As a result of the bombing of the Kerch Bridge the Russian authorities have declared that they will view any vessels lifting and delivering cargos in the Black Sea as potentially carrying weapons and., as such, subject to interdiction. Russia is preventing the free passage of merchant shipping in the Black Sea. This is a highly specious contention. In international maritime law there is the right of free passage. As per the treaty

 SECTION 3. INNOCENT PASSAGE IN THE TERRITORIAL SEA

 SUBSECTION A. RULES APPLICABLE TO ALL SHIPS

Article17

Right of innocent passage

Subject to this Convention, ships of all States, whether coastal or land-locked, enjoy the right of innocent passage through the territorial sea.

Article18

Meaning of passage

1. Passage means navigation through the territorial sea for the purpose of:

(a) traversing that sea without entering internal waters or calling at a roadstead or port facility outside internal waters; or

(b) proceeding to or from internal waters or a call at such roadstead or port facility.
2. Passage shall be continuous and expeditious. However, passage includes stopping and anchoring, but only in so far as the same are incidental to ordinary navigation or are rendered necessary by force majeure or distress or for the purpose of rendering assistance to persons, ships or aircraft in danger or distress.

Article19

Meaning of innocent passage

1. Passage is innocent so long as it is not prejudicial to the peace, good order or security of the coastal State. Such passage shall take place in conformity with this Convention and with other rules of international law.

2. Passage of a foreign ship shall be considered to be prejudicial to the peace, good order or security of the coastal State if in the territorial sea it engages in any of the following activities:

(a) any threat or use of force against the sovereignty, territorial integrity or political independence of the coastal State, or in any other manner in violation of the principles of international law embodied in the Charter of the United Nations;

(b) any exercise or practice with weapons of any kind;

(c) any act aimed at collecting information to the prejudice of the defence or security of the coastal State;

(d) any act of propaganda aimed at affecting the defence or security of the coastal State;

(e) the launching, landing or taking on board of any aircraft;

(f) the launching, landing or taking on board of any military device;

(g) the loading or unloading of any commodity, currency or person contrary to the customs, fiscal, immigration or sanitary laws and regulations of the coastal State;

(h) any act of wilful and serious pollution contrary to this Convention;

(i) any fishing activities;

(j) the carrying out of research or survey activities;

(k) any act aimed at interfering with any systems of communication or any other facilities or installations of the coastal State;

(l) any other activity not having a direct bearing on passage.
 

Russia is not the only nation with a coastline on the Black Sea. Bulgaria, Rumania, Turkey, the Ukraine and Georgia all have a Black Sea coast and have rights of free passage.   There is no basis for the Russians to interdict  the right of free passage to their ports and the interference with the trade in these nations.

Moreover, the unfettered access of vessels to and from the Black Sea to the Mediterranean Sea is regulated by the  Montreux Convention, an international agreement governing the Bosporus and Dardanelles Straits in Turkey, signed on 20 July 1936 at the Montreux Palace in Switzerland. The Montreux Convention regulates maritime traffic through the Turkish Straits. It guarantees "complete freedom" of passage for all civilian vessels in times of peace. In peacetime, military vessels are limited in number, tonnage and weaponry, with specific provisions governing their mode of entry and duration of stay. If they want to pass through the Strait, warships (including Russian warships)must provide advance notification to the Turkish authorities, which, in turn, must inform the parties to the convention.When Turkey is at war, or feels threatened by a war, it may take any decision about the passage of warships as it sees fit.

The Convention consists of 29 Articles, four annexes and one protocol. Articles 2–7 consider the passage of merchant ships. Articles 8–22 consider the passage of war vessels. The key principle of freedom of passage and navigation is laid out in articles 1 and 2. Article 1 provides, "The High Contracting Parties recognise and affirm the principle of freedom of passage and navigation by sea in the Straits". Article 2 states, "In time of peace, merchant vessels shall enjoy complete freedom of passage and navigation in the Straits, by day and by night, under any flag with any kind of cargo".

