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The Conundrum of Defining the Caspian Sea

May 9, 2023

An Overview

The Caspian Sea has been shared for centuries between Russia/USSR and Persia/Iran. Before the dissolution of the Soviet Union in 1991, the international legal status of the Caspian Sea was governed by the International Treaties of 1921 and 1940, in which no specific definition was given to the Caspian Sea and its sui generis status was not named, but it was de facto condominium. These Treaties were accepted and recognized by the international community.[1] Since 1991, the three newly formed littoral states, former Republics of the USSR – Azerbaijan, Kazakhstan and Turkmenistan, have expressed a need to establish the new legal regime of the Caspian Sea.

The legal definition of the Caspian as a sea or a lake was a key point in determining its new legal status and was closely linked to its scientific definition. The increased interest of the three newly formed states and the international community in the legal status of the Caspian Sea was due to the existence of the rich hydrocarbon reserves and the prospect of their redistribution under the new geopolitical conditions. It was fundamentally important to take into account the special nature of functioning of the Caspian Sea as an enclosed body of water with a particularly valuable biodiversity and particularly fragile environmental balance, in view of the non-unified management of its resources and, above all, the production of hydrocarbons from offshore and coastal fields, as well as their transportation.

1. Main characteristics of the Caspian Sea, its natural resources and the environment

A. Physical characteristics

The Caspian Sea is the largest enclosed body of water in the world, which is also referred to as a lake and a sea. Its total area varies between 270 and 390 thousand sq km. Its length from North to South is about 1200 km, the width from East to West – 200-435 km. The maximum depth varies from 25 m in the Northern part of the Sea to 1025 m in the South. The Caspian lies at -26-29 m below mean sea level and its water volume is about 78,000 cubic km. Close to the coast there are 50 relatively small islands with a total area of about 2,000[2]

The Caspian Sea is one of the remnants of the ancient Tethys Sea. Throughout its history and over millions of years, the Caspian Sea had been connected to other seas and lost connection with some of them at times. The seabed of the Southern part of the Sea is of oceanic origin and consists of a basalt layer up to 15 km thick. In the relief of the seabed of the Middle part, the main oceanic-type geomorphological features are clearly distinguished – shelf, continental slope, abyssal plain, thench and, additionally in the Southern part, there are submarine ridges and mud volcanoes.

The water level of the Caspian has had a very large fluctuation since historical times. Now it largely depends on the runoff volume of the enormous hydrographic basin (3 100 000 cubic km). The Caspian has a characteristic regime of water circulation and sea currents with waves up to 11 m. Its average salinity ranges from 3 to 12.66 ‰ compared to 35 ‰ of the World Ocean.[3]

B. Natural Resources

Biological resources include fauna and flora of the Sea and coastal areas. The biodiversity of the Caspian Sea is characterised by the presence of salt and fresh water species, which have adapted to the conditions of the Caspian Sea, mainly after its isolation from the World Ocean. This is the most important reason why the Caspian Sea has a large number of endemic species with a fairly high level of endemism. 331 endemic fauna species have been identified. The Caspian flora consists of 728 species and subspecies of lower plants and 5 species of higher plants. Approximately 90% of the world’s sturgeon stocks (6 species) live in the Caspian Sea, which, along with other species, are on the verge of extinction and have been included in the IUCN Red List of Endangered and Critically Endangered Species. The fauna and flora of the coastal areas include species of continental, warm temperate, subtropical humid and desert climatic zones. Many coastal areas of the Caspian are Protected, mainly wetlands, which are of International, Regional and National importance and host, among others, millions of migratory birds.

The Caspian Sea and its surrounding area are particularly rich in valuable minerals, including energy resources – oil, natural gas, coal and uranium. The most optimistic estimates speak of 15 – 20 billion tons of potential oil reserves,[4] with 2.95 billion tons of them being proven reserves, the largest of which are in Azerbaijan and Kazakhstan, followed by Turkmenistan and Russia (no data for Iran) and over 3.1 trillion cubic meters of proven natural gas reserves (The Ministry of Natural Resources and Environment of the Russian Federation). Kazakhstan also holds 15% of the world’s uranium reserves and 45% of their production, as well as 2.4% of the world’s coal reserves.

The Caspian Sea region is not a world famous tourist destination. However, the region has all the prerequisites for the development of a tourism industry: high-quality natural resources and a rich historical past with thousands of historical and cultural monuments. The region has an important geographical position, as it connects the main energy resource markets (Europe and Asia) and the main energy suppliers (Middle East, North Africa and Russia) with the markets of Africa and Eurasia.

