some sort of mortality event did occur,” Katie Palof, an Alaska Department of Fish and Game biologist who advises the North Pacific council about crab, told the Seattle Times in October. The quota is at its lowest level in 40 years.

The cause of these collapses is unclear. Warming water temperatures are probably a factor, potentially causing migrations, mass die offs, and increased predation on juvenile crabs. Some have speculated that bottom trawl gear from other fisheries have impacted king and snow crab populations, producing unseen bycatch. The crab industry submitted comments in February to the North Pacific Fishery Management Council regarding concerns over unobserved and unaccounted mortality of crab bycatch in the Bering Sea and Aleutian Islands. But the data is relatively new and more research is needed before various theories and mechanisms can be confirmed.

To better understand the best available science, we spoke with Cody Szuwalski, research fishery biologist at NOAA, specializing in stock assessments and management strategy evaluations under climate change scenarios for snow crab and Pribilof Island red king crab.

What kind of precedent is there for these collapses, and to what degree were they predicted?

King crab and snow crab have different issues. Three years ago, we observed a snow crab cohort coming through the population that appeared to be the largest ever recorded. But this year there were fewer snow carb on the shelf than we have ever observed before. It’s snow crab whiplash almost, going from the highest of highs to the lowest of lows.

Red king crab, on the other hand, was not as boom or bust—that population has been declining since the early 2000s, with poor recruitment for about 10 years now.

Snow crab was declared overfished in 1999, so there is some precedent with this fishery. But in 1999 the exploitable biomass was around 52,000 tons. This year there were only 12,000 tons on the survey, just one quarter of the biomass when it was last declared overfished.

When you add up all of the mature male crab surveyed, for all the crab species in the Bering Sea, this is the lowest it has ever been. There is no precedent for this year.

To see the collapse this year was a total surprise, we were not expecting it so soon. We have been working to predict the future distributions and productivity of the three big crab stocks in the Bering Sea (snow crab, red king crab and tanner crab). We thought snow crab would move north and west, and the productivity would go down. We thought Bristol Bay red king crab would move north but we weren’t sure of its potential productivity, and we thought tanner crab would probably back fill for snow crab. When we look at the survey data that’s pretty much what happened this year, except we expected it to happen twenty years in the future and gradually.

We rely on the summer survey, but we weren’t able to perform one in 2020. For snow crab, that’s especially bad. We don’t exactly know what happened between 2019 and 2021. 2018 was the year with more crab than we had ever seen. In 2019, it appeared the biomass was half of what it was the year before. Something was happening from 2018 to 2019, and if we had the 2020 survey, we may have been able to anticipate this a little more quickly.

Was this a mass die-off event?

It is possible that the crab may actually be alive, but they have eluded our surveys for some reason. They could have migrated to new waters, or they are still on the shelf but survey gear didn’t work as predicted. We actually had a survey for the northern Bering Sea and they weren’t there in numbers and sizes that would account for the missing crab in the eastern Bering Sea. The eastern Bering Sea shelf survey worked as expected for tanner crab, so it seems to have done its job and therefore would have found other crab species if they were present. I was able to find an assessment for Russian crab in the western Bering Sea, but the catch per unit effort (CPUE) dipped in 2020. If our crab was walking into Russian waters, you probably wouldn’t expect their CPUEs to drop. The last place they could be is down the slope, but we don’t have a lot of data to confirm or refute that possibility. The slope is very small compared to the shelf, so you wouldn’t expect all the missing crabs to be in such a small area. However, fishers also reported some of the best fishing they had seen in a long time up on the Russian border in deeper water on the slope. This is one of the scenarios we’re hoping to examine further and understand as to how much crab might be on that slope. The last slope survey was conducted in 2016.

On the other hand, it could be that they all died. Predation, disease such as bitter crab syndrome, fishery effects, temperature effects and maybe even cannibalism, are all possible factors.

The missing crab were not of the size to be impacted by the directed fishery, so we don’t think that the decline in the stock is primarily driven by the directed fishery (the pot fishery that targets just crab). Targeted crabs were too large to be the ones that were missing.

Trawl fisheries do sometimes hit the smaller crabs, but we have good observer data to understand the extent of that threat. We also have data from experiments specifically aimed at measuring the amount of crab hit by the trawl gear but not brought up in the nets. Of those crabs, there was only about a 15% mortality rate. Altogether, it’s a vanishingly small amount of mortality associated with trawling bycatch.

The final consideration is rising temperatures. In 2018 and 2019 the Bering Sea was exceptionally warm. There was no cold pool, something often associated with snow crab. While we do know from laboratory studies that snow crab can handle higher temps than we see in the cold pool, we also know higher temperatures can increase the rations that they need, requiring they eat more to stay alive. That’s one of the things we want to track down, look at the prey biomass and if that influenced their metabolic needs.

The lack of a cold pool could also facilitate more predation from Pacific cod and other species. However, I don’t think the large sockeye runs in recent years would be a source of greater predation. The missing crab were in their benthic stages and too large for salmon.

I believe the best available evidence we have suggests that a mortality event contributed to at least some of the decline we’ve seen in snow crab over the last two years. There’s a possibility that some of the crabs have moved elsewhere into deeper waters. But given the increased cod consumption, the disease and the increased temperatures, a mortality event likely played some role in the declines. But while we have seen mortality events of this scale for king crab in the past, we have never seen it for snow crab.

Will these fisheries be managed differently in the future?

These sorts of challenges are likely going to be something that we increasingly have to face as we deal with environmental change, particularly in the northern latitudes. I don’t think that there is a defined path for what we will do differently in the future, but we are working on a framework to ask those sorts of questions.

An interesting management consideration is that as the ice retreats in the northern Bering Sea, more snow crab may be available to be fished. Historically, it was not a place that was fished because there was too much ice there. The crabs in the northern Bering Sea are much smaller than the crabs in the eastern or southern Bering Sea, so they are not a commercial size right now. But we are trying to understand if these reductions in ice are also going to correspond to increases in the size of crab in the northern Bering Sea, which would then precipitate crabbers thinking harder about whether or not they want to make the trek into the northern Bering Sea to catch the crab.

Jack Cheney

Jack Cheney

Jack has sourced, sold, cooked, and sustainably certified seafood over the past 10 years. In addition to his contributions to Sustainable Fisheries UW, he is working to increase traceability into supply chains and educate consumers, chefs and retailers on the value of environmentally sustainable seafood. He earned a Master's in Marine Affairs from the University of Washington in 2015.

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