It has been found that catches of Alaska pollock go up three years after stormy summers. The storms stir up nutrients, and this results in phytoplankton being plentiful for longer, which in turn allows more pollock hatchlings to survive. The Alaska pollock has well-developed drumming muscles that the fish use to produce sounds during courtship, like many other gadids. The primary factor in determining the foraging behavior of the Alaskan pollock is age. Young pollocks can be divided into two sub-groups, with lengths below or above 60 mm (2.4 inches). Both groups mainly feed on copepods. However, the latter group will also forage for euphausiids. Therefore, food depletion has a larger effect on smaller pollocks. The variation in size of each subgroup also affects seasonal foraging behavior. During the winter, when food is scarce, foraging can be costly due to the fact that longer hunting time increases the risk of meeting a predator. The larger young pollocks have no need to hunt during the winter because they have a higher capacity for energy storage, while smaller fish do not, and have to continue foraging, putting them at greater risk. To maximize their chances of survival, large pollock increase their calorie intake in autumn to gain weight, while smaller ones focus solely on growing in size. Alaskan pollock exhibit diel vertical migration, following the seasonal movement of their food. Although pollocks exhibit vertical movement during the day, their average depth changes with the seasons. Originally, the change in depth was attributed to the amount of light or water temperature, but in fact, it follows the movement of food species. In August, when food is abundantly available near the surface, pollocks will be found at lower depths. But in November, they are found deeper, along with their planktonic food source. The Alaska pollock's main habitat are the coastal areas of the Northern Pacific, especially the waters off Alaska (Eastern Bering Sea, Gulf of Alaska, Aleutian Islands) as well as off Russia, Japan and Korea (Western Bering Sea and Sea of Okhotsk). The largest concentrations of Alaska pollock are found in the eastern Bering Sea. Very small populations of fish genetically identical to "Gadus chalcogrammus" are found in the Barents Sea waters of northern Norway and Russia. This fish was initially described as an own species to science under the taxon "Theragra finnmarchica" by Norwegian zoologist Einar Koefoed in 1956.