Pacific cod—already weathering climate change in the Bering Sea and Gulf of Alaska, far from their ancestral home—face new challenges due to global warming, with the answers to their ability to adapt perhaps centered on the impact of the ZP3 protein in their genome.
NOAA Fisheries scientists in Seattle are researching a specific gene region of the genome called zona pellucida, or ZP3, to assess its role in geographic variation among Alaska’s Pacific cod stocks. The fishery comprises the second largest commercial groundfish harvest off Alaska, a multi-million dollar business that provides hundreds of jobs in the hook-and-line, bottom trawl, jig, pot and longline catch- and-processing facilities.
According to NOAA Fisheries, the 2019 commercial harvest alone of Pacific cod totaled about 464 million pounds and was worth $118.7 million. While the fishery is considered to be sustainably managed and responsibly harvested, scientists have found evidence that adaptation to their local environment could potentially make some Pacific cod stocks more vulnerable than others to climate change.
“We don’t know how quickly cod adapt to their environment,” said Ingrid Spies, an assessment scientist and geneticist with the Alaska Fisheries Science Center in Seattle. She’s researching the relationship between Bering Sea and Gulf of Alaska cod, which are managed as separate stocks.
Pacific cod are believed to be the offspring of Atlantic cod that migrated into the Pacific Ocean four million years ago, but new research shows a disconnect between different regions of the Pacific cod genome, the complete sets of genes in a cell or organism.
Evidence from a small region of the genome suggests that the genetic break between the Bering Sea and Gulf of Alaska cod may in fact fall further in the east than is currently designated under existing management measures, NOAA Fisheries explained in a report released in early December.
Researchers have also found genetic evidence to suggest that local adaptation to environmental conditions may be critical to the development and survival of these cod, the report said.
Rapid evolution has been documented in some cases, but is not something predictable in the face of climate change; cod in the Gulf of Alaska during the recent marine heat wave did not adapt and suffered increased mortality, Spies said.
“Much more work is needed to understand how and whether the starkly different ZP3 types affect Pacific cod’s reaction to temperature and other physiological responses,” she said. “I’m working towards that, but it will be a huge research effort.”
The annual feeding migrations of Pacific cod, whose diet includes clams, worms, crab, shrimp and juvenile fish, are not well understood, but scientists do know that Pacific cod typically do return to their natal spawning areas in winter months to reproduce.
“While many of the genetic differences between Pacific cod populations can be explained by reproductive isolation caused by strong homing behavior, the mechanisms underlying these differences have not been specifically explored,” Spies explained. “Such selective processes are increasingly relevant to effective management of Pacific cod, given recent ocean warming events that have resulted in steep declines in Gulf of Alaska populations and anomalous summer feeding migrations into the northern Bering Sea.”
Researchers have found these events emphasize sensitivity to temperature related to growth and survival in Pacific cod. The broad species range of these fish, from temperate climates of Korea and the Pacific Northwest to Arctic and subarctic conditions in the Bering and Chukchi seas also suggests that local cod populations are adapted to thermal profiles specific to their habitat, the report said.
Scientists studying ZP3 in other mammals and fish have found that ZP3 helps prevent polyspermy, which is when two sperm fertilize a single egg. When this occurs the embryo may die. ZP3 protects
the embryo and helps with the fertilization process. In Antarctic fish eggs, the report notes, ZP3 also produces an antifreeze protein to help fish survive in colder water.
AFSC researchers examined the genetic sequence at the ZP3 gene in 230 samples collected at 16 spawning areas during the 2003-2019 spawning seasons. They found that the genetic structure of fish, at the ZP3 gene, from the Aleutians, Bering Sea, Shumagin Islands and Kodiak collections were similar and distinct from fish in Prince William Sound and southward.
“The genetic differences between neighboring populations at the ZP3 gene were extremely stark,” said Wes Larson, a co-author of this study and leader of the AFSC genetics program. “They are similar to those found in highly structured salmon populations, which return to very specific environments to spawn and are locally adapted to those environments. This is not often the case for marine fish which live in a more uniform environment.”
These findings suggest that large differences in habitats across small geographic areas are leading to very different selective pressures.
Further studies are needed on the role of ZP3 in Pacific cod found throughout the Gulf of Alaska, as the answers could play a role in whether some populations of fish are more or less tolerant to warming ocean temperatures. Other work by AFSC stock assessment scientists has found evidence that cod at early life stages are particularly vulnerable to temperature shifts and that warm temperatures also affect prey available for both young and adult fish in the Gulf of Alaska.
Spies said she suspects this may have been the issue during and after the heat wave of 2014-2016 in the Gulf. While the population of cod in the Eastern Gulf of Alaska remained low but stable during the heat wave, cod stocks in the Central Gulf of Alaska declined by about 30% per year, and by 23% per year in the Western Gulf.
“We also need wider-scale sampling of fish caught in winter and summer fisheries from the Shumagin Islands and Kodiak to have a more complete picture of genetic diversity across seasons,” Spies said.
“Currently these areas are managed as part of the Gulf of Alaska stock but results from this work suggest that fish found here at least during the winter, are more of a match for fish found in the Bering Sea,” she said.