photo credit: Courtesy NOAA Fisheries/Kelsey James
Sardine collected on the 2023 Coastal Pelagic Species Survey. Pacific and Japanese sardine
look so similar that they can only be distinguished by genetics. 

New visitors have been appearing up and down the west coast the last two years that may be a harbinger of bigger changes on the way.

Trying to determine differences between northern and southern populations of Pacific sardine, Dr. Gary Longo, a research scientist contracted to NOAA's Southwest Fisheries Science Center, stumbled upon an unusual discovery.

"My initial thought was , oh I may have made a mistake and swapped plates with different samples because the differentiation was so strong. But as we started to dig down into the data we realized that the differentiation was so strong because we were looking at two different species of sardine," Longo told KRCB.

It's a recent phenomenon.

"We only really detected them in 2022 and 2023, and of the samples we sequenced, around 40 percent...were Japanese sardine," Longo said.

The two seem to be getting along fine.

"They're schooling together and definitely co-mingling. One of the important questions that we're looking to answer moving forward is whether or not Japanese and Pacific sardine can interbreed and produce viable offspring. We don't know any of the details of whether or not they can even hybridize at this point," Longo added.

Established studies theorize the two types have been apart for about a quarter million years.

"The North Pacific during times of glaciation became a pretty cold and inhospitable environment for sardine. It acted as a pretty effective dispersal barrier and effectively kept Japanese and Pacific sardine separate. Recently, with global warming, that barrier seems to have become less effective," Longo said.

Although there's no proven explanation, Longo and colleagues have a theory.

"As things have warmed up in the North Pacific, a habitat corridor opened up and Japanese sardines just followed that habitat corridor east across the North Pacific," Longo said.

Whether the newcomers take over, die off or if the two species will produce viable hybrids is unclear. What's also unknown is if other species will follow or what a warmer pacific means for other, less adaptable marine species.

Longo's work will be published in the upcoming issue of the journal Molecular Ecology, it is currently available online and may be found here.