There have been several attempts to change Turkey's domination of passage to and through the Black Sea, but Turkey has always refused. The United Nations Convention on the Law of the Sea (UNCLOS), which entered into force in November 1994, prompted calls for the Montreux Convention to be revised and adapted to make it compatible with UNCLOS's regime governing straits used for international navigation. However, Turkey's longstanding refusal to sign UNCLOS has meant that Montreux remains in force without further amendments. The U.S. is not a party to the Montreux Convention but, with the membership of Turkey in the NATO alliance it considered itself protected.

With the resumption of hostilities between Russia and the Ukraine with Russia's renewed attacks in February 24, 2022, , the Ukrainian government appealed to Turkey to exercise its authority under the Montreux Convention to limit the transit of Russian warships from the Mediterranean to the Black Sea. At least six Russian warships and a submarine had crossed the Turkish straits in February. After initial reluctance, attributed to the country's close ties with both Russia and Ukraine,Turkish Foreign Minister Mevlüt Çavuşoğlu announced on 27 February that his government would legally recognise the Russian invasion as a "war", which provides grounds for implementing the Monteux convention with respect to military vessels. This blockage of naval vessels also applies to NATO powers who cannot now move their vessels from the Mediterranean to the Black Sea.

Just as it was a Turkish initiative to create a "Grain Deal" for the export of Ukrainian and Russian grains from the Black Sea, Turkey is also a key player in the response of the world maritime community to the Russian announcement of the curtailment of free passage in the Black Sea just announced by Russia. Under the terms of the Montreux Convention and UNCLOS it is only Turkey who can control the free passage of vessels (both commercial and military) in and out of the Black Sea.

As has been pointed out much of the commercial shipping in and out of the Black Sea is being carried out by vessels registered in a variety of nations and subject to their rules. There are very good reasons for a shipowner to register his vessels under 'flags of convenience'. Each merchant ship is required by international law to be registered in a registry created by a country, and such a ship is subject to the laws of that country, which are used also if the ship is involved in a case under admiralty law. A ship's owners may elect to register a ship in a foreign country so as to avoid the regulations of the owners' country, which may, for example, have stricter safety standards. They may also select a jurisdiction to reduce operating costs, avoiding higher taxes in the owners' country and bypassing laws that protect the wages and working conditions of mariners. It is also the custom that each ship is a business on its own, incorporated as a company. This is used to protect the fleet of an owner with several vessels from blackmail when one of the vessels is seized, impounded or arrested. What happens to that arrested ship and its liabilities cannot be used against the other vessels of the owner as they are all separate companies on their own. So, if the Russians attack or blockade a marine vessel in the Black Sea flying the flag of Vanuatu there is no reason why the master of that vessel need to pay any attention to the Russian orders as the rights asserted by Russia are themselves not founded in law. Diverting a Russian frigate to attack Port Vilar in Vanuatu is an unlikely occurrence.

The answer lies with Turkey which has the rights to regulate free passage through the Dardanelles. There are a number of rules and customs which have been voluntarily obeyed in the Black Sea as well. I used to ship tens of thousands of tons of Guinean bauxite every year to Nikolaeyev Smelter. We used 35,000 dwt vessels for the trade but we had to be sure that the crew was Russian or Ukrainian because as we passed the Kerch Strait the Soviet authorities declared it a 'protected' zone and we had to conform. There were other, niggly things, about the trade but there was never any thought of a ban on our vessels. In fact, the waters of the Kerch were so shallow that when we loaded steel at Azov in Mariopul for a our return cargo we had to stop in Bulgaria to complete because the draft didn't allow us to travel fully laden from the Ukraine.

So, until Turkey decides to become a participating member of NATO, it is likely that the Russians will continue to 'MauMau' world shipping. The solution to the grain problem lies with Turkey stepping up its role in pursuit of peace and blocking Russian transit of the Black Sea.