Species diversity and critical habitats of offshore and deep-diving cetaceans in the South China Sea

Abstract

Despite being recognized as a global marine biodiversity hotspot, the South China Sea (SCS) is a distinct “cetacean data-deficient area”. In particular, robust scientific knowledge and conservation baseline information available for offshore cetacean species in this region remain scarce. Between 2019 and 2023, we conducted six systematic ship-based surveys to investigate cetaceans occurring potentially in the offshore waters of the SCS, primarily around the Xisha-Zhongsha Islands. During the field surveys, 211 cetacean sightings comprising 187 single- and 24 mixed-species sightings were recorded. We identified 17 odontocete species, all offshore and deep-diving species. Notably, the pantropical spotted dolphin, short-finned pilot whale, and Fraser's dolphin, were recognized as the dominant species because of regular presence, commonness and high relative abundance. Based on the field sightings reported in this study and supplementary opportunistic sightings, we confirm 25 cetacean species living in the SCS. By integrating several additional data sources, such as stranding records and taxidermy/skeletal specimens, we further establish a cetacean species checklist consisting of at least 36 species. Biogeographically, the northern SCS (especially Xisha-Zhongsha waters) not only supports a remarkably high level of cetacean richness, and may also function as nursery and/or feeding grounds for the “Vulnerable” sperm whale and serve as critical habitats for many other offshore and deep-diving species. Therefore, we recommend that the Xisha-Zhongsha waters be proposed as a candidate Important Marine Mammal Area (IMMA), which has great potential to be delineated and managed for conservation.