The Japanese clams are farmed in Taiwan and shipped back to Japan for dispersal in beaches around the bay. But the researchers say they believe Taiwanese clams are being mixed together with their Japanese counterparts during the farming process.
They said DNA tests confirmed that Taiwanese clams have found their way into the bay and pose a threat to the Japanese shellfish by taking away their habitat and through cross-breeding.
Massive landfill projects and the construction of sea walls around the nation destroyed much of the Japanese clams’ habitat. In 2012, the Environment Ministry designated the common orient clam as a species at increased risk of extinction.
The researchers, led by Ayako Yamakawa, a lecturer of marine biology at Okinawa International University, examined the DNA of shellfish gathered at 12 sites in Japan, South Korea, China and Taiwan between 2004 and 2013.
They concluded that the DNA in six of 62 clams collected on man-made beaches at Kasai Rinkai Park in Tokyo’s Edogawa Ward was identical to the DNA of the Taiwanese variety.
The results of the study were carried in a Finnish publication late last year.
One theory holds that common orient clams in Taiwan were originally taken from Japan to the island in the 1920s.
But the recent study showed that more than 8 percent of the DNA of Japanese clams was dissimilar to the DNA of the Taiwanese clams.
“It would take the shellfish more than 2 million years to be different to that extent,” Yamakawa said. “The ones that turned up in the park’s beaches must be the indigenous Taiwanese clams.”
To revive the common orient clam population, the confederation of fishermen’s cooperatives in Chiba Prefecture has been releasing the shellfish off Kisarazu in the prefecture.
The clams, originally from Kumamoto Prefecture in Kyushu, are sent for farming in Taiwan, which fisheries officials say has advanced technology in the field, and then transferred to Chiba Prefecture.
According to the Chiba prefectural government, 100 tons of farmed shellfish were released into Tokyo Bay in fiscal 2012. The harvest was 39 tons the same year.
A private group has been also releasing young clams provided by the Chiba confederation on beaches at Kasai Rinkai Park since 2010.
Yamakawa called for a system to determine whether clams are Japanese or Taiwanese. It would cost only 1,000 yen ($9.07) or less to conduct a simple DNA test per clam, she said.
“It would prove a great challenge to remove the Taiwanese shellfish once they settle in the bay,” she said.
But officials with the Chiba confederation are cool to the idea.
“We believed that the indigenous species did not exist in Taiwan,” an official said. “We are aware of the contents of the study, but we are not thinking about taking any specific measure now because we trust the clam farmers (in Taiwan).”