Complex sentence: Pacific bluefin tuna spawn in the Western Pacific between Okinawa and the Philippines and probably the Sea of Japan, then migrate over 6,000 nautical miles (11,000 km) to the Eastern Pacific, and eventually return to their birth waters to spawn again.
Split into simpler sentences: Pacific bluefin tuna spawn in the Western Pacific between Okinawa and the Philippines and probably the Sea of Japan. They migrate over 6,000 nautical miles (11,000 km) to the Eastern Pacific, eventually returning to their birth waters to spawn again.