The hook came out of a line your eye would normally slide right past. In an analysis of Iran’s wartime arithmetic, a detail flashed by: trade is shifting to the Caspian because the Strait of Hormuz is functioning poorly — but the water there has fallen by roughly two meters since the nineties, and ports are taking ships carrying four thousand tons instead of six.
Two meters and four thousand tons looked like a minor detail in a conversation about war. But behind them lies a question with no short answer: by what right is Iran trading across a sea that, legally, still has no border? I started checking — and found that the Caspian has a convention on its legal status, signed back in 2018, which still has not entered into force because it is awaiting a fifth ratification. That fifth one will be Iran’s.
Until 1991, the Caspian was the private business of two states — the USSR and Iran. The 1921 treaty gave Iran an equal right to sail under its own flag; the 1940 treaty added a ten-mile coastal zone of exclusive fishing rights and closed the waters to every flag but those two. Neither document drew a maritime border or divided the seabed and its resources: throughout, the subject was joint use, not ownership.
Then the Soviet Union ended, and there were five owners on the shore instead of two. Azerbaijan, Kazakhstan, and Turkmenistan emerged from the collapse with offshore oil fields and not a single document stating whose they were. Formal negotiations on a new regime began in early 1992; a working group at deputy-minister level was assembled in 1996.
The legal fork looked elegant and insoluble at once. If the Caspian is a sea, the 1982 UN Convention on the Law of the Sea applies to it, meaning territorial waters, an exclusive economic zone, and everything else. If it is a lake, ordinary law on border lakes applies, and it has to be divided differently. The real answer turned out to be unpleasant for lawyers: the Caspian is neither a sea nor a lake in the terms of international law, and its status must be determined by a separate agreement of all five parties. That is what they eventually did.
The Iranian public clings to the figure of 50 percent, and that faith has a pedigree. When the Caspian was divided only with the Soviet Union, “equal rights” could easily be read as “equal ownership”: two countries, split down the middle. The problem is that the treaties of 1921 and 1940 contain neither a half, nor percentages, nor a border. Ahmad Vakshiteh puts it bluntly: they emphasized joint use, not division into shares of ownership.
There was a second foundation for that belief — the Astara–Hossein-Gholi line. Border agreements of 1954 and 1957 fixed two points on the coast, western and eastern, and subsequent practice gradually turned the line between them into a de facto delimitation. Iran’s side was left with roughly 12–14 percent of the sea. The lawyers at Tehran Bureau carefully note that this line has never been tested in court as a maritime border and has never been openly challenged by anyone — which is to say, it exists precisely as long as nobody touches it.
Iran’s negotiating position traveled from a condominium arrangement — common ownership by all five — to a demand for exactly 20 percent for each country, beginning in December 1998 and, according to former head of the Majlis National Security Committee Heshmatollah Falahatpisheh, on the basis of a decision by the Supreme National Security Council. The other four parties rejected both.
The convention was signed on 12 August 2018 in Aktau at the fifth Caspian summit, after 51 meetings of the working group and four presidential meetings. And then comes the most interesting part for any discussion of “Iran’s share”: there is not a single percentage in the text.
The treaty divides the water into four belts. Internal waters — behind the baseline. Territorial waters — up to 15 nautical miles, their outer boundary declared a state border. A 10-mile fishing zone adjoining territorial waters, where the country has the exclusive right to fish. And common maritime space, open to all. The seabed and subsoil are divided into sectors by separate agreements between neighbors, and percentages are mentioned nowhere. On the fishing zone, incidentally, sources are already beginning to diverge: Article 9 says 10 miles adjoining territorial waters, while the head of the Caspian Secretariat, in an official explanation, gives the figure as 25 miles. They agree only if both belts are counted from the baseline — 15 plus 10. Careless retelling or differing interpretations — I never did find out.
