Theragra chalcogrammaPacific pollock(Also: Walleyed pollock)

Ge­o­graphic Range

Ther­a­gra chalcogramma (Alaska pol­lock) oc­cu­pies a wide ge­o­graphic range and is found in a polar cli­mate across the north­ern Pa­cific Ocean, from 68° north to 34° north, and from 129° east to 120° west (Kooka 1998). Alaska pol­lock in­habit a broad niche span­ning the North Pa­cific, from the east­ern Paleartic re­gion to the west­ern Neartic. North Amer­i­can pop­u­la­tions lie along the west coast from north­ern Cal­i­for­nia to north­west Alaska with major pop­u­la­tions lo­cated south­east of Alaska and Canada and in the west­ern-cen­tral Gulf of Alaska. Alaska pol­lock pop­u­la­tions span the coast­line of West Asia to South-West Asia and South Asia with the largest pop­u­la­tions con­cen­trated in the north­west­ern Bering Sea, the west­ern Bering Sea, east­ern Kam­chatka, and the north­ern Sea of Okhotsk. (Bai­ley, et al., 1999; Kooka, 1998)

Habi­tat

Ther­a­gra chalcogramma is a salt­wa­ter fish that thrives in seas, oceans, and gulfs. It is broadly dis­trib­uted with greater pop­u­la­tions ex­ist­ing in the outer shelf and slope re­gions of oceanic wa­ters. These fish, semi-de­m­er­sal in some re­gions and pelagic in oth­ers (due to feed­ing on a va­ri­ety of pelagic and de­m­er­sal prey), are able to sur­vive in large es­tu­ar­ies, coastal areas, and open basins. Alaska pol­lock are lim­ited to north­ern re­gions re­flect­ing this species' lim­ited tem­per­a­ture tol­er­ance. They can only with­stand a range of ten to twelve de­grees Cel­sius (Bai­ley 1999). Com­mon trends ex­hibit an age-de­pen­dent depth dis­tri­b­u­tion. Fish less than one year old live through­out the water col­umn, one-year-olds live at bot­tom trawls, two- to three-year-olds live higher in the col­umn, and fish at least four years old dis­play de­m­er­sal be­hav­ior. The ther­mal range of these fish de­creases with age. Younger fish have a greater range of tem­per­a­ture tol­er­ance than older fish, which re­main at lower depths where the tem­per­a­ture is a con­stant one to six de­grees Cel­sius (Avdeev 2001). (Avdeev, G.V and Avdeev, 2001; Bai­ley, et al., 1999)

  • Range depth
    1280 (high) m
    4199.48 (high) ft
  • Average depth
    366 m
    1200.79 ft

Phys­i­cal De­scrip­tion

Ther­a­gra chalcogramma has an olive-green belly. Its dor­sal side dis­plays a grad­u­ally dark­en­ing color scheme from the bot­tom to the top, even­tu­ally fad­ing to a dark brown. This fish ex­hibits char­ac­ter­is­tic blotches all over its en­tire body, which is slen­der with a large head. The head con­sists of var­i­ous lat­eral pores, large eyes, and a pro­trud­ing lower jaw which lacks a bar­bel (Bai­ley 1999). Ther­a­gra chalcogramma can be rec­og­nized by its two anal fins, three widely dis­persed fins on its back, and out­stretched pelvic fins. The fins are all a dark grey to black color. A lat­eral line runs across the fish to its cau­dal fin. At three to five years old, the av­er­age length of this fish is 30.5 cen­time­ters (Har­mann 2005). By the time it reaches ages of eight to twelve years, the av­er­age length is about 61 cen­time­ters. The max­i­mum length of T. chalcogramma is 91 cen­time­ters. Weights of this fish range from 600 grams to 800. The max­i­mum weight was recorded at 1400 grams (Cohen 1990). (Bai­ley, et al., 1999; Cohen, et al., 1990; Har­mann, 2005)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    600 to 800 g
    21.15 to 28.19 oz
  • Range length
    30.5 to 91 cm
    12.01 to 35.83 in
  • Average length
    61 cm
    24.02 in

