Gadids are teleosts that belong to the order Gadiformes, which includes oceanodromous, benthonic and benthopelagic marine fish. Within the order, the species of interest Melanogrammus aeglefinus (haddock) and Gadus morhua (cod) belong to the family Gadidae and are distributed in polar and temperate waters of the northern hemisphere. In contrast to salmonids, their pelvic fins are located further forward than the pectoral fins, and they have 3 dorsal fins and 2 anal fins; usually they present a mentonian barbell.

They are typically predators, feeding on fish and invertebrates, living in waters with temperatures ranging from 4 to 15ºC, and swimming long distances in shoals.

The haddock is fusiform and has a dark spot on each flank below the lateral line and above the pectoral fin, and a slightly curved, dark-coloured lateral line. The cod has a relatively large head compared to the body; usually lacks spots on the body, though when they are present, they are numerous, and has a light-coloured lateral line curved at the level of the pectoral fin.

Gadids may be as long as 200 cm, weigh as much as 96 kg and live up to 25 years. Both species are important fishing resources (sport and commercial) or production (commercial or experimental aquaculture) resources, and commercial cod capture surpasses 150,000 tons.