One technical detail is mentioned, though, and it is worth more than any percentage. The normal baseline is defined literally as “the line of the multi-year average level of the Caspian Sea, measured at minus 28.0 meters in the Baltic System of 1977 from the zero of the Kronstadt tide gauge.” This is an exceptionally rare case in which a maritime border in an international treaty is tied to a specific water-level mark. While the water stays put, the border stays put. When the water recedes, the line begins to mean something else — and the convention says nothing about that contingency.
A separate victory for Iranian negotiators is a proviso in the first article: if the configuration of the coast puts a state at a clearly disadvantageous position in determining internal waters, that is to be taken into account when developing the methodology. Iran is not named, but the description is unmistakable, and the country later secured written acknowledgment from its partners that this clause refers to Iran. The reason is geography: Iran’s coast is 657 kilometers long, the shortest on the Caspian, and it is concave, with no headlands or coastal islands convenient for linking the points of straight baselines. Azerbaijan and Turkmenistan have convex coastlines, and Azerbaijan also has islands to boot — they have something with which to mark out a baseline. Iran has nothing.
By calculation, if the seabed is divided along the modified median line that the four countries agreed on among themselves in bilateral formats, it comes out like this: Kazakhstan 27–30 percent, Turkmenistan 19–23, Azerbaijan 18–21, Russia 16–19, Iran 11–14. Former Iranian diplomat Kourosh Ahmadi does not consider these figures final and points to another scenario: if the neighbors accept the straight Astara–Hossein-Gholi baseline, Iran’s share may approach those very 20 percent. The difference between eleven and twenty is not arithmetic, but two fundamentally different seas.
Rouhani signed the convention, but in the three remaining years of his administration, the ratification bill was never submitted to the Majlis. It was not submitted under Raisi either. Only on 1 August 2026 did Pezeshkian’s government send the document to parliament with urgent status — during a war, while parliament was meeting remotely.
The Foreign Ministry’s public case rests on security. Deputy Foreign Minister for Legal Affairs Kazem Gharibabadi gives three arguments, the first being the situation: over the past year or two, outside powers have been drawn toward the Caspian, while Article 3 of the convention declares the sea a zone of peace and prohibits the presence of armed forces from non-Caspian states, including transit. The second argument is the economy, where cooperation is stalling precisely because the core legal framework has not entered into force. The third is unresolved legal questions.
Critics respond that the order of operations is wrong. Falahatpisheh: “At present, this convention is Iran’s only lever. If Iran adopts it before defining its share and before settling disputes with Azerbaijan and Turkmenistan, there will be no lever left.” He also points out that the treaty has no mechanism for compulsory dispute settlement: Article 21 leaves everything to negotiations and consultations, while arbitration is possible only by mutual consent — meaning Iran cannot unilaterally take a dispute anywhere.
Another of his arguments is that ratification will nullify the treaties of 1921 and 1940. Here, the sources diverge, and there is no point smoothing that over: Article 20 of the convention expressly says that the document does not affect rights and obligations under other international treaties, so formally Falahatpisheh is wrong. But where the new five-party regime replaces the old bilateral arrangements, Iran will in practice find it harder to rely on an interpretation like “the Caspian is jointly owned by Iran and Russia.” Formally, the rights are preserved; in fact, they are blurred. No one knows which legal scholars are closer to the truth — and that is the honest answer, not a rhetorical one.
On 22 September 2026, MP Hadi Haji Delaigani announced that he would not vote on the convention, without explaining why. One refusal does not decide the fate of the bill — but it came three weeks after Iran hosted the seventh conference of the Tehran Convention in Tehran, attended by deputy environment ministers from all five countries. The symbolism of the situation was almost mocking: Iran chairs the regional environmental process and has not ratified the document on which that process rests.
