Pacific cod (Gadus macrocephalus), also known as gray cod, have been fished commercially in Alaska since 1865 (Rigby 1995). They are found from the northern Yellow Sea in China through the Bering Sea as far north as the Chukchi Sea, and south along the Gulf of Alaska and the coast of North America to Monica Bay, California (Love 1991, Westrheim 1996).

Adults inhabit continental shelf and upper continental slope waters (Hart 1973, Bakkala et al. 1984, Love 1991, Westrheim 1996). They have been found at depths ranging from 10 m to 875 m, but are more typically at depths between 50 m and 300 m (Allen and Smith 1988, Love 1991). During the summer, they tend to move to waters less than 100 m deep (Kruse et al. 2000). Adults and large juveniles seem to prefer soft clay, sand, or mud bottom habitats (Garison and Miller 1982).

Pacific cod are opportunistic feeders (Kruse et al. 2000). They feed mainly on shrimp, crab, polychaetes, eelpouts and other fish, and amphipods (Yang 2004, Kruse et al 2000). They are preyed upon by halibut, sharks, seabirds, and pinnipeds such as fur seals (Kruse et al. 2000).

Compared to many other groundfish found in Alaska, Pacific cod is a moderately fast growing, short-lived species. Female maturity occurs between four and five years of age in Alaska (Stark 2007), and fish typically recruit to the fishery at about seven years (Kruse et al. 2000). Estimates of annual natural mortality in the Gulf of Alaska range from 0.27 to 0.50 (Thompson and Zenger 1993, 1995). The NMFS currently uses an estimate for natural mortality of 0.38 in the Gulf of Alaska stock assessment (Thompson et al. 2007). A maximum age for Pacific cod has been estimated at 21 years using the break-and-burn method (ADF&G fishery data 1987–2007). However, aging of Pacific cod has been problematic, which makes historic age data difficult to interpret. Recent study has validated some aging methods, which may allow current and future researchers to age Pacific cod with more confidence (Roberson et al. 2005).

 
Pacific cod in northern waters are slower growing and longer lived than those found farther south, and attain a greater size (Tok 1994). The longest fish sampled from the Southeast Alaska commercial fishery was 95 cm, and the average length is 61 cm (ADF&G fishery data 1987–2007). The estimated length at which half of females are mature enough to spawn is 503 mm in the Gulf of Alaska (Stark 2007).

Pacific cod are oviparous and eggs are fertilized externally (Hart 1973). Each female produces between 225,000 and 5 million eggs per year (Klovach et al. 1995) and releases them once annually over the course of a few minutes (Sakurai 1989). Eggs are demersal, are approximately 1 mm across, and are weakly adhesive (Hart 1973, Palsson 1990). Most spawning occurs from January to April (Klovach et al. 1995), although spawning can extend into early summer for Gulf of Alaska fish (Stark 2007). Spawning generally occurs in waters 40 m to 120 m deep (Klovach et al. 1995). Eggs and winter concentrations of adults have been found associated with coarse sand and cobble bottom type. Therefore, it has been inferred this is optimal spawning habitat (Palsson 1990). Hatching occurs eight to 28 days after eggs are released, depending on water temperature and salinity (Alderdice and Forrester 1971, Hart 1973).

Larvae are approximately 3–4 mm long at the time of hatching and are born with a yolk sac that is absorbed within ten days (Palsson 1990). There is some evidence that both larvae and juveniles are transported to nursery habitats by the current (Garrison and Miller 1982). Pacific cod nursery habitats are shallow waters and sandy-bottomed intertidal areas (Miller et al. 1976). Small juveniles dwell in kelp beds and eelgrass, and begin moving to less vegetated habitat when they are greater than 8 cm long. Juveniles remain near shore at least through their first summer (Laurel et al. 2007).

REFERENCES

Allen, M. J. and G. B. Smith. 1988. Atlas and zoogeography of common fishes in the Bering Sea and north Pacific. NOAA Tech. Rep. NMFS-NWFSC 66, 151p.

Alderdice, D. F., and C. R. Forrester. 1971. Effects of salinity, temperature, and dissolved oxygen on the early development of Pacific cod (Gadus macrocephalus). J. Fish. Res. Board Canada 28: 883–902.

