FIRST COMES THE “wind shower.” Wearing overalls and rubber boots, employees at Fusheng Food Co. stand in a narrow room as air jets in the walls blow away any dust on them. They wade through ankle-deep disinfectant and don caps, gloves and surgical masks.

Only then can they enter the chilly, white-tiled room where they pack Alaska salmon for American dinner tables and Russian cod for McDonald’s fish sandwiches in Japan. The Japanese-owned company says the fish already has been tested for more than 100 banned chemicals.