During recent periods of unusually warm water in the Gulf of Alaska, young Pacific cod in coastal safe havens where they typically spend their adolescence did not experience the protective effects these areas typically provide, a new study from Oregon State University.
Instead, during the 2014-2016 and 2019 marine heatwaves in these ‘farms’ around Alaska’s Kodiak Island, juvenile cod experienced significant changes in their abundance, growth rate and diet, with researchers estimating that only the largest 15-25% of the The island’s cod population survived the summer. Even after high temperatures subsided, cod have yet to return to pre-heat wave size and diet.
The findings, published today in the journal Scientific reportscould have broader implications for marine fish populations worldwide as marine heat waves become longer and more frequent due to climate change, the researchers said.
“These coastal habitats no longer support fish in the same way they used to due to marine heat waves,” said lead author Hillary Thalmann, a graduate student in OSU’s Department of Fisheries, Wildlife and Conservation Sciences. “That’s a new finding because we don’t always think of farms as a place where size-based mortality can occur quickly.”
Pacific cod, a popular choice for fish and chips, is the second largest commercial groundfish fishery off the coast of Alaska. The 2022 commercial harvest totaled 403 million pounds and was valued at $225 million, according to NOAA Fisheries. Cod also has a long history in Alaskan culture and is important to the region’s indigenous communities.
The nurseries are shallow areas along the shoreline with many aquatic plants, including seagrass, algae and kelp, which attract plenty of food for the fish and provide hiding places where they can avoid predators. Typically, they are considered safe havens for small Pacific cod – areas where the fish go around three months of age to feed and grow as much as possible during their first summer and fall.
But during the two recent marine heat waves in the Gulf of Alaska, water temperatures were recorded at 58 degrees Fahrenheit, almost 6 degrees above normal. Together, the two heat waves are considered the most extreme warming events ever recorded in the northeastern Pacific Ocean, and the impacts on the cod population were so severe that the fishery was closed in 2020 and a federal disaster was declared in 2022.
Previous OSU research found that warmer temperatures triggered faster reproduction and higher mortality among young Pacific cod. The new study focuses on the physiological disturbances that the young cod experienced while occupying the coastal farms.
Researchers used juvenile Pacific cod collected by the NOAA Alaska Fisheries Science Center Fisheries Behavioral Ecology program at 16 sites around Kodiak Island in mid-July and late August for the years 2006-2019. This sampling was part of routine population monitoring for the cod fishery.
For the July sample, researchers looked at the fish’s otoliths, small bony structures that describe a fish’s growth, similar to the rings of trees. By measuring the otoliths, researchers were able to calculate the fish’s precise growth rate up to the July sampling date, and then calculate their expected size based on maintaining the same growth rate through August.
However, when they looked at the August sample, the fish were 30% larger than the size predicted by the established growth rate, and there were almost no small fish present in the sample. The only way researchers could account for the size of the fish in August was to remove all small fish from the July sample, leaving only the largest 15-25% of fish following the expected growth rate trajectory.
“If we took out the little guys and let the big guys – the top 15-25% – grow through August based on the growth rates we saw earlier this summer, we would get the size range we see during that heat wave . years,” said Thalmann. “It is important to show that in heat events like this, selective mortality in the cod population can continue beyond their early life in the open water,” where the larvae spend their first three months.
Size-selective mortality is the phenomenon in which survival is determined by the size of an organism; here only the largest fish seem to have survived.
“We saw these size differences at the farm and we tried to explain them with growth rates and with nutrition, but we couldn’t explain it all,” Thalmann said. “There was something out there, probably size-selective mortality, that was the main driver for what we saw.”
In the future, researchers say that changing ocean conditions could mean that Pacific cod will have to move further north to find optimal growing environments, or that there could be a shift toward larger cod, which are the only ones that survive and have genetic information contributes to future generations.
“If marine heat waves continue, there will likely be some changes in both the distribution and quality of these populations,” Thalmann said. “I don’t think this is the end of fish and chips, but I do think it is a cautionary tale for climate change and the changing dynamics of fishing in warm temperatures.”
Co-authors of the study were Zoe Almeida, Kaitlyn Osborne, Kaylee Marshall and Jessica Miller of OSU and Benjamin Laurel of the NOAA Alaska Fisheries Science Center in Newport, Oregon.
