Tuna belong to the family Scombridae. They are large, oceanic fish and are seasonally migratory, some making trans-oceanic journeys. Bluefin tuna are the largest of the tuna species, reaching upwards of 680 kg. There are three species in each of the Pacific, Atlantic and Southern Oceans. In all oceans bluefins are known for their impressive migrations, routinely crossing ocean basins. Pacific bluefin tuna are generally smaller than their Atlantic cousins, reaching a maximum length of 3m and a maximum weight of 540kg. Not only do they have a hydrodynamic shape, their pectoral (side) fins can be retracted and, unlike other fish, their eyes are set flush to their body. Pacific bluefin tuna is capable of swimming at speeds of 12 to 18 miles per hour (20-30 km per hour) for brief periods. In the Pacific, tagging studies indicate there is only one stock with a spawning ground off southern Japan. Pacific bluefin tuna spawn between Okinawa (Japan) and the Philippines, in April and August, then migrate over 6,000 nautical miles to the eastern Pacific, eventually returning to their birth waters to spawn. They reach reproductive maturity at around 5 years and 60kg.