Surprising results of a genetic study on Pacific cod in Alaska
Much of the success in managing fish species in the United States has come from the recognition of local differences and genetic variability within species. Pacific Cod is an important commercial fish species caught in Alaskan waters. Cod caught in the Gulf of Alaska and Bering Sea are managed as separate stocks. A new genetic study has found that the relationship between the Bering Sea and the Gulf of Alaska may be a bit more complicated than originally thought.
A genetic break between the Bering Sea and the Gulf of Alaska could be further east than what is currently designated in existing management measures. Scientists have also found genetic evidence suggesting that localized environmental conditions may be essential for the development and survival of Pacific cod.
The evolution of Pacific cod
Pacific cod are said to originate from an ancestor of the Atlantic cod that moved to the Pacific Ocean 4 million years ago. Today, Pacific cod are found throughout the western Pacific. They are found in the eastern Pacific along the Korean Peninsula. In the United States, their range extends from the Chukchi Sea to northern California. Populations at the southern edge of the eastern and western Pacific exhibit distinct genetic differences from northern populations.
Pacific Cod undertake annual foraging migrations that are not well understood. However, scientists know that Pacific cod typically return to their native spawning grounds during the winter months to breed.
“Although many genetic differences between Pacific cod populations can be explained by reproductive isolation caused by extensive research behavior, the mechanisms underlying these differences have not been specifically explored,” said Ingrid Spies, Stock Assessment Scientist and Geneticist at the Alaska Fisheries Science Center. . “Such selective processes are increasingly relevant to the effective management of Pacific cod, given recent ocean warming events that have resulted in significant declines in Gulf of Alaska populations and abnormal summer migrations in the north of the Bering Sea. “
NOAA fisheries scientists have found that these events emphasize the temperature sensitivity associated with the growth and survival of Pacific cod. There is a wide range of Pacific cod species, from the temperate climates of Korea and the Pacific Northwest to the arctic and subarctic conditions of the Bering and Chukchi Seas. This also suggests that local cod populations are adapted to the thermal profiles specific to their habitat.
In this study, Spies and his colleagues looked at a specific genetic region called zona pellucida or ZP3. They wanted to assess its role in the geographic variation between the two Pacific cod stocks in Alaska.
Scientists have studied ZP3 in other mammals and fish. In other species, it plays a key role in reproductive isolation. This is when related species are not able to reproduce successfully. This may be because they breed at different times of the year or have other behavioral or physiological differences.
ZP3 also helps prevent polyspermia, in which two sperm fertilize a single egg. When this happens, the embryo can die. ZP3 protects the embryo and facilitates the fertilization process. In Antarctic fish eggs, ZP3 also produces an antifreeze protein that helps fish survive in colder waters.
The role of ZP3 in defining Alaska Pacific cod stocks
Scientists examined the genetic structure of 230 samples taken from 16 spawning sites during the winter spawning season from December to May (2003-2019). Samples were taken during this fish spawning period to avoid the risk of stock mixing.
They found that the genetic structure of fish, at the ZP3 gene level, collections from the Aleutian Islands, Bering Sea, Shumagin, and Kodiak was similar and distinct from fish found in Prince William Sound and south. . This suggests that selective pressures such as water temperature may affect the development of Pacific cod at small spatial scales in the Gulf of Alaska.
According to Spies, more work is needed to study the role of ZP3 in Pacific cod found throughout the Gulf of Alaska. This could play a role in whether some fish populations are more or less tolerant of warming ocean temperatures. In other work from NOAA Fisheries, stock assessment scientists have found evidence that cod in the early stages of its life is particularly vulnerable to temperature changes. Warm temperatures also affect the availability of prey for young and adult fish in the Gulf of Alaska.
Spies suspects this may be the case during and after the Gulf of Alaska heat wave (2014-2016). During this period, the population size of the eastern Gulf of Alaska cod stock remained small but stable. In contrast, the cod stock in the central Gulf of Alaska has declined by about 30 percent per year and 23 percent per year in the western Gulf of Alaska.
âWe also need larger-scale sampling of fish caught in winter and summer fisheries on Shumagin and Kodiak Islands to get a more complete picture of genetic diversity over the seasons. Currently, these areas are managed as part of the Gulf of Alaska stock, but the results of this work suggest that the fish found here, at least during the winter, more closely match the fish found in the Bering Sea.