The study analyzed the data from six South China Sea deep-diving/remote-sea cetacean research cruises carried out from 2019 to 2023, providing key information on the distribution, species composition, relative abundance, social behavior and habitat characteristics of deep-diving/remote-sea cetaceans in the South China Sea, and preliminarily revealed the biogeographic pattern of cetaceans in the South China Sea. The results of the study indicate that the sea area around Xisha and Zhongsha Islands is an “Important Marine Mammal Area (IMMA)”. The research results provide important scientific support for the spatial zoning and protection management of the regional marine ecosystem, and lay a scientific foundation for the protection of the flagship cetacean-based marine mammal area in the South China Sea and the formulation of specific initiatives for the protection of the marine ecosystem in the South China Sea.

Through the integrated and comprehensive systematic survey, the marine mammal research team found a large number of cetaceans in the northern waters of the South China Sea; and also found that the Xisha and Zhongsha Islands and their adjacent waters along the continental slope of Qiongdongnan, and the Xisha Trough highly satisfy the criteria for becoming an IMMA. The following characteristics meet the IMMA criteria for marine mammal endangerment, key life-history activities, and diversity: (1) sperm whales with an IUCN Red List rating of “Vulnerable” (VU) inhabit these waters; (2) at least a dozen cetacean species including sperm whales breed and raise their young; and (3) the area is home to a number of cetacean species. nursery behavior; and (3) field surveys in the region have documented at least 17 species, which is well above the diversity level of the vast majority of existing IMMAs of ≥5 marine mammal species. In terms of the number of marine mammal species, the northern part of China's South China Sea is comparable to many existing well-known IMMAs, such as the Bururao and East Kutai Zone IMMAs in Indonesia (25 species), the Hikurangi Trench IMMA in New Zealand (22 species), the South Cook Islands IMMA (15 species), and the Great Pemba Channel IMMA (13 species).

Despite the important value of marine mammal conservation-research-management in the Xisha and Zhongsha Seas, there is currently a lack of compatible spatial zoning for marine conservation in this area. Therefore, in the near future, it is recommended that the flagship cetacean be the main target for protection, and that its key habitats, such as the Xisha-Zhongsha IMMA, be protected, so as to systematically and comprehensively conserve the marine ecosystem and marine biodiversity of the South China Sea.

A variety of deep-diving/remote-sea cetacean species recorded on the South China Sea cetacean research cruises