Funafuti, providing training to approximately 120 marine cadets each year so that they have the skills necessary for employment as seafarers on merchant shipping. The Tuvalu Overseas Seamen's Union (TOSU) is the only registered trade union in Tuvalu. It represents workers on foreign ships. The Asian Development Bank (ADB) estimates that 800 Tuvaluan men are trained, certified and active as seafarers. The ADB estimates that, at any one time, about 15% of the adult male population works abroad as seafarers. Job opportunities also exist as observers on tuna boats where the role is to monitor compliance with the boat's tuna fishing licence. Government revenues largely come from sales of fishing licences, income from the Tuvalu Trust Fund, and from the lease of its ".tv" Internet Top Level Domain (TLD). Tuvalu began deriving revenue from the commercialisation of its ".tv" Internet domain name, which was managed by Verisign until 2021. Tuvalu also generates income from postage stamps by the Tuvalu Philatelic Bureau, and from the Tuvalu Ship Registry. The Tuvalu Trust Fund was established in 1987 by the United Kingdom, Australia, and New Zealand. The value of the Tuvalu Trust Fund is approximately $100 million. Financial support to Tuvalu is also provided by Japan, South Korea and the European Union. Australia and New Zealand continue to contribute capital to the Tuvalu Trust Fund, and provide other forms of development assistance. The U.S. government is also a major revenue source for Tuvalu. In 1999, the payment from the South Pacific Tuna Treaty (SPTT) was about $9 million, with the value increasing in the following years. 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) for 18 months. The United Nations designates Tuvalu as a least developed country (LDC) because of its limited potential for economic development, absence of exploitable resources and its small size and vulnerability to external economic and environmental shocks. Tuvalu participates in the Enhanced Integrated Framework for Trade-Related Technical Assistance to Least Developed Countries (EIF), which was established in October 1997 under the auspices of the World Trade Organisation. In 2013, Tuvalu deferred its graduation from least developed country (LDC) status to a developing country to 2015. Prime Minister Enele Sopoaga said that this deferral was necessary to maintain access by Tuvalu to the funds provided