UNCLOS says much about maritime jurisdiction but little about substantive criminal law. UNCLOS parties are obliged to cooperate in suppressing only three particular offences in international waters: piracy, illicit narcotics traffic and unauthorized broadcasting. Of these, high seas jurisdiction to enforce is only generally granted over piracy and unauthorized broadcasting, and the latter only in a qualified form. All UNCLOS parties are also obliged individually to ‘prevent and punish’ the use of their flag vessels in the slave trade, but there is no general duty of cooperation to this end, let alone universal high seas enforcement jurisdiction.

These are significant preliminary distinctions to make. Discussion of non-flag-state interdictions on the high seas usually begins with UNCLOS Article 110. Article 110 provides for visit and inspection upon suspicion of:

• piracy;

• slave trading;

• unauthorised radio broadcasting;

• statelessness; and

• a vessel being of the same nationality as the interdicting vessel, despite ‘flying a foreign flag or refusing to show its flag’.

In respect of each of the grounds listed in Article 110 a duly authorised State vessel ‘clearly marked and identifiable as being on government service’ may exercise a ‘right of visit’ to board the vessel and inspect its papers and, where suspicion remains, may proceed to ‘further examine’ the vessel. This list may be apt to mislead; as discussed above, not all such suspect vessels are necessarily subject to seizure. The grant of enforcement jurisdiction may be limited.