Here’s a little known fact. Chinese commercial ships, particularly freighters and ocean-going fishing ships, are considered part of a military maritime reserve force and are expected to follow orders from navy or coast guard ships whenever called upon. The commercial ships are expected to collect intelligence and even risk damage and injuries by using their ships to block the movement of foreign ships (including warships). In return the Chinese navy and coast guard will come to the assistance if Chinese commercial ships get in trouble with foreign navies or coast guards. But this arrangement does not always work out as it should
An example of this occurred on March 15th 2016 when an Argentinian coast guard ship sank a Chinese trawler that was illegally fishing in Argentinian waters. The coast guard rescued five of the crew, including the captain and arrested them. China complained but did nothing else. In fact, within weeks China publicly reaffirmed its growing economic and diplomatic ties with Argentina. Meanwhile the owners of the lost fishing trawler will be quietly compensated.
This sort of illegal fishing is a worldwide problem and Chinese trawlers are probably the biggest offenders. In waters closer to China there will often be Chinese warships near areas where Chinese trawlers fish illegally. This sometimes becomes a problem as Chinese warships will often try to rescue Chinese trawlers seized for illegal fishing. This doesn’t always work but it sets a scary precedent. This has happened several times in Indonesian waters, even in areas where China does not dispute ownership. China justifies their armed intervention because the Chinese trawlers were in “traditional Chinese fishing grounds.”
In the case of Argentina the Chinese did nothing because most South American nations have an illegal fishing problems and would unite in opposition to Chinese bullying. Any Chinese economic threats could backfire because Chinese firms are currently investing a lot money in South America and don’t want those investment threatened because of widespread local anger over Chinese poachers.
The trawlers involved in these incidents are formally called "freezer trawlers." These ships are up to 100 meters (320 feet) long and have facilities on board to store hundreds of tons of frozen fish. These ships normally stay at sea months at a time and have crews of 14-30. The number of Chinese trawlers has expanded enormously since 1985 (when there were 13) and there are currently over 2,400 of them operating worldwide. China helped with this expansion by subsidizing ocean going fishing boats. Those subsidies have since been withdrawn but meanwhile the number of larger (than 100 meter) freezer trawlers has grown and these are meant for use in far distant waters. The trawler sunk off Argentina was 66 meters long but was in the company of similar Chinese vessels, which picked up most (23) of the crew from the sunk trawler and fled the area.