Since the text ties borders to the minus-28-meter mark, it is worth looking at where that mark stands now. Azerbaijani Deputy Environment Minister Rauf Hajiyev gave Reuters the figures in August 2025: over five years, the level had dropped by 0.93 meters; over ten, by 1.5; over thirty, by 2.5. He put the current rate at 20–30 centimeters a year. Kazakh statistics are even more specific: in 2025, the average level in Kazakhstan’s sector stood at minus 29.35 meters, while in some areas the shoreline had retreated 30–35 kilometers compared with 2006.
The Kazakh forecast, drawn up by Kazhydromet, reads like a menu with no good options. The optimistic scenario is minus 29.8 meters by 2030 and minus 32.4 by 2050. The pessimistic one is minus 30 and minus 34 respectively. The Hudson Institute adds a farther horizon: by the end of the century, a decline of 9–18 meters, at which point an area roughly the size of Jordan turns into desert or marshland.
The parties name different culprits for the drop, and that, too, is part of the story. Alongside climate, Azerbaijan points to Russian dams on the Volga, which supplies 80 percent of the inflow. Russia points mainly to climate change. Formally, both causes operate at once, but politically one has to choose, because the other means sending the bill to a neighbor.
The practical consequences are already being counted in tons. In the first half of 2025, 810 thousand tons passed through the Dubendi oil terminal in Azerbaijani waters, against 880 thousand in the same period a year earlier. The director of the Port of Baku calls the decline insignificant and explains it as ordinary fluctuations in orders, but immediately begins talking about dredging: in 2024, more than 250 thousand cubic meters were excavated, and in April 2025 the dredger Engineer Soltan Kazimov was launched, capable of deepening the seabed to 18 meters. Baltic and Black Sea ports do not think about such things at all — their water is where it belongs.
There is a more unpleasant layer, too. As the water recedes, the former seabed becomes dry land, and a dispute over a maritime border turns into a dispute over territory. The Hudson Institute calls this the third geopolitical risk of falling levels — and the most explosive. Claims to exposed areas, if there are hydrocarbons beneath them, will be filed not over sea but land, and people can litigate them for decades.
The sturgeon is the clearest indicator in this story: its condition is measured in biomass, not kilometers of exposed land. According to data from VNIRO, cited by Rossiyskaya Gazeta, commercial fishing for beluga was banned in 2000, for Russian sturgeon and stellate sturgeon in 2005, and since 2012 all Caspian littoral countries have renewed the ban annually and maintained zero quotas for caviar exports. And still, from 2012 to 2024, beluga biomass fell 11.6-fold, Russian sturgeon 2.6-fold, and stellate sturgeon fourfold.
A ban observed by everyone does not work.
Since 2009, specialists have recorded no beluga larvae drifting down from the Volga spawning grounds: there is no natural replenishment; the population survives on artificial breeding and is sustained by it. The calculation for recovery is no quick one either: with the release of 10–15 million juveniles a year and a total end to poaching, beluga needs 40–50 years. And that is without accounting for the fact that as the level falls, sturgeon lose up to 45 percent of their summer and autumn habitats and are cut off from spawning grounds in the rivers.
The Caspian seal, an endemic species found nowhere else, is following the same path. Over a century, its numbers have dropped by roughly 90 percent: around 1.2 million at the beginning of the 20th century; today estimates range from 75 to 270 thousand, and no one knows the precise figure. The seal breeds on northern ice, and the dependency is direct: with a five-meter drop in level, up to 81 percent of breeding sites are lost; with ten, nearly all of them. This is no longer about forecasts for the future: the species is running out of places for its pups.
A picture emerges in which two speeds have diverged beyond repair. The legal question of the Caspian’s status has been discussed since 1992 and is handled through working groups, deputy-minister meetings, and 51 sessions; at the last summit in Tehran, where they agreed on water and seals, nobody signed anything on borders. The physical question of the water itself is being resolved at a rate of 20–30 centimeters a year, and within that gap lies the entire history of the dispute: the convention ties the border to the minus-28-meter mark, and that mark was passed a meter and a half downward long ago.