De­vel­op­ment

Newly fer­til­ized pelagic eggs of Ther­a­gra chalcogramma are ap­prox­i­mately 3.4 mm in di­am­e­ter. The eggs hatch in ap­prox­i­mately 28 days at a tem­per­a­ture of three de­grees Cel­cius. The length of each larva when it set­tles out is 3.8 to 5.1 cen­time­ters (Love 1996). The lar­val de­vel­op­men­tal stage is con­sid­ered com­plete within ap­prox­i­mately 50 days. Fe­males grow at the same rate as, or slightly faster than, males. Im­ma­ture fish swim to­wards the sur­face at night to cap­ture prey. The pre-spawn­ing stage be­gins at age 2 years. Fe­male T. chalcogramma reach ma­tu­rity at three to four years at an av­er­age length of 36 cen­time­ters, while males ma­ture at ages four to five years (Bai­ley 1999). (Bai­ley, et al., 1999; Love, 1996)

Re­pro­duc­tion

These fish spawn in large groups. The mat­ing sys­tem of Ther­a­gra chalcogramma con­sists of fe­males re­leas­ing their eggs and then males ran­domly fer­til­iz­ing them (Love 1996). Fe­males are re­ferred to as batch spawn­ers that re­lease eggs every few days for one month per year. (Cohen, et al., 1990; Love, 1996)

Spawn­ing of Ther­a­gra chalcogramma oc­curs in water depths of 90 to 200 me­ters. Spawn­ing, which oc­curs for an en­tire month, is an­nual and the peak spawn­ing sea­son fluc­tu­ates from re­gion to re­gion. Early spawn­ing, in the months of Feb­ru­ary, March and April, oc­curs in the Bering Sea. Later spawn­ing, in the months of April, May, and June, oc­curs fur­ther in­ward over the con­ti­nen­tal slope and shelf. Spawn­ing fish (three- to four-year-old fe­males and four- to five-year-old males) and pre-spawn­ing fish (two-year-old fe­males) swim high to mid-wa­ter in dense schools. The amount of eggs that each fe­male holds is pro­por­tional to her body size. Fe­males of ap­prox­i­mately 60 cen­time­ters pro­duce 500,000 to 600,000 eggs while fe­males with lengths of 40 to 45 cen­time­ters pro­duce about one third of that. The largest of fe­males have been known to pro­duce up to one mil­lion eggs (Har­mann 2005). Ther­a­gra chalcogramma eggs, which are trans­ported by cur­rents, are plank­tonic (Love 1996). The eggs take any­where from 9 to 28 days to hatch. (Love, 1996; Har­mann, 2005; Love, 1996)

  • Breeding interval
    Theragra chalcogramma spawn once yearly
  • Breeding season
    March to August in eastern Bering Sea and January to March in Aleutian Basin
  • Range number of offspring
    100,000 to 1,000,000
  • Range time to hatching
    9 to 28 days
  • Range age at sexual or reproductive maturity (female)
    3 to 4 years
  • Range age at sexual or reproductive maturity (male)
    4 to 5 years

Be­sides the en­ergy put into ga­mete pro­duc­tion and spawn­ing, Ther­a­gra chalcogramma makes no in­vest­ment in its off­spring. (Love, 1996)

  • Parental Investment
  • no parental involvement

Lifes­pan/Longevity

The mor­tal­ity co­ef­fi­cient for Ther­a­gra chalcogramma is 0.3 on the east­ern Bering Sea shelf (0.3 im­ply­ing that that the an­nual nat­ural moral­ity rate is 30%) and 0.2 on the Aleut­ian Basin. Ther­a­gra chalcogramma usu­ally lives from 12 to 16 years. The old­est T. chalcogramma ever re­ported was 31 years old (Bai­ley 1999). (Bai­ley, et al., 1999)

  • Range lifespan
    Status: wild
    31 (high) years
  • Typical lifespan
    Status: wild
    12 to 16 years
  • Average lifespan
    Status: captivity
    17 years

