Giant Chinese seafood supplier Zoneco Group on Tuesday said it found more than 80 percent of the scallops at its farm in the sea between China and Korea have died because of “unidentified” causes, according to a statement on the Shenzen Stock Exchange.

The book value of the affected mollusks is CNY 300 million (€38.8 million/$42.7 million), the company said. It’s still assessing the damage, and the cause of death is unknown though believed to be natural, it said.

The average yield for the mollusks that started seeding in 2018 is expected to have dropped to about 3.5 kilograms as a result of the deaths, down from 25.61 kilograms per mu (2.4 square kilometers) in the first 10 months of this year, the company said.

Zhangzidao’s shares tumbled past their daily trading limit of 10 percent in Shenzhen on Tuesday. They’re down 17 percent this year.