Coronavirus lingering on chilled salmon may be infectious for more than a week, according to researchers in China, where imported fish have been investigated as a potential source of infections.

Researchers at the South China Agricultural University and Guangdong Academy of Agricultural Sciences in Guangzhou found SARS-CoV-2, the virus that causes Covid-19, collected from salmon samples could survive for eight days at 4 degrees Celsius (39 degrees Fahrenheit). That’s roughly the temperature at which the fish are transported. The research was released Sunday in a report ahead of publication and peer review.

Chinese authorities have been investigating imported meat, packaging and containers as a potential source of Covid-19 since June after repeatedly finding traces of the pathogen on packaging and food. Six of more than 500,000 samples tested have been positive for the coronavirus, China’s customs agency said in a report Tuesday.

“SARS-CoV-2-contaminated fish from one country can be easily transported to another country within one week, thus serving as one of the sources for international transmission,” the researchers said, noting the fish must be kept at a temperature of 0-4 degrees Celsius during shipping.

The government is striving to prevent contamination by strengthening cooperation with meat exporting countries. As of Monday, China had halted imports from 56 companies in 19 countries after employees there were infected.

The first outbreak of the pandemic-causing disease was initially associated with the Huanan Seafood Market in the central Chinese city of Wuhan in late 2019, and an outbreak in June was associated with the Xinfadi Seafood Market in Beijing, they said.