Red Crab Habitat: A Deep Dive Into Where Red Crabs Thrive
September 26, 2024Red Crab Habitat: The Oceanography Behind Where King, Snow, and Dungeness Crabs Thrive
The distribution of red crab species is not random -- it is determined by specific oceanographic conditions: water temperature, depth, bottom substrate type, dissolved oxygen levels, salinity, and the productivity of the food web that supports them. King crabs, snow crabs, and Dungeness crabs each occupy distinct ecological niches shaped by the specific current systems, seafloor geology, and water mass characteristics of their respective habitats. Understanding the oceanography behind these habitats explains why these species are found where they are, why their populations fluctuate with ocean conditions, and why climate change poses such a specific threat to cold-water specialists like snow crabs.
Golden King Crab | Snow Crab Legs | Dungeness Crab
King Crab Habitat: The Bering Sea and North Pacific
The Oceanographic Setting
The eastern Bering Sea -- the primary habitat of red king crab (Paralithodes camtschaticus) -- is one of the most productive marine ecosystems on Earth. Its productivity is driven by: (1) The Bering Sea shelf: a broad, shallow continental shelf (average depth approximately 50-100 meters / 165-330 feet) that receives intense seasonal upwelling of cold, nutrient-rich water from the deep Bering Sea basin; the upwelling fuels massive phytoplankton blooms in spring and summer, which support the zooplankton, benthic invertebrates, and fish that king crabs feed on; (2) The Alaska Coastal Current: a southwestward-flowing current that carries cold, relatively fresh water from the Gulf of Alaska along the Alaska Peninsula; this current maintains the cold temperatures that king crabs require and transports nutrients along the shelf; (3) The cold pool: a body of water on the eastern Bering Sea shelf that remains below 2°C (36°F) year-round due to winter sea ice formation; the cold pool is a critical habitat feature for both snow crabs and juvenile king crabs, which use it as a refuge from predators that cannot tolerate the cold temperatures.
Depth, Temperature, and Substrate Preferences
Red king crab: 90-600 feet (27-183 meters); water temperature 34-39°F (1-4°C); sandy or muddy seafloors with shell hash (fragments of broken shells) that provide camouflage and foraging substrate. Golden king crab (Lithodes aequispinus): found at greater depths (300-1,500 feet / 90-460 meters) in the Aleutian Islands and Gulf of Alaska; prefers rocky, high-relief seafloor habitat -- a significant difference from red king crab's preference for soft-bottom habitat. Blue king crab (Paralithodes platypus): found around St. Matthew Island and the Pribilof Islands in the Bering Sea; similar depth and temperature preferences to red king crab.
Why King Crabs Are Found in Alaska and Not Further South
King crabs require water temperatures below approximately 10°C (50°F) for survival; they cannot tolerate the warmer waters of the Pacific Coast south of Alaska. The cold, nutrient-rich waters of the Bering Sea and North Pacific provide the specific combination of temperature, depth, and food availability that king crabs require. The same oceanographic conditions that make the Bering Sea one of the world's most productive fisheries -- cold temperatures, strong upwelling, and a broad shallow shelf -- are what make it the center of the global king crab industry.
Snow Crab Habitat: The Cold Pool and Continental Shelves
The Oceanographic Setting
Snow crabs (Chionoecetes opilio) are cold-water specialists with one of the most specific temperature requirements of any commercially harvested crab species -- they require water temperatures below approximately 2°C (36°F) and are physiologically stressed at temperatures above 5°C (41°F). Their distribution is therefore tightly linked to the extent of the cold pool on the eastern Bering Sea shelf: (1) The cold pool as habitat boundary: the cold pool (water below 2°C) forms each winter as sea ice cools the shallow shelf water; in years with extensive sea ice, the cold pool is large and snow crabs can occupy a wide area of the shelf; in years with reduced sea ice (warm years), the cold pool shrinks and snow crabs are compressed into a smaller area of suitable habitat; (2) The 2018-2019 marine heat wave: an exceptionally warm period in the Bering Sea nearly eliminated the cold pool; snow crabs were compressed into a small area of suitable habitat, their metabolic rates increased (requiring more food), and their prey abundance declined; the result was mass starvation and a 90% population decline between 2018 and 2021 -- the most dramatic documented collapse of a commercially important crab species in US history; (3) North Atlantic distribution: snow crabs also inhabit the continental shelves of eastern Canada (Gulf of St. Lawrence, Newfoundland, Labrador), Greenland, Iceland, and Norway; the Canadian snow crab fishery is one of the largest in the world and many Canadian fisheries hold MSC certification.
Depth, Temperature, and Substrate Preferences
Snow crab: 150-1,200 feet (45-365 meters); water temperature below 2°C (36°F) preferred, physiologically stressed above 5°C (41°F); soft, muddy or sandy bottoms on the continental shelf; juveniles are found in shallower, colder water (often within the cold pool); adults move to deeper water as they mature. The soft-bottom habitat preference reflects their feeding strategy -- snow crabs forage for mollusks, worms, echinoderms, and organic debris buried in or on the sediment surface.
