Cod are saltwater fish, belonging exclusively to the marine environment for their entire life cycle. True cod species, such as the commercially prominent Atlantic Cod (Gadus morhua) and Pacific Cod (Gadus macrocephalus), are demersal fish that thrive in the cold, deep waters of the North Atlantic and North Pacific oceans. Their biology and physiology are specifically adapted to the high salinity of the sea, making long-term survival in freshwater systems impossible.
Defining True Cod
True cod belong to the genus Gadus, which is nested within the family Gadidae and the order Gadiformes. This taxonomic classification establishes a clear identity for the fish, separating it from other species that have co-opted the common name “cod” over time. The genus Gadus currently includes the Atlantic Cod, the Pacific Cod, and the Alaska Pollock (Gadus chalcogrammus), all of which are exclusively marine species.
Physically, true cod possess a distinct set of characteristics. They have three separate dorsal fins running along their back. These are complemented by two anal fins located on the underside of the fish, giving them a unique fin configuration among popular commercial species.
A pronounced barbel, a fleshy whisker-like sensory organ, hangs from the chin, which they use to forage for food along the seabed. This barbel, along with a prominent, pale lateral line running from the gills to the tail, helps distinguish true cod from other members of the Gadidae family, such as haddock or pollock. The combination of these specific features is what defines a fish as a member of the Gadus genus.
The Marine Habitat of Cod
The habitat of cod is defined by its requirement for high salinity and low temperatures, which is why the species is found throughout the world’s northern oceans. Atlantic Cod are widely distributed across the North Atlantic, from the coasts of North America to the Barents Sea, while Pacific Cod are found in the northern Pacific, including the Bering Sea and the Gulf of Alaska. These fish are benthopelagic, meaning they live near the bottom of the ocean but also venture into the open water.
The necessity of a saltwater environment is rooted in the cod’s physiological process of osmoregulation. Marine fish, including cod, have an internal salt concentration that is lower than the surrounding seawater. This osmotic difference causes water to constantly move out of the fish’s body through its gills and skin. To counteract this constant dehydration, cod must actively drink large amounts of seawater and use specialized chloride cells in their gills to excrete the excess salt. The constant energy expenditure required for this process is necessary to maintain the correct internal water-salt balance, which is only viable in a high-salinity environment.
Addressing Freshwater “Cod” Imposters
The confusion regarding whether cod can be found in freshwater often stems from numerous unrelated species that have the word “cod” in their common name. These fish do not belong to the true Gadus genus. The Burbot (Lota lota) is one example that adds to the confusion, as it is the only freshwater fish species within the entire Gadiformes order, making it a distant relative of true cod.
The Burbot is a cold-water fish that lives in large rivers and lakes across the Northern Hemisphere. While it possesses a chin barbel, its body shape is more serpentine, and it belongs to a separate family, Lotidae. A more extreme example of a misnomer is the Murray Cod (Maccullochella peelii), a large Australian apex predator found exclusively in the Murray-Darling river system.
Despite its common name, the Murray Cod is not related to the true cod of the Northern Hemisphere at all, belonging instead to the order Perciformes and the family Percichthyidae. Other fish that are marine but not true cod also contribute to the confusion, such as Black Cod (Sablefish, Anoplopoma fimbria), which belongs to a different family entirely, and Lingcod (Ophiodon elongatus). These instances illustrate that the presence of the word “cod” in a fish’s name is often a historical or colloquial convenience rather than an indicator of a true biological relationship.