On Friday the International Tribunal of the Law of the Sea (ITLOS) delivered its verdict on demarcating the maritime boundaries between Mauritius’ Chagos islands and Maldives. Here is why it decided to split the claim, the role that the Blenheim reef question played in that verdict and why the ITLOS decided not to decide on Mauritius’ claims to a continental shelf on the northern Chagos.

The split

On Friday, Jin-Hyun Paik, president of the special chamber of the International Tribunal of the Law Sea (ITLOS), delivered the verdict on the maritime boundary dispute between Mauritius and Maldives. At stake were 92,563 km2 of sea, claimed by both Mauritius and Maldives as falling within their exclusive economic zones (EEZs). In many ways the case was an unusual one: both Mauritius and Maldives are two of 22 states globally that are self-declared archipelagic states, with the United Nations Convention on the Law of the Sea (UNCLOS) having provision on how to decide on maritime boundaries between neighbouring archipelagos.

At the ITLOS, both states agreed to apply an ‘equidistance’ formula (similar to the formula applied by the UK in the 1990s to demarcate Chagossian waters from Maldivian ones, but which were never ratified formally). To figure out just where to draw the one both states also agreed on a three-step process: coming up with a provisional boundary splitting the disputed area, adjusting that boundary to make sure it’s fair to both states and then checking whether the proportion of waters allocated to each state were in proportion to each state’s coast.

This is where Blenheim Reef comes in. A smattering of reefs, underwater at high tide and partially over the water at low tide, was the main bone of contention between the two states. Mauritius wanted Blenheim to be counted as part of its coast and wanted it to be used to calculate the equidistance line between it and Maldives. For that it produced a geodetic survey carried out in February last year as part of a scientific trip to the Chagos islands. Maldives argued against considering the 36 km2 Blenheim Reef to calculate the share of waters to be divided between the two states; instead where to draw the line should be counted from islands such as Peros Banhos and Salomon.

Maldives pointed to satellite images to argue that Blenheim was underwater at high tide with just patches of 57 coral heads sticking up at low tide, whereas Mauritius argued that Blenheim was part of a single “underwater structure”. Should Blenheim be included in calculating the maritime boundary between the two states, it would mean pushing half the proposed boundary further north by 11 miles, netting Mauritius an additional 4,690 km2 of water, or 4.9 per cent of the waters under dispute, something that Mauritius saw as “extremely modest”, but which Maldives saw as “extraordinarily disproportionate”.

While the ITLOS decided that “Blenheim Reef cannot be considered a site of base points for the construction of the provisional equidistance line”, it did count part of the reef – that falling within 12 nautical miles of the nearby Île Takamaka – to use to draw the boundary. Why the ITLOS decided to do this is simple: by counting only part of Bleinheim Reef, the ITLOS tried to come up with a division of the maritime disputed zone which corresponded as closely as possible to the 40.3-km coast of Mauritius (excluding most of Blenheim from the calculation) and Maldives’ 39-km coast. Out of the 92,563 km2 of ocean being contested by the two states, the ITLOS’ verdict gave Maldives 47,232 km2 and Mauritius, 45,331 km2 , with the ITLOS arguing “that there is no significant disproportion between this ratio and the ratio of the lengths of the respective coasts of the parties”.

Besides adding an additional 45,331 km2 to Mauritius’ territorial waters, Prime Minister Pravind Jugnauth in a televised address on Friday greeted this as a “historic judgment” that in a legally binding way re-confirmed Mauritian sovereignty over the Chagos islands. “The whole world has an obligation to respect it” as well as ending a simmering boundary dispute with Maldives.

The continental shelf question

Demarcating the EEZ’s of the two states was not the only thing that the ITLOS was being asked to decide. Mauritius also wanted it to decide on its claim of a continental shelf in the northern Chagos. This was a dispute that was bound to occur sooner or later. In 2010 Maldives submitted its continental shelf claim to the UN’s Commission on the Limits of the Continental Shelf (CLCS). The problem was that Maldives was drawing up its continental shelf claim while looking at the maritime zone that was being claimed by the UK as part of its British Indian Ocean Territory (BIOT). The zone being claimed by the UK did not count Blenheim Reef when drawing up its maritime zone claims but Mauritius did. In its Maritime Zones (Baselines and Delineating Lines) regulations 2005 that were passed under its Maritime Zones Act of the same year, Port Louis outlined the maritime zone it was claiming as its own. By including Blenheim Reef in its calculation, Mauritius was claiming a bigger maritime zone than the UK was. The result was that when Mauritius submitted its claim to an enlarged maritime zone to the UN in 2008, with Maldives ignoring the Mauritian claim and sticking to the UK’s one, an overlap occurred with Maldives’ claim to a continental shelf eating a bit into the maritime zone claimed by Mauritius.

On October 21, 2010, both states discussed this looming boundary issue with Ahmed Shaheed (then Maldives’ foreign minister) acknowledging that “the parties discussed Maldives’ submission to the CLCS, which had improperly claimed an outer continental shelf entitlement within 200 miles of the baselines of Mauritius” because as the Maldivian side acknowledged, they had not taken the EEZ claimed by Mauritius into account – just that claimed by the UK. According to Port Louis, Maldives promised to amend its CLCS submission which it did not do. Now with the ITLOS drawing up the line between the two states – and the continental shelf overlap occurring 224 km2 into the Maldivian zone of the now-demarcated boundary, the ITLOS has settled any such claims that eat into the other states’ EEZs on either side of the new maritime boundary of the two states.

