Marine heatwaves trigger shift in hatch dates and early growth of Pacific cod

Marine heat waves appear to trigger earlier reproduction, high mortality in early life stages and fewer surviving juvenile Pacific cod in the Gulf of Alaska, according to a new study from the University of Alaska. Oregon State.

These changes in hatching cycle and early growth patterns persisted in the years following marine heatwaves, which may have implications for the future of Gulf of Alaska Pacific cod, a economically and culturally important species, said Jessica Miller of OSU’s Oregon Coastal Marine Experiment Station. at Hatfield Marine Science Center in Newport and lead author of the study.

“We found that the fish were hatching two to three weeks earlier. To see such a dramatic change in the hatch dates of a species due to a one- or two-year event is pretty remarkable,” Miller said . “The fact that these changes continue to persist suggests that marine heatwaves could have long-term impacts that also influence the likely trajectory of species in the face of climate change.”

The results, which have just been published in the journal Elementa: Science of the Anthropocenecould also have implications for future management of the fishery.

Pacific cod, perhaps best known as the key ingredient in fish and chips, is the second largest commercial groundfish fishery off the coast of Alaska. The 2022 commercial harvest was 403 million pounds and was valued at $225 million, according to NOAA Fisheries. The Gulf of Alaska stock is one of four Pacific cod stocks. It has a long history in Alaskan culture and is important to the region’s Native communities.

From 2014 to 2016 and in 2019, marine heatwaves, which are periods of unusually high ocean temperatures, led to a sharp decline in the abundance of adult Pacific cod in the Gulf of Alaska. As a result, the fishery was closed in 2020 and a federal disaster was declared in 2022.

To better understand the impact of these heat waves on the Gulf of Alaska Pacific cod population, researchers studied the otoliths, or ear stones, of young Pacific cod. The tiny bony structures begin to grow during the embryonic development stage and tell the story of a fish’s life in a way similar to tree rings.

Most fish die in the larval stage. The events of the animal’s first year of life impact its survival and its ability to reproduce. Understanding the impact of marine heatwaves on the early growth cycle of fish provides critical information for researchers and fisheries managers, Miller said.

“Stones are a common tool in fish ecology. They are a time capsule that can be very useful for tracking what fish ate and how quickly they grew over time,” said Miller, professor at the Department of Fisheries, Wildlife and Flora. Conservation Sciences in the OSU College of Agricultural Sciences.

Researchers found that Pacific cod hatched earlier during and after the start of the 2014 to 2016 marine heatwave, and that these earlier hatchings continued even as ocean temperatures cooled in 2017 and 2018.

“Fish responded differently to temperature during and after marine heat waves,” said Zoe Almeida, who worked on the research as a postdoctoral researcher at Oregon State and is now at the Cornell University. “Warmer temperatures only partially explain earlier hatch dates in 2017 and 2018, and faster growth was not always associated with warmer temperatures, as we often assume.”

Overall, fewer juveniles survived the first year of life during marine heatwaves.

“These are complex and unexpected consequences that we are seeing and will continue to see in the future as the climate changes,” Miller said. “It’s not just simple changes in growth, with young fish growing faster because the ocean is warmer, as several models predict. Changes in hatch timing have influenced their body size as much, if not more, than moderately faster growth, which may affect the ability of Pacific cod to survive beyond the first year. There may also be future impacts on reproductive times.

The results suggest that fisheries managers may want to continue monitoring marine heatwaves and take a more conservative approach in subsequent years when fish stocks are likely to decline, Miller said. In the future, monitoring programs may also need to be redesigned, in terms of timing or types of nets used, to account for changes in spawning timing and body size.

Researchers are also working on three other projects to further explore the impact of marine heatwaves on Pacific cod, including the characteristics of fish that survived the first year after a marine heatwave and the cascading effects of changes growth pattern as fish age.

Other co-authors are Hillary Thalmann of Oregon State and Benjamin Laurel of the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center.