The U.S. and China began charging additional port fees on ocean shipping firms that move everything from holiday toys to crude oil, making the high seas a key front in the trade war between the world’s two largest economies. China said it had started to collect the special charges on U.S.-owned, operated, built or flagged vessels but clarified that Chinese-built ships would be exempted from the levies. In details published by state-run broadcaster CCTV, China spelled out spec