Theragra chalcogramma (Pallas, 1814)
The Alaska pollock or walleye pollock (Gadus chalcogrammus, formerly Theragra chalcogramma) is a marine fish species of the cod family Gadidae. Alaska pollock 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 Pollachius genus. Alaska pollock was long put in its own genus Theragra, 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.
Ecology and behaviour
The speckled colouring 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. Both groups mainly feed on copepods. A limited supply of copepods may lead to food depletion. However, the larger group is also capable of foraging for euphausiids and are able to diversify their food sources. Therefore, food depletion has a larger impact on smaller pollocks.
The variation in a size of each subgroup also impacts 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 wintertime because they have a higher capacity for energy storage while smaller individuals do not and, therefore, have to continue foraging putting themselves at a greater risk. To maximize their chances of survival, the larger group increases their calorie intake to gain weight in autumn, while the smaller group focuses solely on growing in size.
Lastly, Alaskan pollock exhibit diel vertical migration, following the seasonal movement of their food. Although pollocks exhibit vertical movement during a day, their average depth changes following the seasons. Originally, the change in depth was attributed to the amount of light or the ambient water temperature, but currently, it is exclusively connected to 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 at greater depths along with their planktonic food source.
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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[according to whom?] to worry that Alaska pollock could be about to repeat the kind of collapse experienced by Atlantic cod, which could have negative consequences for the world food supply and the entire 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 returning to above average in 2011 and remained so through to 2014. Greenpeace however has long been critical of the management of Alaska pollock, placing the fish on its "red list" of species and retaining it through to the present day (October 2015), the stated reason being damage of trawling to the seabed.
Other groups have hailed the fishery as an example of good management, and the Marine Stewardship Council declared it "sustainable". The Marine Conservation Society rates Alaska pollock trawled from the Gulf of Alaska, Bering Sea, and Aleutian Islands as sustainable, but not those from the Western Bering Sea and Okhotsk Sea.
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Compared with pollock, Alaska pollock has a milder taste, whiter color and lower oil content.
High-quality, single-frozen whole Alaska pollock fillets may be layered into a block mold and deep-frozen to produce fish blocks that are used throughout Europe and North America as the raw material for high-quality breaded and battered fish products. Lower-quality, double-frozen fillets or minced trim pieces may also be frozen in block forms and used as raw material for lower-quality, low-cost breaded and battered fish sticks and portions.
Single-frozen Alaska pollock is considered to be the premier raw material for surimi; the most common use of surimi in the United States is imitation crabmeat (also known as crab stick).
Alaska pollock is commonly used in the fast food industry, in products such as McDonald's Filet-O-Fish sandwich and (now-discontinued) Fish McBites, Arby's Classic Fish sandwich, Long John Silver's Baja Fish Taco, and Birds Eye's Fish Fingers in Crispy Batter.
Pollock roe is consumed in Russia as a sandwich spread. The product, resembling liquid paste due to the small size of eggs and oil added, is sold canned. Pollock roe is also known in Japan as tarako and mentaiko.
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- Assessment of the walleye pollock stock in the Eastern Bering Sea, Alaska Fisheries Science Center, December 2014
- North Pacific Groundfish Stock Assessments, Alaska Fisheries Science Center
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- NOAA NMFS: U.S. Seafood Facts
- FISHINFOnetwork Market Report 04/07
- FishBase: Alaska Pollock
- Alaska pollock-Theragra chalcogramma fishery profiles Status of these fisheries, summarised on FisheriesWiki