C. Environmental problems

Natural phenomena (sea level fluctuations, periodic seismic activity, climate change) and human activities (hydrocarbon production and transportation, industrial activity, fishing, river flow regulation, shipping, agriculture, livestock industry, etc.) have caused the following problems in the Caspian Sea: depletion of stocks of certain commercial fish species, including sea sturgeon, threat to biodiversity, overall degradation of the quality of the marine environment, degradation of coastal zones and disturbance to coastal habitats, damage to coastal infrastructure, negative impact on the health and well-being of the population. According to the estimates of many institutions and scientists, the environmental state of the Caspian Sea is catastrophic.[5] Any alteration or change in one sector of the complex system of the sea entails chain changes, mostly negative, in the entire system. Due to the closed nature of the Caspian Sea the consequences can hardly be predicted and assessed.

2. Caspian Energy Resources as an instrument of Geopolitics

Hydrocarbons are the primary means of determining a geopolitical situation. Control of distribution and producer-consumer interdependence ensure power to policy makers. The battles are fought for dominance in the areas with strategic energy resources and the areas of existing or potential passage of the “geopolitical pipelines”. [5] After the disintegration of the Soviet Union, these factors “suddenly” made the Caspian region of such great geopolitical importance. Since then, the wider region has witnessed many armed conflicts (Chechnya, Ossetia, Abkhazia, Nagorno-Karabakh, Dagestan (Russia), Afghanistan, Georgia, Ukraine and the Middle East).

3. The Geopolitical Interests of the Great Global and Regional Powers and of the new littoral states

The geopolitical situation in the wider Caspian region is reminiscent of the Great Game (competition between the British and Russian Empires in Central Asia, 1800-1907). This is where the interests of the US, the EU, Russia, China, Iran, Turkey, their intelligence agencies and transnational corporations converge. The US seeks control over the distribution of energy resources. The EU is concerned about ensuring its energy sufficiency. Russia is trying to secure its borders and maintain its influence in the post-Soviet space. China is ensuring its stable, secure and long-term supply of large volumes of Caspian hydrocarbons at fixed prices. Iran, as a major regional power, despite Western sanctions, seeks to maintain friendly relations with countries in the region in the hope of becoming a hub for energy and trade. Turkey wants to strengthen its position as a transit hub for energy resources of global importance and transform itself from a regional into a global power.

The new littoral states have much in common. Most importantly, their economies are largely based on the energy resources of the Caspian Sea and, as former Republics of the USSR, they are seeking independence from Russia. Azerbaijan has managed to develop into an important energy and trade hub. Kazakhstan and Turkmenistan pursue the multi-vector policy, simultaneously participating in many International and Regional Organizations with competing geopolitical plans. This policy is based on their political and economic uncertainty and does not contribute to stability and security in the region or sustainable partnerships. It ultimately leads to an unpredictable development of the geopolitical and geoeconomic situation.[6]

4. Addressing the Peculiarities of Defining the Character of the Caspian Sea in the Framework of UNCLOS. The Caspian as a “Residual Sea”

The UN Convention on the Law of the Sea of 10.12.1982 and the Agreement of 28.07.1994 on the implementation of Section XI of the Convention were signed by Russia in 1997 and Azerbaijan in 2016. On the other hand, the UN Fish Stocks Agreement (Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks) was signed by Russia in 1997 and Iran in 1998.

Before the dissolution of the USSR, the Caspian Sea had no legal definition, while in scientific sources it was referred to as a sea-lake. The question of whether it is a sea or a lake arose along with the question of the new legal status. From the point of view of many scientific characteristics of the Caspian Sea (such as its oceanic origin, oceanic crust, the oceanic geomorphology of the seabed, the chemical composition of the water, the hydrological regime, the fauna and flora, size, etc.), this constitutes a “sea”. However, the sea in general is part of the World Ocean and therefore, according to the definition given by the majority of researchers, it must have a connection with it. Since all oceans and seas are connected to each other, they have the same mean sea level. Thus, the basic principle of classifying a body of water as a “sea” is its connection to the World Ocean and the nature of that connection.

Since there is no natural connection of the Caspian Sea to the World Ocean (therefore the Caspian Sea has no exchange of water masses with it and does not have the same water/sea level) it could be classified as a “lake” or a “residual lake”. However, in this case, its “birth certificate”, undeniable sea characteristics and its enormous size would be ignored. Thus, this unique body of water does not fully meet the conditions to be classified solely as a “sea” or a “lake”, as it combines (although not equally) properties of both.

