JAKARTA — The world’s seaborne trade exceeded 10 billion tons in a single year for the first time in 2015, according to the United Nations Conference on Trade and Development, with about 60% passing through Asia. Sitting between the Indian and Pacific Oceans, Southeast Asia’s big archipelagos should be well placed to capitalize as trade expands. Indonesia and the Philippines comprise about 17,000 and 7,500 islands respectively, while Indonesia, home to the world’s fourth-biggest population — about 260 million people — has the second-longest coastline after Canada. However, the bulk of this seaborne trade is moving between Europe and Asia’s powerhouse economies in China and Japan, mainly through the South China Sea and the Strait of Malacca, which lies between Malaysia and the Indonesian island of Sumatra. “The largest archipelagic countries in the world are not being optimized,” said Fauziah Zen, an economist with the Economic Research Institute for ASEAN and East Asia, at the recent launch of a report on Southeast Asia’s maritime infrastructure published by The Habibie Center, a Jakarta-based research organization.