Be­hav­ior

Ther­a­gra chalcogramma is a mo­bile species that is so­cial and trav­els in schools. Dense schools of Ther­a­gra chalcogramma form dur­ing the spawn­ing sea­son. Depth, tem­per­a­ture, and cold fronts con­tribute to the ver­ti­cal dis­tri­b­u­tion of ju­ve­nile Alaska pol­lock. These fac­tors also ef­fect the hor­i­zon­tal dis­tri­b­u­tion of adult schools, which usu­ally stay within a range of 0 to 2 de­grees Cel­cius. At night, ju­ve­nile fish can be found near the sur­face, in­di­cat­ing their dis­tinc­tive eat­ing pat­tern. This is called ver­ti­cal mi­gra­tion, and oc­curs when fish travel up­ward in the water col­umn to the sur­face to at­tain a larger pop­u­la­tion of po­ten­tial prey. School­ing co­he­sion de­creases at night as well as times of lim­ited food. (Har­mann, 2005; Har­mann, 2005; Avdeev, G.V and Avdeev, 2001; Bai­ley, et al., 1999; Har­mann, 2005)

Home Range

No in­for­ma­tion on the home range of Ther­a­gra chalcogramma could be found.

Com­mu­ni­ca­tion and Per­cep­tion

Lim­ited in­for­ma­tion per­tain­ing to the com­mu­ni­ca­tion and per­cep­tion of Ther­a­gra chalcogramma ex­ists. Most school­ing fish, how­ever, use vi­sual cues for school­ing and feed­ing (Bai­ley 1999). (Bai­ley, et al., 1999)

  • Communication Channels
  • visual

Food Habits

Ther­a­gra chalcogramma feeds on both fish and in­ver­te­brates (Bai­ley 1999). The diet of this fish varies by de­vel­op­men­tal stage, sea­son, and body size. Lar­vae tend to con­sume zoo­plank­ton such as lar­val cope­pods, while older T. chalcogramma tend to eat larger food items such as adult Acar­tia and Pseudo­calanus. In win­ter, adult pol­lock feed mostly on fishes and eu­phausiid krill. In spring, T. chalcogramma feed on Ap­pen­dic­u­laria. In sum­mer, the diet con­sists of eu­phausi­ids and cope­pods and in au­tumn, they feed on mainly eu­phausi­ids (Kooka 1998). Large T. chalcogramma have a diet rich in small pol­lock (specif­i­cally in the east­ern Bering Sea) and shrimp. In areas where the ju­ve­nile pop­u­la­tion is ex­tremely large, such as in the east­ern Bering Sea, canna­bal­is­tic adults prey on the ju­ve­niles. Smaller-sized in­di­vid­u­als are more likely to con­sume cope­pods and eu­phausi­ids (Avdeev 2001). (Avdeev, G.V and Avdeev, 2001; Bai­ley, et al., 1999; Kooka, 1998)

  • Animal Foods
  • fish
  • aquatic crustaceans
  • other marine invertebrates
  • zooplankton

Pre­da­tion

Ther­a­gra chalcogramma lar­vae are preyed on by north­ern rock­fish, polka-dot snail­fish, and atka mack­eral. One-year-olds are preyed on by green­land tur­bot, plain sculpin, and ar­row­tooth floun­der. Those that are two years old are hunted by green­land tur­bot, big­mouth sculpin, sable­fish, pa­cific cod, plain sculpin, pa­cific hal­ibut, and great sculpin. Adult T. chalcogramma fall prey to yel­low irish lords and plain sculpin (Cohen 1990; Kooka 1998). In the Gulf of Alaska, T. chalcogramma is the pri­mary prey of stel­lar sea lions (Cohen 1999). (Cohen, et al., 1990; Kooka, 1998; Love, 1996)

Ecosys­tem Roles

In the Bering Sea, Ther­a­gra chalcogramma plays a key role as both preda­tor and prey. The re­pro­duc­tive suc­cess of stel­lar sea lions is de­pen­dent on T. chalcogramma (Avdeev 2001).

Preda­tor-prey in­ter­ac­tions for T. chalcogramma fluc­tu­ate with sea­sonal con­di­tions. One-year-old pol­lock oc­cupy dif­fer­ent areas de­pend­ing on tem­per­a­ture. This change in local abun­dance af­fects the an­nual pop­u­la­tions of other species in the re­gion.

