Two years after Japan and China sealed a historic natural gas joint development deal intended to turn the East China Sea into a “sea of peace [and] friendship”, the waters between the powers look more like a sea of friction and animosity.

A widening dispute over Japan’s detention of a Chinese fishing boat captain in contested waters is threatening to derail efforts to implement the 2008 deal, just as Japanese suspicions grow that China is expanding development of an undersea gas field that could draw fuel from areas to which it claims exclusive economic rights.

China has threatened to take “strong countermeasures” if Japan does not immediately release fishing captain Zhan Qixiong. Meanwhile, Tokyo is considering what measures if might take if China expands drilling at its controversial Chunxiao field close to the “median line” between the two nations.