Atlantic Cod (Gadus morhua) and Pacific Cod (Gadus macrocephalus) are cold-water fish widely distributed across the northern hemispheres. Known for their flexible and opportunistic feeding habits, they thrive in diverse marine environments. Krill, small crustaceans, form dense swarms and serve as a major energy source in the ocean food web. Understanding whether these commercially significant cod species rely on krill is key to understanding the dynamics of their ecosystems.
Cod’s Consumption of Krill
Cod consume krill, but the importance of this prey shifts based on the cod’s size, season, and geographical location. Krill are a particularly significant component of the diet for juvenile cod, providing a nutrient-rich food source during their early growth stages. In Arctic regions, such as the Svalbard fjords, krill can dominate the stomach contents of young Atlantic cod.
The species Meganyctiphanes norvegica (Northern Krill) is a key prey item for Atlantic cod in areas like the North Sea and the Norwegian Sea. Consumption is highest when larger, preferred prey (such as small fish) are less available, or when krill form concentrated swarms that are easy to target. As generalist predators, cod switch to the most energetically favorable food source they can find.
Cod actively engulf swarms of krill, often in mid-water zones where the krill aggregate. For larger, adult cod, krill represent a smaller proportion of their total diet biomass compared to fish. However, krill remain a reliable food source when cod forage close to the seafloor or migrate through pelagic zones. Reliance on krill acts as an important buffer, sustaining young cod until they are large enough to hunt bigger prey.
Other Primary Prey Sources for Cod
While krill are part of the diet, cod shift to a diet dominated by fish as they mature. Adult Atlantic cod, particularly those in the Barents Sea, consume large quantities of small, schooling fish like capelin (Mallotus villosus) and polar cod. This transition to piscivory often begins when the cod reaches a length of approximately 20 centimeters.
The diet of adult Pacific cod is similarly dominated by fish and larger invertebrates, including crustaceans and mollusks. Key prey often include commercially important species such as walleye pollock and various species of decapod crustaceans. Shrimp, such as crangonid shrimps, are also a significant food source for smaller individuals before they shift to primarily eating fish.
Cod are opportunistic feeders, meaning their diet reflects what is most abundant and accessible in their immediate environment. Their ability to consume a broad spectrum of prey, ranging from small crustaceans and mollusks to larger fish, is a defining trait of their ecological success. This dietary flexibility helps explain their wide distribution and persistence in marine ecosystems that experience seasonal or long-term changes in prey availability.
Krill’s Importance in the Marine Food Web
Krill occupy a foundational position in the marine food web, connecting primary producers to larger predators. These crustaceans feed on phytoplankton and zooplankton, converting energy into a form accessible to higher trophic levels. Krill represent a large reservoir of energy in the ocean, especially in polar and sub-polar regions.
Krill are a primary food source for numerous species.
Krill Predators
- Baleen whales
- Seals
- Penguins
- Seabirds
- Many species of fish
Fluctuations in krill populations can have widespread effects, directly impacting the feeding success and reproductive output of dependent predators. Krill also contribute to global biogeochemical cycles, including the carbon cycle, by transporting nutrients through vertical migrations and waste products.
The conservation of krill populations is recognized as ecologically important due to their role as a keystone species supporting entire marine ecosystems. Changes in their distribution or abundance, whether due to climate change or human harvesting, can destabilize the food web structure, affecting predators like cod and other commercially important species. Maintaining healthy krill stocks is fundamental to the stability of cold-water marine environments.