2019 Assessment of the Pacific Cod Stock in the Gulf of Alaska

Pacific cod (Gadus macrocephalus) is a transoceanic species, occurring at depths from shoreline to 500 m. The southern limit of the species’ distribution is about 34° N latitude, with a northern limit of about 63° N latitude. Pacific cod is distributed widely over Gulf of Alaska (GOA), as well as the eastern Bering Sea (EBS) and the Aleutian Islands (AI) area. The Aleut word for Pacific cod, atxidax, literally translates to “the fish that stops” (Betts et al. 2011). Recoveries from archeological middens on Sanak Island in the Western GOA show a long history (at least 4500 years) of exploitation. Over this period, the archeological record reveals fluctuations in Pacific cod size distribution which Betts et al. (2011) tie to changes in abundance due to climate variability (Fig. 2.1). Over this long period colder climate conditions appear to have consistently led to higher abundance with more small/young cod in the population and warmer conditions to lower abundance with fewer small/young cod in the population.

Tagging studies (e.g., Shimada and Kimura 1994) have demonstrated significant migration both within and between the EBS, AI, and GOA outside of spawning season (Fig. 2.2). There appears to be substantial migration between the southern Bering Sea and the western GOA based on tagging data, however little movement has been observed from the central GOA to the Western GOA. Two recent genetics studies using Restriction-site Associated DNA sequencing have indicated significant genetic differentiation among spawning stocks of Pacific cod in the Gulf of Alaska and the Bering Sea (Drinan et al. 2018; Spies et al. 2019). The first study (Drinan et al. 2018) used 6,425 single-nucleotide polymorphism (SNP) loci to show high assignment success >80 percent of five spawning populations of Pacific cod throughout their range off Alaska. Further work using 3,599 SNP loci and spawning samples throughout the range of Pacific cod off Alaska, as well as a summer sample from the Northern Bering Sea in August 2017 showed significant differentiation among all spawning groups (Spies et al. 2019). The three spawning groups examined in the Gulf of Alaska, Hecate Strait, Kodiak Island, and Prince William Sound, were all genetically distinct and could be assigned to their population of origin with 80–90 percent accuracy (Fig. 2.3; Drinan et al. 2018). Cod that spawned at Unimak Pass in 2003 and 2018 were genetically distinct from the Kodiak Sample (spawning year 2003), FST=0.004 and FST=0.001. There was strong evidence for selective differentiation of some loci, including one that aligned to the zona pellucida glycoprotein 3 (ZP3) in the Atlantic cod genome. This locus had the level of differentiation of any locus examined (FST=0.071). ZP3 is known to undergo rapid selection (Drinan et al. 2018), and completely distinct haplotypes have been observed in spawning cod from Kodiak Island westward vs. Prince William Sound and samples to the east.

Although there appears to be some genetic differentiation within the GOA management area and some cross migration between the Western GOA and southeastern Bering Sea the Pacific cod stock in the GOA region is currently managed as a single stock. Further work is needed to understand the genetic stock structure of cod in the GOA and its relationship with the Bering Sea stock of cod during spawning and feeding periods.