When a ship puts into port, it comes under the jurisdiction of the State on whose territory the port is situated. For a long time, these powers of jurisdiction were only exercised for the purpose of ensuring the safety of the people concerned. Since the 3rd UNCLOS , States have used their powers of jurisdiction as port States to ensure that rules regarding the protection of the marine environment are observed. And since more recently, to ensure compliance with rules regarding the preservation of halieutic resources: it is much easier to conduct investigations of a vessel in a port than on the high sea, especially in the hope of limiting activities that may be in violation of the law applied by the port State (cf. flag State and coastal State).
Unilaterally or, more generally, by means of a regional agreement, some States have thus compelled vessels wanting to put into their ports to comply with legal rules and regulations aimed at ensuring a better level of protection for the marine environment (double hull) or improved conservation of resources (prohibiting the unloading of certain species). Checks conducted by port States help towards giving legal regulations additional effectiveness, as they no longer depend on the capacities of the flag State or coastal State to have them enforced.
In the case of IUU fishing (illegal, unreported and unregulated fishing), the FAO Conference approved an agreement to fight against such activity , which is of particular interest as it recognizes not only the port State's authority to ensure that regulations on the conservation of resources are observed, but also, and most importantly, the obligation of said State to conduct such controls and exercise its authority as a port State with regard to fishing vessels, either national or foreign. Community regulations based on the principle of trackability, extremely restrictive for the import and export of halieutic products in or out of the European Union, are founded on the same principle.
However, all else being equal, port States, within the EU or elsewhere, may be faced by enormous difficulties when attempting to conduct the investigations they are supposed to carry out: cargoes suspected by the port State to be IUU still manage to penetrate the Common Market .
 Montego Bay, 1982.
 Agreement regarding the measures available to the port State designed to prevent, thwart and eliminate illegal, unreported and unregulated fishing. http://www.fao.org/fileadmin/user_upload/legal/docs/1_037t-e.pdf