Article: Tuvalu participates in the operations of the Pacific Island Forum Fisheries Agency (FFA) and the Western and Central Pacific Fisheries Commission (WCPFC). The Tuvaluan government, the US government, and the governments of other Pacific islands, are parties to the South Pacific Tuna Treaty (SPTT), which entered into force in 1988. Tuvalu is also a member of the Nauru Agreement which addresses the management of tuna purse seine fishing in the tropical western Pacific. In May 2013 representatives from the United States and the Pacific Islands countries agreed to sign interim arrangement documents to extend the Multilateral Fisheries Treaty (which encompasses the South Pacific Tuna Treaty) to confirm access to the fisheries in the Western and Central Pacific for US tuna boats for 18 months. Tuvalu and the other members of the Pacific Island Forum Fisheries Agency (FFA) and the United States have settled a tuna fishing deal for 2015; a longer term deal will be negotiated. The treaty is an extension of the Nauru Agreement and provides for US flagged purse seine vessels to fish 8,300 days in the region in return for a payment of US$90 million made up by tuna fishing industry and US-Government contributions. In 2015 Tuvalu has refused to sell fishing days to certain nations and fleets that have blocked Tuvaluan initiatives to develop and sustain their own fishery.

Now answer this question: Of what did Tuvalu agree to the extension ?
Multilateral Fisheries Treaty