For the case of the Caspian Sea the scientific (geographical) term “Residual Sea” has been proposed,[7] which underlines that this specific body of water was in the past part of the World Ocean and maintains at present many natural characteristics of the sea, although it has no connection with it, therefore, it cannot have the legal implications of a ‘sea’. As evidence of the disconnection between the geographical and legal concepts, one can encounter the use of the following terms in official and scientific texts regarding the Caspian Sea: sea water”, “marine species”, “marine research”, “marine environment”, “sea currents”, “sea-level”, “sea pollution”, “seabed activities”, “sea ecosystem” etc., although the Caspian was not recognised as a ‘sea’. That is, these terms are and will be used regardless of the legal classification of the Caspian Sea. In the case of common lakes, terms such as water spaces”, “water area”, “lake species” etc. would be used.

If there was a possibility to recognise the Caspian as a “sea”, according to some scientists, it could be classified as an “enclosed or semi-enclosed sea” (Articles 122, 123, UNCLOS). During the negotiations in the deliberations of the UN Plenary Meetings on the Law of the Sea (1974-1982) regarding the definition of the terms “enclosed” and “semi-enclosed seas”,[8] which seemed to overlap in some cases, there were many proposals and concerns, of which the following are notable for the case of the Caspian Sea.

In the discussion of the 3rd United Nations Conference on the Law of the Sea, 2nd session, 19th plenary meeting, (1974, Caracas), the following enclosed and semi-enclosed bodies of water were mentioned: the Andaman Sea, the Baltic Sea, the Black Sea, the Caribbean Sea, the Celebes/Sulawesi Sea, the East China Sea, the Mediterranean Sea, the Persian Gulf, the Red Sea, the Sea of Japan, the Sea of Okhotsk and the South China Sea. Nowhere was the Caspian referred to as a “sea”. On the contrary, as it is still the case in many scientific sources, it was stated that the Caspian cannot be classified as a sea. At the 43rd Meeting of the Second Session in 1974, the representative of Iran noted that the Caspian Sea and the Aral Sea, which have no connection with the World Ocean, do not belong to the category of enclosed seas. In 1975, it was specified that the enclosed and semi-enclosed seas will be treated as a single category. During the discussion of the 3rd United Nations Conference on the Law of the Sea, 4th session, 69th plenary meeting (1976, New York), the following enclosed and semi-enclosed bodies of water were mentioned representative of Iraq proposed that the Convention should only deal with the semi-enclosed seas, which are connected to the World Ocean through a “natural narrow outlet”. It was the only proposal that specified that the narrow must be natural. But «… in light of dissatisfaction with the provision requiring the coordination of activities” (article 130, now article 123), it was decided «not to make the definition of enclosed or semi-enclosed seas more restrictive».[9] Article 122 (“Definition”) of the 1982 Convention, states that the term “enclosed or semi-enclosed sea” means a gulf, basin or sea surrounded by two or more States and connected to another sea or the ocean by a narrow outlet or consisting entirely or primarily of the territorial seas and exclusive economic zones of two or more coastal States”.

After the dissolution of the USSR, some scientists of the new littoral states expressed the opinion that, since in case where enclosed or semi-enclosed sea is «…a gulf, basin or sea surrounded by two or more States and connected to another sea or the ocean by a narrow outlet …» of Article 122 of the UNCLOS do not specify whether the narrow outlet must be natural or artificial, rivers and canals could be considered as “the narrows”. However, rivers and artificial canals located on the territory of a state, are not subject to the International Law of the Sea, such as the Volga-Don, Volga-Baltic Sea and Volga-White Sea canals, which are located in the territory of Russia and therefore governed by its National Legislation (“inland waterways”).

After all, as mentioned above, the main condition for classifying a body of water as a sea is its connection with the World Ocean and the nature of this connection, e.g. the Great Lakes of North America are connected to the Atlantic Ocean by rivers and lakes, but this does not make them seas, because it is not a two-way connection due to the different elevation. The same applies to the Caspian Sea, which is 25-29 m below the mean level of the World Ocean, therefore it is not a “regular” sea.

