Abstract:

Aims: Despite the rich marine mammal resources in the Beibu Gulf, comprehensive information on their species composition and distribution remains lacking.

Methods: Our study integrates historical records, literature, monographs, as well as verified stranding, field sighting, and bycatch data to systematically review the species composition and distribution patterns of marine mammals in this region.

Review Results: The updated checklist identified 26 marine mammal species, belonging to 3 orders, 8 families, and 19 genera, including resident species such as the Indo-Pacific humpback dolphin (Sousa chinensis) and the Indo-Pacific finless porpoise (Neophocaena phocaenoides), seasonal residents such as the Eden’s whale (Balaenoptera edeni) and the common dolphin (Delphinus delphis), visitor species such as the spinner dolphin (Stenella longirostris), and vagrant species such as the spotted seal (Phoca largha). Marine mammal species diversity in the Beibu Gulf follows a north-high-south-low gradient. Specifically, the coastal waters of Beihai, Guangxi represent a high-density area, while the nearshore waters of Fangchenggang and Qinzhou constitute medium-density distribution zones. According to historical sources, marine mammals such as the dugong formerly occupied a much broader range in the Beibu Gulf compared to their current distribution. Modern records can be divided into two phases: 1949-2000 and 2000-2025. A comparison between these phases shows that the number of recorded marine mammal species in the Beibu Gulf increased from 21 to 22. Five new species were added, including the bearded seal (Erignathus barbatus) and Omura’s whale (Balaenoptera omurai), while four species, such as the gray whale (Eschrichtius robustus) and the sei whale (Balaenoptera borealis), have not been recorded since 2000. Among the 26 marine mammal species, 10 were classified as national grade I protected wildlife, and the remaining 16 were classified as grade II. Of the total species, five (19.23%) were IUCN-listed as threatened, comprising one Endangered (EN, 3.85%) and four Vulnerable (VU, 15.38%) species. A sharp reduction in seagrass beds has led to the corresponding disappearance of the dugong from its historical habitat in Hepu, Beihai. The Indo-Pacific humpback dolphin is primarily distributed in the Dafengjiang-Nanliujiang River estuary, the Shatian (Guangxi)-Caotan (Guangdong) coastal waters, and southwestern Hainan. However, there is no consensus on its total population size. Eden’s whales aggregate around Weizhou and Xieyang Islands from December to April, and over 80 individuals have been documented there. The Indo-Pacific finless porpoise is widely distributed throughout the coastal waters of the Beibu Gulf, with a large population likely exceeding 1,000 individuals.

Conclusion: It is recommended to strengthen baseline surveys of marine mammals, optimize the monitoring system, scientifically regulate coastal engineering projects, promote modified fishing gear and acoustic deterrent devices, establish dynamic marine protected areas, strengthen ecological restoration, implement responsible whale/dolphin- watching guidelines, and mitigate the impacts of human activities.

Key words: Beibu Gulf, marine mammals, species composition, geographical distribution, conservation strategies