Tuna are predators. They herd smaller fish to the surface, where they can be picked off one by one. They are high-speed fish - the cheetahs of the ocean. They're also the wildebeest - they herd together in their thousands and undertake epic migrations across the Pacific in search of their prey. If these animals lived on land, they'd be famous for providing the greatest wildlife spectacle on Earth. Instead, they're better known as a filling for a sandwich. In an ocean with no marine reserves, migratory fish have nowhere to hide. Up to two kilometres long and 200 metres deep, ''purse seine'' nets are designed to encircle schools of tuna. A fisherman checks his nets, breathing air pumped down a tube from the vessel above. It's not only tuna that get caught in these nets. A lone turtle was in the wrong place at the wrong time. She can only hold her breath for a few minutes and the path to the surface isn't clear. She begins to panic.


金枪鱼是掠食性鱼类。它们将小鱼赶到海面。在那里把小鱼逐一吞食。它们游动迅速－是大海里的猎豹。它们也是牛羚－常常成千上万地聚集，并为寻找猎物在太平洋各处进行史诗般的迁移。如果生活在陆地上，它们将会因为地球提供最壮丽的野生动物奇观而闻名。然而，现在人们更多地认识它们是作为三明治的填充物。在没有海洋保护区的大海中，洄游鱼类无处藏身。长2公里深200米的大型围网被专门设计用来围捕金枪鱼。一个渔民检查他的网，呼吸的空气由一根管子从上方的船泵压下来。这些网捕捉到的不只是金枪鱼，一只孤独的海龟在错误的时间来到了错误的地方。它只能屏息几分钟。它辨不清往海面的路，它开始惊慌。