Distribution Aspects of Walleye Pollock

Walleye pollock is one of the most abundant and widespread Gadid fishes in the Northern Pacific. Walleye pollock, a temperate cold-water species, mostly lives at low temperatures a little above or even below 0°C, and its upper temperature limit is 12-14°C. Eggs development and larval growth take place in the deep water layers at 200-400 m.

Pollock is a fast-growing fish with a relatively short life cycle. According to various sources of data Walleye Pollock live up to 16-25 years. The growth of Pollock is characterized by a slower rate in the cold part of the areal and, on the other hand, more rapid in warm-temperate areas.

Walleye Pollock (Theragra chalcogramma)

At the first year of life juvenile Pollock have sizes from 4-6 up to 12-14 cm. The average size of two-year specimens in the coldest part of the areal (the western part of the Bering Sea, a large part of the Sea of Okhotsk, the Tatarsky Strait, waters off East Kamchatka) is about 19-23 cm, and in the south (in Korean and Japanese waters) – 23-26 cm. Three-year specimens reach sizes of 25-31 cm in the north and 30-34 cm in the southern areas.

There are evidence of maximum length of 93 cm (single specimens marked with length of 1 meter) and weight of 5 kg. However, such specimens are rare, usually the maximum size of fish in the catch are 65-75 cm and at weight of 2-3 kg.

In terms of diet Walleye Pollock feeds plankton, however, cannibalism especially of juvenile fish is not often. Pollock is a favorite subject in the diet of such predatory fishes like halibut, cod, sable fish and grenadiers, as well as Pacific herring, and sea mammals.  

Spawning period of walleye pollock usually continues for 6-7 months, and the most massive spawning, involving a majority of producing individuals, takes 2-3 months. Spawning period in the Sea of Okhotsk usually goes during the winter and spring seasons. The reproduction of walleye pollock in the Bering Sea starts in March showing the maximum in May. 

It occurs in all marine coastal areas in the northern part of Pacific Ocean. Its distribution extends from the Chukotka Sea to the south along the coast of the North American continent down to Monterey Bay and along the Asian coast down to Korea Strait. Across this vast distribution range, there are several regions that have quite remarkable abundance and high commercial impact of a walleye pollock. They include Korea Bay, Peter the Great Bay, Alaska Bay, Sakhalin, Hokkaido, South Kuril and East Kamchatka regions. 

The most abundant populations of Walleye Pollock live in the Sea of Okhotsk and in the Bering Sea. They are called North Okhotsk Sea and East Bering Sea super-populations, respectively, because of their huge biomass. There are no other massive fish populations like these two. Populations with such immense and complex structures usually include the core and several lateral subpopulations. The core subpopulation of the North Okhotsk Sea population is located in the north-eastern part of the sea that is the main reproductive area for walleye pollock, at some years having nearly 80 % of all producers and eggs being produced during one spawning season, and nearly 90 % of all juveniles. The lateral subpopulations inhabit the western part of the Sea of Okhotsk. Due to the separation of the super-population between spawning and feeding areas, which jointly occupy almost the whole sea, the walleye pollock sub-populations continually invade new feeding locations that are less favorable for their reproduction. Similar pattern is observed in the East Bering Sea super-population of walleye pollock, which is usually half larger than the North Okhotsk Sea super-population. However, at some periods, including the present time, the North Okhotsk Sea super-population can take the leadership by biomass. 

The West Alaskan, Olyutor-Karaginsk, East Kamchatka and East Korean populations are rather abundant, at some years reaching several million metric tons in biomass. Moreover, there are many small populations of walleye pollock at different regions, including Peter the Great Bay, Tatar Strait, Commander Islands, East Alaska, Vancouver and four assemblages near Hokkaido. The distinctiveness of all other populations is still presumptive and questionable. Despite of incoming migrants from Hokkaido and South Kuril regions during feeding season, there is a high evidence for isolated South Okhotsk from North Okhotsk walleye pollock. The survey data analysis has demonstrated the distinctiveness of the East Kamchatka walleye pollock population and its isolation from the Olyutor-Karaginsk population.

Walleye pollock from the northern assemblages can be characterized by a high migration activity, while the extend of migrations in the southern assemblages is relatively small. The northern populations reproduce mostly in spring time, while the south populations reproduce in fall (Korean waters) or, which is more often, in winter. The majority of population assemblages occurs in the southern and, especially, in the south-western parts of the walleye pollock distribution area that is explained by their local adaptations to complex marine ecosystems, including the intricate structure of marine currents in that region, located on the border between the temperate and subtropical zones. 
 Pollock map.jpg
The generalized scheme of the walleye pollock population structure in the North Pacific                       (Shuntov et al., 1993).

Total biomass of population: a – 10 million metric tons and larger; б – from hundreds of thousands to millions of tons; в– from tens of thousands to hundreds of thousands tons.
Arrows show migration routes for walleye pollock.
Open circles represent fall or winter spawning; black – spring spawning.
Populations: 1) East Korea, 2) Peter the Great Bay, 3) Shima; 4) Toyama, 5) West Hokkaido, 6) Tatar Strait, 7) South Hokkaido, 8) East Hokkaido, 9) South Kuril Islands, 10) North Hokkaido or Raus, 11) North Okhotsk Sea,12) East Kamchatka, 13) Commander Islands, 14) Olyutor-Karaginsk, 15) Koryak, 16) East Bering Sea, 17) East Aleutian, 18) West Alaska, 19) East Alaska, 20) Vancouver