But what Mauritius wanted the ITLOS to do was draw up boundaries for continental shelves that straddle – but fall outside the EEZs of both states – where there are still overlaps in the claims of both states. The Maldives argued that this was a new problem that Mauritius was bringing into the case which was supposed to be about EEZs and furthermore that the ITLOS did not have the power to decide on continental shelf questions. While the ITLOS rejected Maldives’ objections and deemed itself fit to decide on the continental shelf, it refused to wade into this question. In a separate document, the president of the special chamber Paik, explained why the ITLOS was refusing the Mauritian request to delimit the continental shelf entitlements of Mauritius and Maldives. Citing the scientific and technical aspects involved in deciding on continental shelf issues with both Mauritius and Maldives coming up with competing assessments of the seabed and subsoil of northern Chagos, there was just too much uncertainty for the ITLOS itself to decide that particular question. The risks he cited were: the effect that such a judgement would have on the rights of other states in international waters and the risk of the CLCS when deciding on Mauritian and Maldivian claims coming up with a different decision than that of the ITLOS. Hence, Paik argued, the ITLOS decided not a get an expert to decide on this and conclude that “there is significant uncertainty as to Mauritius’ entitlement to the continental shelf beyond 200 nm and that, given such uncertainty, it is not in a position to make a determination on that matter”.

While the ITLOS has decided the respective EEZs of Mauritius and Maldives by dividing the disputed zone between the two, both states will still have to reach a deal on the continental shelf that both states are claiming just outside each other’s EEZs.

Timeline of the case

❚ June 18, 2019: Mauritius sends notification to Maldives indicating it is starting proceedings at the ITLOS.

❚ August 23, 2019: Mauritius’ Solicitor-General informs ITLOS that it is bringing a case against Maldives.

❚ September 24, 2019: Following meetings in Hamburg, both states agree to submit dispute to the special chamber of ITLOS.

❚ September 27, 2019: ITLOS decides to form special chamber of nine judges with Mauritius and Maldives allowed to pick one judge each. Maldives picks judge Bernard Oxman. UN Secretary General informed of the case.

❚ October 9, 2019: Mauritius picks Judge Nicolaas Schrijver to sit in chamber.

❚ December 18, 2019: Maldives files preliminary objections in the case.

❚ August 26, 2020: Judge Cot resigns from the chamber.

❚ September 15, 2020: Judge Cot is replaced by Judge Stanislaw Pawlak.

❚ January 28, 2021: ITLOS rejects Maldives’ preliminary objections and denies that UK has any sovereign rights over the Chagos.

❚ May 25, 2021: Mauritius submits its memorial.

❚ October 18, 2021: Mauritius informs ITLOS that Dheerendra Kumar Dabee, who retired as Solicitor-General on September 27, 2021, and now an advisor at the Attorney General’s Office, to continue to act as Mauritius’ representative.

❚ November 24, 2021: The Chagossian Committee Seychelles seeks permission to file its own submissions in the case.

❚ November 25, 2021: Maldives files its counter-memorial.

❚ December 15 & 16, 2021: Both Maldives and Mauritius state that it is up to IOTC to decide whether to allow submissions by the Chagossian Committee Seychelles in the case.

❚ January 11, 2022: ITLOS decides not to include submissions by the Chagossian Committee Seychelles in the case.

❚ January 12, 2022: Mauritius informs ITLOS of its intention to carry out a scientific survey of Blenheim Reef and other islands in the Chagos and asks ITLOS’s help in arranging trip from Maldives’ Gan in Addu atoll.

❚ January 17, 2022: Mauritius says Maldives putting unacceptable restrictions on its scientific survey, including excluding its lawyers and Mauritian government officials from survey team, as well as asking Mauritius to get clearances from UK for the expedition.

❚ February 8, 2022: ITLOS informed that Mauritius had made other arrangements for the expedition and no need to further engage with Maldives. Mauritius also notes its intention to claim resulting additional costs – €460,000 – from Maldives.

❚ February 14, 2022: Maldives tells ITLOS that Mauritius “misrepresenting” its stand over the survey and rejects calls for compensation from Port Louis.

❚ April 14, 2022: Mauritius submits its reply to Maldive’s counter-memorial.

❚ August 15, 2022: Maldives files its rejoinder.

❚ August 16, 2022: ITLOS informs both states that it is considering whether or not to arrange for expert opinion to decide on continental shelf claims in the case.

❚ August 30, 2022: Mauritius welcomes proposal to appoint experts to investigate continental shelf claims.

❚ August 31, 2022: Maldives objects to appointing experts as “unnecessary”.

❚ September 30, 2022: Mauritius tells ITLOS that Maldives has changed its position in the UN over Mauritian sovereignty over the Chagos, and that President of Maldives has pledged to facilitate future travel of Mauritian Prime Minister from Maldives to the Chagos. Mauritius tells ITLOS it is dropping its claim for additional costs from Maldives over expedition.

❚ October 6, 2022: Maldives writes to ITLOS reiterating its willingness to arrange for future trip of Mauritian Prime Minister to Chagos via Maldives.

❚ October 10, 2022: Maldives objects to Dr. Mohammed Rezah Badal, Director General of the Department of the Continental Shelf, Maritime Zones Administration and Exploration at Mauritius’ Prime Minister’s Office from testifying on Mauritius’ behalf.

❚ October 12, 2022: Mauritius denies that Dr. Badal will introduce new evidence in the case.

❚ October 16, 2022: ITLOS submits list of questions to both parties to answer in their oral hearings.

❚ October 17-24, 2022: ITLOS holds seven public sittings.

❚ April 28, 2023: ITLOS delivers its verdict.