Here's what you need to know about the fish and their role in the dispute:

Koi are beautifully hued and expensive carp formally called nishikigoi in Japan. The fish, appreciated as “swimming jewels,” represent good luck in life and business. They're often fixtures of garden ponds for wealthy and influential families in Japan. In recent years, koi have become hugely popular in Asia, with Japan's koi exports doubling over the past decade to 6.3 billion yen ($43 million) — one-fifth of them shipped to China, the top Japanese koi importer, followed by the United States and Indonesia.