As mentioned above, the same article of the UNCLOS also provides for cases of an “enclosed sea”, which is “consisting entirely or primarily of the territorial seas and exclusive economic zones of two or more coastal States”. According to some researchers (Yu. Fedorov et al.), this case “completely corresponds to the case of the Caspian Sea“. However, S. Vinogradov considered that «a distinction must be made between enclosed seas defined by Article 122 (UNCLOS) and the so-called “enclosed” seas or “quasi-seas” (such as the Caspian and the Aral Seas), which, despite some of their natural characteristics (i.e., dimensions and salinity) do not fall within the former category. They have no connection with the ocean and sometimes are regarded as lakes. However, due to specific geographical, political and historical circumstances, their legal regimes differ from that of most other international lakes and thus deserve special consideration…».[10]

The UNCLOS provides no definitions for the concepts of “sea”, “gulf”, “basin/water body” etc., but these terms are cited in it with reference to the following sources: B. Charten, B. Seas and Oceans (1989); Tver, D.F. Ocean and Marine Dictionary (1979); and Beach, E.L. & Noel, J.V. Naval Terms Dictionary (4th ed., 1978). In these sources, a “sea is the subdivision of oceans, a large marginal portion of a particular ocean, also, a large inland body of water, having a connection with the Ocean (excluding the Aral and the Caspian Sea)“.

In conclusion, the Caspian was never referred to as a sea in the numerous discussions and, on the contrary, it was emphasised that it does not belong to enclosed or semi-enclosed seas both in discussions and in scientific sources cited in the UNCLOS. Since the UNCLOS had not been acceded to by all littoral states, it was at their discretion whether or not to give a legal designation to the Caspian Sea.

5. Defining the Character of the Caspian Sea in the Framework of its Regional Evolution

The Caspian Sea, along with the lands around it, had been the subject of wars between Russia and Persia for centuries, after which these lands changed hands. The British and Ottoman Empires had a significantly negative influence on their relations. Bilateral agreements between Russia and Persia, including the last ones between the USSR and Iran (1921 and 1940), governing their relations in the Sea, did not specifically mention the Caspian Sea in terms of its status or borders, but were instead emphasising on the free movement of merchant ships and citizens between the two states. In these historical agreements, the Caspian Sea was treated as an undefined inland body of water between the two states with a legal status of joint ownership. W. Butler reported that «Soviet jurists regard the Caspian as a large lake which historically has been called a sea. General norms of international law relative to the high seas, to vessels and their crews sailing on the high seas do not extend to the Caspian, whose regime is governed by Soviet-Iranian treaties and agreements”.[11]

After the dissolution of the USSR and the occurrence of the three new littoral states, there was a need to establish the new legal regime for the Caspian Sea in order to solve many complex political, economic and environmental problems in the region, in particular to determine their sovereign rights regarding the rational use of its natural resources. In the framework of the Alma-Ata Declaration of 1991, the former Republics of the USSR, as successor states, assumed all the obligations of the USSR Conventions. Subsequently, all matters relating to the use of the Caspian Sea could be regulated in accordance with the principles and objectives of the Treaties of 1921 and 1940 until a new agreement on its legal status was reached. Nevertheless, the legal status of the condominium and the validity of the aforementioned Treaties were disputed by the new littoral states. Amongst the issues raised were the determination of the Caspian Sea as a sea or a lake, its legal status and regime, the use of the Sea and its natural resources by the littoral states and the urgent issue of the protection of its marine environment.

Since 1996, the issue of the legal designation of the Caspian Sea has practically not been a concern of the littoral states. In 2003, the states agreed that the Caspian Sea will not acquire the status of a “sea” or a “lake” in the negotiated new regime.[12] Despite this, for many years the issue of the legal designation of the Caspian Sea remained topical for various reasons. S. Vinogradov characteristically stated that «… in the absence of a definitive conventional determination of the legal regime of the Caspian Sea its legal status will continue to be the subject of scientific speculations».[13]

6. “Sea” or “lake”?

According to the UNCLOS, if the Caspian was recognised as a “sea”, each littoral state could have 12 n.m. of territorial waters, 12 n.m. contiguous zone and up to 200 n.m. Exclusive Economic Zone and continental shelf. Since the maximum width of the Caspian Sea is about 270 n.m./435 km, the outer limits of the EEZ and the continental shelf could be determined based on the median line method. There would be no international waters. All states of the world would have the right of “innocent passage” of ships, then Russia’s rivers and canals could acquire international legal status (which Russia would rightfully never agree to). Russia and Iran have always been against the presence of third parties in the Caspian Sea, because they would disturb the security of the Sea and the wider region. It is clear that the UNCLOS could not be fully implemented, except for some of its provisions.

If the Caspian Sea was recognised as a “lake”, it could be divided into national sectors and each littoral state would have full and exclusive sovereignty over all activities in its sector. The majority of Caspian states would have restrictions on navigation and interstate cooperation among them.[14] In any case, the particularly vulnerable Caspian environment would suffer, since activities in one national sector would affect other sectors with extremely limited possibilities of natural recovery in the event of an accident.

