The polar cod (Boreogadus saida) is a small, silvery fish found throughout the Arctic and sub-Arctic. It can inhabit waters as cold as -1.8 °C due to antifreeze proteins in its blood. Polar cod are distributed across the entire North Polar Basin, from the Barents Sea to the Canadian Arctic, making them one of the most widely distributed fish in the region. Its abundance and high energy content establish it as a foundational prey species for a wide range of Arctic animals.
Central Role in the Arctic Food Web
The polar cod’s importance in the Arctic ecosystem stems from its function as an energy conduit, efficiently transferring sustenance from lower to higher trophic levels. The fish feeds extensively on zooplankton, specifically Calanus copepods and amphipods like Themisto species, which themselves feed on ice algae and phytoplankton. This process funnels the energy-rich lipids produced by these small crustaceans into a much larger, more accessible form for predators.
The fish are particularly abundant, forming large schools that represent a significant, high-fat food source for cold-adapted predators. Polar cod are considered one of the most energy-rich prey species in the Arctic due to their high lipid concentration. Scientists estimate that over 70% of the energy flow from zooplankton to higher predators in high-Arctic regions passes through the polar cod.
Major Marine Mammal and Avian Predators
A diverse array of marine mammals and seabirds rely heavily on the dense aggregations of polar cod for survival, especially during the long Arctic winter. The ringed seal (Pusa hispida) is a specialized ice-associated predator that depends on polar cod as a primary food source. Polar cod is often the main diet item for seals during the ice-free period and remains a significant food source during winter months.
Other marine mammals, including the narwhal and the beluga whale, also consume polar cod in large quantities. These deep-diving cetaceans are known to hunt the fish in deep aggregations below the landfast ice. Belugas are capable of diving over 350 meters for their prey. The importance of this fish is so pronounced that a decline in the polar cod population is likely to affect ice-associated species such as harp seals more significantly than other predators.
Seabirds also consume a massive volume of polar cod, with some species being highly adapted to hunt them near the ice edge. The Thick-billed Murre (Uria lomvia) is a deep-diving avian predator that regularly pursues the fish to depths exceeding 100 meters, making it a highly effective hunter. Surface-feeding seabirds, such as the Black-legged Kittiwake, the Northern Fulmar, and the Black Guillemot, also target the fish, especially when they are aggregated near the surface or in open water leads within the ice.
Secondary Consumers and Intraspecies Predation
The consumption of polar cod extends beyond the apex predators, involving numerous other fish and even members of its own species. Other fish, such as the Arctic char and the Greenland halibut, are locally dominant predators of polar cod. The larger Atlantic cod, a species expanding its range northward due to warming waters, has also become a significant predator of polar cod.
Intraspecies predation, or cannibalism, is a notable aspect of the polar cod’s life cycle. Adult polar cod are known to prey on smaller, juvenile members of their own species. This behavior helps regulate the population, which is one reason why juvenile fish often seek refuge in cavities and channels within the sea ice.
Predators are highly opportunistic, with the prey’s life stage often determining the predator’s success. Seabirds often preferentially capture age-1 cod, which are typically between 6 to 11.5 cm long. The largest marine mammals consume both juvenile and adult fish. The sheer volume of consumption is staggering, with estimates suggesting that seabirds and marine mammals collectively consume approximately 75% of the annual polar cod production in regions like the Chukchi Sea. Changes in sea ice cover due to a warming climate affect the cod’s habitat and the hunting success of its predators, potentially causing cascading effects throughout the entire Arctic marine food web.