The coast guard said it released one fishing boat but confiscated a second boat along with fishing gear and fish.
One boat with seven people aboard was seized by Chinese authorities on June 20 and a second with six aboard on July 3, both on charges of fishing illegally in Chinese waters.
Tensions between the two countries spike after Beijing deployed an oil rig in May near the disputed Paracel Islands.
In the July 3 incident, Beijing said the boat was seized 12 kilometres from its southern island of Hainan, which would place the fishermen within Chinese territorial waters.