The Caspian Sea, as a unique enclosed body of water, had to have a special legal status. As expected, the Parties, guided by common interests, did not define the Caspian as a sea or a lake in the Convention on the Legal Status of the Caspian Sea of 2018, in which it is referred to as «the body of water surrounded by the land territories of the Parties». The Convention, which is a framework, reflects the basic principles of the activities of the littoral states in the Caspian Sea (issues of ownership, exploitation and transportation of energy resources, shipping, navigation, fishing, preservation of the unique ecosystem of the Caspian Sea, etc.). In addition, it provides for a special regulation on the transit of goods of the littoral states through internal waterways of the Volga-Baltic Sea, Volga-White Sea and Volga-Don-Azov and Black Sea canals on the territory of Russia. The presence of armed forces not belonging to the Parties was excluded in line with the historical continuity of the previous international agreements. The new international legal regime of the Caspian Sea is unique and is the result of an agreement between the littoral states through lengthy negotiations (1991-2018) and is based on existing regulations and internationally recognised rules of International Law.


The new Caspian regime, as defined by Convention on the new Legal Status of the Caspian Sea of 2018, seems to be the best solution to the issue of its determination. The Parties very wisely avoided the designation of the Caspian Sea as an enclosed sea or an international lake. Ιn the author’s thesis, the geographical term “residual sea” is proposed exclusively for scientific use which is not referring to the legal meaning of “sea”. The flexible and skilful application and adaptation of existing International customs, rules and principles of International Law, their functional combination with amendments and the establishment of the new legal regime of an enclosed body of water as a kind of “sea-lake hybrid” (Grozin A.),[15] were the result of a creative international negotiation process of international governance for international common interest,[16] contributing significantly to the practical and theoretical aspects of scientific knowledge in the fields of Public International Law, International Environmental Law, International Relations and Diplomacy.

The Convention on the Legal Status of the Caspian Sea (2018), along with the Convention on the Protection of its Marine Environment (2003) and other existing and future international agreements, are expected to contribute substantially and effectively to the security in the region and the governance of the unique environment of the Caspian Sea by preserving and protecting its valuable natural resources as World Natural Heritage.



  1. Barsegov, Yu.G. Каспий в международном праве и мировой политике / The Caspian Sea in International Law and World Politics, Moscow, 1998.
  2. Zonn, I.S., Kostianoy, A.G., Kosarev, A.N. & Zhiltsov S.S. Каспийское море. Энциклопедия, Восточная книга / Caspian Sea, Encyclopedia, Vostochnaya Kniga, ISBN 978-5-7873-0732-0, 2013.
  3. Ibid.
  4. Zhiltsov, S.S., Zonn, I.S., Kostianoy, A.G. Hydrocarbon Potential of the Caspian Region, In book: Oil and Gas Pipelines in the Black-Caspian Seas Region, 2016, Springer, 10.1007/698_2015_382.
  5. Kostianoy, A.G. On the need for the organization of integrated monitoring of the Caspian Sea,
  6. Soilemezidou M. PhD Thesis “International Governance of the Caspian Sea Regime: Political, Legal, Negotiating and Scientific Aspects”, 2022.
  7. Ibid., p. 330-332.
  8. Nordquist, Myron H. Shabtai Rosenne, Nandan, Satya N. United Nations Convention on the Law of the Sea, 1982: A Commentary, Martinus Nijhoff Publishers, 1985,
  9. Ibid.
  10. Vinogradov, S., & Wouters, P. (1995). The Caspian Sea: current legal problems. Zeitschrift für ausländisches öffentliches Recht und Völkerrecht, 55, 604-623,
  11. Butler, W.E. The Soviet Union and The Law of The Sea, The John Hopkins Press, 1971.
  12. Mamedov, R.F. Международно-правовой статус Каспийского моря: вчера, сегодня, завтра / International Legal Status of the Caspian Sea: yesterday, today, tomorrow, Baku, 2006.
  13. Supra n. 10.
  14. Butayev, A.M. Статус Каспия / Status of the Caspian Sea, 1999,
  15. 15. Подписана конвенция о правовом статусе Каспийского моря / Convention on the legal status of the Caspian Sea was signed, 12.08.2018,
  16. Raftopoulos, E. International Negotiation. A Process of Relational Governance for International Common Interest, Cambridge University Press, 2018,

About the author

Marina Soilemezidou

Research Fellow of MEPIELAN Centre, Academy of Athens

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