Japanese sardines may be signaling major shifts in the ocean

When research scientist Gary Longo first analyzed the genomic data from sardine samples collected off the West Coast, he thought he had made a mistake. 

Initially, it appeared that, besides the Pacific sardines that are common in the area, many of the fish were a different species. 

“It was a total shock,” Longo recalled. After further comparing the genetic code to other known species, he had a revelation: “That was the ‘aha moment’ when we realized we were looking at a second species of sardine.”

Mix of Japanese and Pacific sardines

The analysis revealed that in the 345 sardine samples collected during NOAA Fisheries’ 2021 and 2022 Coastal Pelagic Species Surveys, all of the samples from 2021 were Pacific sardines. 

However, the 2022 samples contained a mix of Pacific sardines and Japanese sardines. Further examination of 825 additional sardine samples collected the following year showed a similar result, with many being Japanese sardines – previously undocumented in this part of the ocean.

Long journey of Japanese sardines

Japanese sardines were previously known only from the west side of the North Pacific Ocean near Asia. The findings, published in the journal Molecular Ecology, showed that Japanese sardines have made their way to the waters off the United States. 

These sardines, which look nearly identical to their Pacific cousins, can only be distinguished through genetic analysis. 

The team used advanced genomic sequencing techniques, analyzing millions of genetic markers and constructing full mitochondrial genomes to confirm the identity of the sardines.

Genetic testing of sardines

In 2022 and 2023, Japanese sardines were found from Washington State to southern California, sometimes schooling with Pacific sardines. 

The newcomers were typically one to three years old, common adult ages for sardines. Wondering if these fish had been overlooked in previous surveys, the researchers turned to the genetic archives at the Southwest Fisheries Science Center. 

Genetic testing of sardine samples collected between 2013 and 2021 showed no signs of Japanese sardines, suggesting that their arrival on the West Coast is a recent phenomenon.

Changing ocean conditions

The detection of Japanese sardines coincided with a period of low Pacific sardine populations, which may have made their presence easier to spot. 

The 2022 NOAA Fisheries survey was the first to record Japanese sardines, suggesting that the species arrived around that time. 

More were found in 2023, though it remains uncertain how long they have continued to move eastward. Researchers are currently analyzing samples from the 2024 survey to track their movements further.

The arrival of Japanese sardines raises many questions about the evolving conditions of the North Pacific. 

Previous genetic studies estimated that Pacific and Japanese sardines diverged around 200,000 to 300,000 years ago, separated by glacial periods that cooled the North Pacific. 

While there have been examples of western Pacific species reaching the U.S. coast – often transported by debris carried by currents – this appears different. 

Japanese sardines leading ecological shifts

The study suggests that recent marine heatwaves warmed the North Pacific, creating a suitable habitat corridor that Japanese sardines may have followed across the ocean.

“Small coastal pelagic fish such as sardines are good indicators of change,” explained Longo. “Often they are the first ones we see responding to shifting temperatures because they have short generation times, are highly mobile, and can use a variety of habitats.” 

According to the researchers, the Japanese sardines could represent the “leading edge” of broader ecological shifts.

Uncertain future for Japanese sardines

The presence of Japanese sardines along the West Coast poses more questions than answers. 

“A lot of fish distributions change as ocean conditions change. Now the question is whether Japanese sardines will die out over time, or will they persist in this new part of their range,” said co-author Matthew Craig, a research scientist at the Southwest Fisheries Science Center.

One crucial unknown is whether the two sardine species can interbreed, which would significantly affect their interaction in the California Current ecosystem. 

Understanding these dynamics is key to determining how the influx of Japanese sardines could impact the management of sardine populations along the West Coast.

Importance of long-term monitoring

Longo and Craig have returned to the lab to analyze around 700 new samples from the 2024 Coastal Pelagic Species Survey. They aim to sequence the DNA of these samples, tracing the ongoing presence and movement of Japanese sardines in the region. 

The findings could reshape scientists’ understanding of sardine distribution and how changes in ocean conditions impact marine species.

“This is one of those remarkable discoveries that never would have happened if scientists had not been out there, looking,” said Kristen Koch, director of the Southwest Fisheries Science Center. “The long-term records from earlier surveys also help us understand how unusual it is.”

The unexpected presence of Japanese sardines in the Pacific Ocean highlights the importance of continued monitoring and research, offering new insights into how marine species may respond to a rapidly changing world.

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