Bakkala, R. G. 1984. Pacific cod of the eastern Bering Sea. Int. North Pac. Fish. Comm. Bull. 42:157–179
Garison, K. J. and B. S. Miller. 1982. Review of the early life history of Puget Sound fishes. Fish. Res. Inst. University of Washington, Seattle, WA. 729p.

Hart, J. L. 1973. Pacific fishes of Canada. Bull. Fish. Res. Board of Canada 180, 730p.
Klovach, N. V., O. A. Rovnina, and D. V. Kol'stov. 1995. Biology and exploitation of Pacific cod, Gadus macrocephalus, in the Anadyr_Navarin region of the Bering Sea. J. Ichthyol. 35: 9–17.

Kruse, G.H., F.C. Funk, H.J. Geiger, K.R. Mabry, H.M. Savviko, and S.M. Siddeek. 2000. Overview of state-managed marine fisheries in the central and western Gulf of Alaska, Aleutian Islands, and southeastern Bering Sea, with reference to Steller sea lions. Alaska Department of Fish and Game, Regional Information Report No. 5J00-10, Juneau.

Laurel, B.J., A.W. Stoner, C.H. Ryer, T.P. Hurst, and A.A. Abookire. 2007. Comparative habitat associations in juvenile Pacific cod and other gadids using seines, baited cameras and laboratory techniques. Journal of Experimental Marine Biology and Ecology 351:42–55.

Love, M. S. 1991. Probably more than you want to know about the fishes of the Pacific coast. Really Big Press, Santa Barbara, Cal. 215p.

Miller, B. S., C. A. Siemenstad and L. L. Moulton. 1976. Puget Sound baseline: near shore fish survey. Fish. Res. Inst., University of Washington, Seattle, WA. 196p.

Palsson, W. A. 1990. Pacific cod (Gadus macrocephalus) in Puget Sound and adjacent water: biology and stock assessment. Wash. Dept. Fish. Tech. Rep. 112, 137p.

Rigby, P.W., D.R. Ackley, F. Funk, H.J. Geiger, G.H. Kruse, and M.C. Murphy. 1995. Management of marine fisheries resources of Alaska: A report to the Northern Forum. Alaska Department of Fish and Game, Commercial Fisheries Management and Development Division, Regional Information Report No. 5J95-04, Juneau.

Roberson, N.E., D.K. Kimura, D.R. Gunderson, and A.M. Shimada. 2005. Indirect validation of the age-reading method for Pacific cod (Gadus macrocephalus) using otoliths from marked and recaptured fish. Fish. Bull. 103:153–160.

Sakurai, Y. 1989. Reproductive characteristics of walleye pollock with special reference to ovarian development, fecundity and spawning behavior. In Proceedings of the International Symposium on the Biology and Management of Walleye Pollock, Anchorage, Alaska, USA, November 14–16, 1988, p. 97–115. Lowell Wakefield Fisheries Symposium, Alsk. Sea Grant Rep. No. 89-1.

Stark, J.W. 2007. Geographic and seasonal variations in maturation and growth of female Pacific cod (Gadus macrocephalus) in the Gulf of Alaska and Bering Sea. Fish. Bull. 105:396–407.

Tok, K. S. 1994. Dynamics of size-age structure of the Tatar Strait Pacific cod stock. Russ. J. Mar. Biol. 19:248–253
Thompson, G. G., J.N. Ianelli, M.W. Dorn, and M. Wilkins. 2007. Assessment of the Pacific cod stock in the Gulf of Alaska. NPFMC Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska, November, Anchorage, pp 169–194.

Thompson, G. G. and H. H. Zenger. 1993. Pacific cod. In plan team for groundfish fisheries of the Gulf of Alaska (editor), Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska as projected for 1994, chapter 2. North Pacific fishery Management Council, 605 W.4th Avenue Suite 306, Anchorage, AK 99501

Thompson, G. G. and H. H. Zenger. 1995. Pacific cod. In plan team for groundfish fisheries of the Gulf of Alaska (editor), Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska as projected for 1994, chapter 2. North Pacific fishery Management Council, 605 W.4th Avenue Suite 306, Anchorage, AK 99501

Westrheim, S. J. 1996. On the Pacific cod (Gadus macrocephalus) in British Columbia waters, and a comparison with elsewhere, and Atlantic cod (G.morhua). Can. Tech. Rep. Fish. Aquat. Sci. 2092. 390 p.