Researchers in the United States have reported that a sardine species from waters near Japan has been found off the US West Coast. They say warmed seawater may have helped the fish cross the Pacific Ocean.

The US National Oceanic and Atmospheric Administration, or NOAA, published the results of its genomic analysis of sardines caught off the West Coast in 2022 and 2023.

The researchers found that the samples collected in those years were a mixture of Pacific sardines, a species common on the US West Coast, and Japanese sardines that had been known only to inhabit the west side of the North Pacific Ocean near East Asia, including Japan.

The researchers say analyses of sardines collected from 2013 to 2021 off the US West Coast found no sign of Japanese sardines, suggesting that their cross-ocean travel is a more recent development.

Research scientist Gary Longo noted that the temperature of the North Pacific has been rising. He said he thinks that Japanese sardines made their way across the Pacific after spending some time in the southern Bering Sea and waters near the Aleutian Islands.

He suspects their long trip was made possible after waters have become warm enough for them to live through the winter.