Take a look at a map
The Chinese launch site is located at roughly latitude: 41°19' N, longitude: 100°19' E. Take a look at a map of China. The launch site is well inland, so trawlers seem out of the question.
The US monitors all launches around the world. We do that to avoid any possibility, no matter how remote, that a space launch could be confused with a missile launch. We use a variety of collection means to gather intel data, especially on space systems that can have military applications. You have a problem with that?