On the contrary, in Annexure III of UNCLOS, under the heading of Convention of High Seas, 1958, Article 2 stipulates certain freedoms that are recognised by the general principles of international law, wherein freedom of fishing is a part.
To combat piracy, a modern threat to shipping, the International Maritime Organization (IMO) adopted a resolution in 1986 on the Suppression of Unlawful Acts against the Safety of Maritime Navigation, then known as the Rome Convention. This was the immediate outcome of the Achille Lauro case, in which a passenger ship with Italian crew was hijacked by Palestinians in October 1985. The U.N. then requested the IMO to address the problem. The action required to be taken was against persons committing unlawful acts against ships.
In the instant case, even this does not apply as there was no unlawful act committed against the ship by the fishermen. The action by the ship's crew was on suspicion that the fishermen could be pirates.
Therefore, this case definitely does not come under any of the provisions of UNCLOS or any other convention connected with international piracy. Killing someone is a crime; the accused has to face charges. But how and where must be decided by the authorities keeping in view diplomatic conditions.