Weren't they saying foreign fish were the cause? Those fish are slippery ones, all right: no passport, no visa. Swedish fish, I heard. (Which country a certain strata of humanity affects to like.)
Norwegian salmon, though I'm sure you knew that. They are backing off on this though. Chinese media reported today that coronavirus was not detected in salmon before it entered the market, and in fact other areas of the market had positive results as well. Here is a Chinese article http://finance.sina.com.cn/chanjing/cyxw/2020-06-17/doc-iircuyvi8910522.shtml