Red Crabs and Climate Change: Christmas Island, the Gulf of Alaska, and the Bering Sea Collapse

September 02, 2024

Red Crabs and Climate Change: Christmas Island, the Gulf of Alaska, and the Bering Sea Collapse

"Red crab" refers to two very different animals facing very different climate-related threats. The Christmas Island red crab (Gecarcoidea natalis) is a land crab famous for its spectacular annual migration — tens of millions of crabs crossing the island to breed at the coast. The Alaskan red king crab (Paralithodes camtschaticus) is a commercially fished marine species that has experienced two major population collapses in recent decades. Both are being affected by climate change, though in different ways and through different mechanisms. This article covers both.

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Part 1: Christmas Island Red Crabs and Climate Change

The Christmas Island red crab (Gecarcoidea natalis) is endemic to Christmas Island in the Indian Ocean. An estimated 40–50 million red crabs live on the island, and each year they undertake one of the most spectacular wildlife migrations on Earth — traveling from the island's forests to the coast to breed, then returning. The crabs play a critical role in the island's ecosystem: their burrowing aerates the soil, their droppings fertilize the forest floor, and they consume leaf litter and seedlings, shaping the forest's composition.

How Climate Change Is Affecting Christmas Island Red Crabs

1. Disrupted Migration Timing

The red crab migration is triggered by the onset of the rainy season and the lunar cycle. Rising temperatures are causing the rainy season to shift, which can trigger early migrations — crabs arriving at the coast before conditions are right for breeding. This mistiming has two consequences: reduced offspring survival (eggs require moist conditions to hatch; dry conditions increase mortality) and disrupted breeding synchronization (the mass simultaneous release of eggs into the ocean is critical for overwhelming predators — desynchronization reduces survival rates).

2. Increased Droughts

Red crabs are moisture-dependent — they must keep their gills moist to breathe and can die from dehydration during extended dry periods. Climate change is contributing to more frequent and prolonged droughts on Christmas Island, increasing mortality during the migration and reducing the availability of food and shelter in the forest habitat.

3. Ocean Acidification

Red crab larvae spend several weeks in the ocean before returning to land. Ocean acidification — caused by elevated CO₂ dissolving into seawater and lowering its pH — reduces the availability of calcium carbonate, which the larvae need to form their exoskeletons. Weaker exoskeletons make larvae more vulnerable to predation and environmental stress, reducing survival rates during the critical larval stage.

4. Yellow Crazy Ant Invasion

While not directly caused by climate change, the yellow crazy ant (Anoplolepis gracilipes) invasion of Christmas Island has killed an estimated 10–20 million red crabs since the 1990s. Climate change may be exacerbating this threat by creating conditions that favor ant population growth. The Australian government has conducted extensive ant eradication programs to protect the red crab population.

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Part 2: Alaskan Red King Crab and Climate Change

The Gulf of Alaska Collapse (1983)

The Gulf of Alaska was once home to a thriving red king crab fishery. In the early 1980s, the population began a catastrophic decline, leading to the closure of the fishery in 1983. Despite decades of research, the exact causes remain unclear. Suspected factors include:

• Climate variability: Changes in water temperature and ocean currents disrupted the crab's habitat and food sources
• Overfishing: Intensive fishing pressure before the collapse likely prevented recovery
• Predation: Increases in Pacific cod populations, which prey heavily on juvenile king crabs, may have suppressed recruitment

The Gulf of Alaska red king crab population has never recovered. Despite conservation efforts and fishing closures spanning four decades, the species remains largely absent from the Gulf, particularly around Kodiak where it was once most abundant. This remains one of the most puzzling unsolved problems in Pacific fisheries science.

The Bering Sea Collapse (2021–2022)

The Bering Sea supports the largest red king crab fishery in the world. In 2021, NOAA surveys revealed that the population had dropped from approximately 8 billion crabs in 2018 to fewer than 1 billion — a decline of nearly 90% in just three years. The fishery was closed for the 2021–2022 season and again for 2022–2023, devastating the Alaskan fishing industry.

The leading hypothesis for the collapse is starvation driven by climate change: warming Bering Sea temperatures reduced the cold-water habitat that juvenile king crabs depend on for survival (the "cold pool" — a layer of water below 2°C that forms in winter and persists through summer). As the cold pool shrank due to warming, juvenile crabs were forced into warmer waters where their metabolism increased, their food supply was insufficient, and mortality rates spiked. A 2023 study in the journal Science found that starvation — not disease, predation, or fishing — was the primary driver of the collapse.

Economic impact: the closure resulted in the loss of thousands of jobs across fishing, processing, transportation, and related industries, with estimated economic losses exceeding $200 million.

Fisheries Management Failures

While climate change is a major factor, the Bering Sea collapse also exposed failures in fisheries management:

• Inadequate monitoring: The population decline was not detected early enough to allow for a gradual reduction in catch limits before the collapse
• Crab rationalization program: The 2005 crab rationalization program, which consolidated fishing permits among fewer vessels, has been criticized for reducing the industry's resilience and its ability to adapt to population changes
• NPFMC oversight: The North Pacific Fishery Management Council failed to predict or adequately respond to early warning signs of the decline

A comparison with Norway, Japan, and Russia — where red king crab populations (introduced or native) remain relatively stable — suggests that fisheries management practices, not just climate change, play a critical role in population outcomes. A holistic management approach that incorporates climate projections, predator-prey dynamics, habitat monitoring, and adaptive catch limits is essential for preventing future collapses.

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FAQs: Red Crabs and Climate Change

What caused the Bering Sea king crab collapse in 2021?

The leading scientific explanation is starvation driven by climate change. Warming Bering Sea temperatures reduced the "cold pool" — a layer of water below 2°C that forms in winter and persists through summer, which juvenile king crabs depend on for survival. As the cold pool shrank, juvenile crabs were forced into warmer waters where their metabolism increased, their food supply was insufficient, and mortality rates spiked. A 2023 study in the journal Science identified starvation — not disease, predation, or fishing — as the primary driver. The population dropped from approximately 8 billion crabs in 2018 to fewer than 1 billion by 2021 — a decline of nearly 90% in three years.

Will the Bering Sea king crab fishery reopen?

The fishery was closed for the 2021–2022 and 2022–2023 seasons. Whether and when it reopens depends on NOAA stock assessment surveys conducted each year. Recovery requires the cold pool to return to sufficient size to support juvenile crab survival — which depends on Bering Sea temperatures in any given year. As of the most recent available data, the population has shown some signs of stabilization but has not recovered to levels that would support a commercial fishery. Check NOAA's Alaska Fisheries Science Center for current stock assessment data.

Are Christmas Island red crabs endangered?

Christmas Island red crabs (Gecarcoidea natalis) are not currently listed as endangered, but their population has been significantly reduced by the yellow crazy ant invasion, which has killed an estimated 10–20 million crabs since the 1990s. The Australian government has conducted extensive ant eradication programs to protect the population. Climate change poses an additional long-term threat through migration timing disruption, increased droughts, and ocean acidification affecting larval survival.

Is Alaskan king crab sustainable to eat?

The sustainability of Alaskan king crab depends on the current status of the fishery. The Bering Sea red king crab fishery has been closed since 2021 due to the population collapse. Other king crab fisheries — including Bristol Bay red king crab and golden king crab — may be open depending on current stock assessments. Always check the Monterey Bay Aquarium's Seafood Watch for current sustainability ratings before purchasing. When buying king crab, look for products that clearly state the species and fishing region, and buy from reputable suppliers who source from well-managed fisheries.

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