When in­fected with the par­a­sitic cope­pod Haemobaphes dicer­aus, there is a de­crease in the body weight of T. chalcogramma as well as an in­crease in the weight of its spleen. If T. chalcogramma is in­fected dur­ing the ju­ve­nile stage of de­vel­op­ment, it suf­fers from de­layed ma­tu­rity, and if it is in­fected as an adult, it ex­pe­ri­ences a de­crease in re­pro­duc­tive suc­cess (Bai­ley 1999). (Avdeev, G.V and Avdeev, 2001; Bai­ley, et al., 1999)

Com­men­sal/Par­a­sitic Species

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

This species is a valu­able fish for fish­eries on the Russ­ian, Japan­ese, and Ko­rean coasts. The fish is sold frozen, as fil­lets, fish sticks, surimi, and roe in Japan (Love 1996). The de­sire for this fish has gone up in re­cent years; how­ever, it still lacks ap­pre­ci­a­tion on the North Cal­i­forn­ian coast where its abun­dance is high. In ad­di­tion to being used for food, it is also uti­lized for fish meal and in­dus­trial prod­ucts. (Love, 1996)

  • Positive Impacts
  • food

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

There are no ad­verse ef­fects of Ther­a­gra chalcogramma on hu­mans (Kooka 1998). (Kooka, 1998)

Con­ser­va­tion Sta­tus

This species is not threat­ened and is not listed on the 2004 IUCN Red List of Threat­ened Species nor is it listed as en­dan­gered by the U.S. Fish and Wildlife Ser­vice. The most im­por­tant site of re­pro­duc­tion for this species, the She­likof Strait in the Gulf of Alaska, has an an­nual pe­riod of time in which fish­ing is pro­hib­ited. This en­forced fish­ing re­stric­tion was put in place to pre­vent po­ten­tial de­creases in this species due to this re­gion hav­ing such a large T. chalcogramma den­sity dur­ing spawn­ing sea­son (Bai­ley 1999). (Bai­ley, et al., 1999; Bai­ley, et al., 1999)

Con­trib­u­tors

Al­li­son Poor (ed­i­tor), Uni­ver­sity of Michi­gan-Ann Arbor.

Parisa Ijadi-Magh­soodi (au­thor), Uni­ver­sity of Michi­gan-Ann Arbor, Kevin Wehrly (ed­i­tor, in­struc­tor), Uni­ver­sity of Michi­gan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

piscivore

an animal that mainly eats fish

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

visual

uses sight to communicate

zooplankton

animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)

Ref­er­ences

Avdeev, G.V, G., E. Avdeev. 2001. Pato­genic [path­o­genic] in­flu­ence ren­dered by par­a­sitic cope­pod Haemobaphes dicer­aus on Alaska pol­lock. Zo­o­log­i­cal Record Plus, 128(1): 287-292, 336, 342.

Bai­ley, K., D. Pow­ers, J. Quat­tro, G. Villa, J. Traynor, G. Wal­ters. 1999. Pop­u­la­tion Ecol­ogy and Struc­tural Dyam­ics of Wall­eye Pol­lock. Pp. 581-590 in T Lough­lin, K Ohtani, eds. Dy­nam­ics of the Bering Sea. Fair­banks, Alaska: Uni­ver­sity of Alaska Sea Grant.

Cohen, D., T. Inada, T. Iwamoto, N. Scial­abba. 1990. "Ther­aga chalcogramma" (On-line). Ac­cessed Oc­to­ber 12, 2005 at http://​www.​fishbase.​org/​Summary/​SpeciesSummary.​php?​id=318.

Har­mann, A. 2005. "A Bio­phys­i­cal Model of She­likof Strait" (On-line). Ac­cessed Oc­to­ber 16, 2005 at http://​www.​pmel.​noaa.​gov/​foci/​spem-ibm.​html.

Kooka, K. 1998. Ver­ti­cal dis­tri­b­u­tion and prey of wall­eye pol­lock in the north­ern Japan sea. Fish­eries Sci­ecne, 64 (5): 686-693.

Love, M. 1996. Prob­a­bly more than you want to know about the fishes of the Pa­cific coast.. Sante Fe, Cal­i­for­nia: Re­ally Big Press.