The cod icefishes or notothens are the family Nototheniidae of acanthopterygian fishes. They are traditionally placed in the Perciformes together with their relatives, but like every lineage in the order, their actual relationships are not yet determined with certainty.
They are largely found in the Southern Ocean and off the coast of Antarctica. As the dominant Antarctic fish taxa, they occupy both sea-bottom and water-column ecological niches. Although lacking a gas bladder, they have undergone a depth-related diversification, such as increased fatty tissues and reduced mineralization of the bones, resulting in a body density approaching neutral, to fill a variety of niches. The spleen may be used to remove ice crystals from circulating blood. As the chilly Antarctica and Subantarctic waters average −1 to 4 °C (30–39 °F), most species of these regions have antifreeze glycoproteins in their blood and other body fluids.
As the major fish resource in the Southern Ocean, notothens are under increasing pressure from commercial fishing.