Dungeness Crab Habitat: The California Current System
The Oceanographic Setting
Dungeness crabs (Metacarcinus magister) inhabit the eastern boundary of the North Pacific -- the California Current System -- one of the four major eastern boundary upwelling systems in the world ocean. The California Current System drives the productivity of the Pacific Coast from British Columbia to Baja California: (1) Coastal upwelling: prevailing northwesterly winds push surface water offshore (Ekman transport); cold, nutrient-rich water from depth upwells to replace it; the upwelled water fuels intense phytoplankton blooms that support the food web from zooplankton to Dungeness crabs; (2) Seasonal productivity: upwelling is strongest in spring and summer (March-August) along the Oregon and Washington coasts; this seasonal pulse of productivity drives the growth and reproduction of Dungeness crabs; (3) Estuaries as nurseries: the bays and estuaries of the Pacific Coast (San Francisco Bay, Humboldt Bay, Coos Bay, Willapa Bay, Grays Harbor, Puget Sound) serve as critical nursery habitat for juvenile Dungeness crabs; the calm, shallow, nutrient-rich waters of estuaries provide ideal conditions for juvenile crabs to grow through their early molting stages before moving to deeper coastal waters as adults; (4) Hypoxia risk: intense upwelling can bring oxygen-depleted (hypoxic) water onto the continental shelf; hypoxic events (dissolved oxygen below 1.4 ml/L) have been documented on the Oregon shelf and can cause mass mortality of benthic invertebrates including Dungeness crabs; the frequency and intensity of hypoxic events on the Pacific Coast has increased in recent decades.
Depth, Temperature, and Substrate Preferences
Dungeness crab: 30-300 feet (9-90 meters); water temperature 45-55°F (7-13°C); sandy or muddy bottoms in bays, estuaries, and nearshore coastal waters; eelgrass beds provide important habitat for juveniles (eelgrass provides both food and shelter from predators). Range: Alaska (Kodiak Island) to Baja California, Mexico; most abundant from northern California to British Columbia; the center of the commercial fishery is the Oregon and Washington coast.
Climate Change Impacts on Red Crab Habitats
- Snow crab (most vulnerable): the 2018-2021 population collapse demonstrated the direct link between ocean warming and snow crab survival; NOAA research has documented that the eastern Bering Sea cold pool has been shrinking in extent and duration as ocean temperatures rise; continued warming is projected to further reduce suitable snow crab habitat in the Bering Sea; the North Atlantic populations (Canada, Greenland) may be less immediately affected but face similar long-term risks
- King crab: warming Bering Sea temperatures are shifting the distribution of red king crab northward and into deeper water; the Bristol Bay red king crab stock (the largest US king crab fishery) has shown significant population fluctuations correlated with ocean temperature; the fishery was closed in 2021-2022 due to low stock abundance
- Dungeness crab: warming California Current waters are increasing the frequency and intensity of harmful algal blooms (HABs) that produce Domoic acid; Domoic acid accumulates in Dungeness crab tissue and is toxic to humans; HAB-related fishery closures and season delays have become more frequent on the Pacific Coast; ocean acidification (driven by CO2 absorption) is also a concern -- more acidic water makes it harder for crabs to build and maintain their calcium carbonate shells
Golden King Crab | Snow Crab Legs | Dungeness Crab | Dungeness Crab Clusters
Also see: Sustainable Red Crab Harvesting: Fisheries Management Explained | The Secret Life of Red Crabs: Biology and Behavior
Watch seafood preparation videos on the Global Seafoods YouTube Channel
Frequently Asked Questions About Red Crab Habitats
Why are king crabs only found in Alaska and not further south along the US coast?
King crabs require water temperatures below approximately 10°C (50°F) for survival and cannot tolerate the warmer waters of the Pacific Coast south of Alaska. The cold, nutrient-rich waters of the Bering Sea and North Pacific -- maintained by the Alaska Coastal Current and seasonal upwelling -- provide the specific combination of temperature, depth, and food availability that king crabs require. The same oceanographic conditions that make the Bering Sea one of the world's most productive marine ecosystems are what make it the center of the global king crab industry.
What is the cold pool and why does it matter for snow crabs?
The cold pool is a body of water on the eastern Bering Sea shelf that remains below 2°C (36°F) year-round, formed each winter as sea ice cools the shallow shelf water. Snow crabs require temperatures below approximately 2°C and are physiologically stressed above 5°C; the cold pool defines the boundaries of their suitable habitat. In warm years with reduced sea ice, the cold pool shrinks and snow crabs are compressed into a smaller area. The 2018-2019 marine heat wave nearly eliminated the cold pool, compressing snow crabs into a small area while simultaneously increasing their metabolic rate and reducing their prey abundance -- causing the mass starvation that drove a 90% population decline between 2018 and 2021.
Why are estuaries important for Dungeness crabs?
Estuaries (bays and river mouths where freshwater and saltwater mix) serve as critical nursery habitat for juvenile Dungeness crabs. The calm, shallow, nutrient-rich waters of estuaries provide ideal conditions for juvenile crabs to grow through their early molting stages -- the period when they are most vulnerable to predators and most dependent on abundant food. Eelgrass beds within estuaries provide both food and shelter. Juvenile crabs typically spend 1-2 years in estuarine habitat before moving to deeper coastal waters as adults. The health of estuarine habitats (which are threatened by development, pollution, and sea level rise) is therefore directly linked to the long-term productivity of the Dungeness crab fishery.
How does ocean acidification affect crabs?
Ocean acidification -- the decrease in seawater pH caused by the absorption of atmospheric CO2 -- reduces the availability of calcium carbonate ions that crabs use to build and maintain their shells. More acidic water makes it harder for crabs to calcify their shells after molting, potentially producing thinner, weaker shells that are more vulnerable to damage and predation. Juvenile crabs are more vulnerable than adults because they molt more frequently and their shells are thinner. The Pacific Coast -- where Dungeness crabs are most abundant -- is particularly affected by ocean acidification because upwelling brings naturally more acidic deep water to the surface; this natural acidity is being amplified by anthropogenic CO2 absorption.