Theragra

The Alaska pollock or walleye pollock (Gadus chalcogrammus) is a marine fish species of the cod family Gadidae. It is a semipelagic schooling fish widely distributed in the North Pacific with largest concentrations found in the eastern Bering Sea. While belonging to the same family as the Atlantic pollock, the Alaska pollock is not a member of the same genus, Pollachius. Alaska pollock was long put in its own genus, Theragra and classified as Theragra chalcogramma, but more recent research has shown it is rather closely related to the Atlantic cod and should be moved back to genus Gadus, in which it was originally described. Furthermore, Norwegian pollock (Theragra finnmarchica), a rare fish of Norwegian waters, is likely the same species as the Alaska pollock. The speckled coloring of Alaska pollock makes it more difficult for predators to see them when they are near sandy ocean floors. They are a relatively fast-growing and short-lived species, currently representing a major biological component of the Bering Sea ecosystem. 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 has been said to be 'the largest remaining source of palatable fish in the world.' Around 3 million tons of Alaska pollock are caught each year in the North Pacific from Alaska to northern Japan. Alaska pollock is the world's second most important fish species in terms of total catch. The Alaska pollock landings are the largest of any single fish species in the U.S, the average annual Eastern Bering Sea catch between 1977 and 2014 being 1.174 million tons. Alaska pollock catches from U.S. fisheries have been quite consistent at about 1.5 million tons a year, almost all of it from the Bering Sea. Each year's quota is adjusted based on stock assessments conducted by the Alaska Fisheries Science Centre. For instance, stock declines in 2008 meant decreased allowable harvests for 2009 and 2010. This decline led some scientists to worry that Alaska pollock could be about to repeat the collapse of the Atlantic cod, which could have negative consequences for the world food supply and the Bering Sea ecosystem. Halibut, salmon, endangered Steller sea lions, fur seals, and humpback whales all eat pollock and rely on healthy populations to sustain themselves. Alaska pollock stocks (and catch levels) subsequently returned to above average in 2011 and remained so through to 2014. However, Greenpeace has long been critical of Alaska pollock management, placing the fish on its 'red list' of species due to damage of the seabed from trawling.