Over 17 mammals, 198 types of birds, 8 types of amphibians, 8 types of reptiles, and 947 insect species have been catalogued in the nature reserve. Endangered species include the "Capreolus capreolus pygargus" and "Felis bengalensis Manchuria", and a resident population of Indo-Pacific bottlenose dolphins and finless porpoises. Historically, the island and adjacent waters had been migration colliders and resting areas for large whales such as western gray whales, North Pacific right whales, humpback whales, blue whales and fin whales. Now possibly extinct Japanese sea lions might have colonized on the island as well. Some pinnipeds still occur occasionally. Since the island was last connected to the Korean Peninsula 10,000 years ago, animals endemic to the island appeared at that time and this separation from the mainland is also of biological significance.