Sperm whales (Physeter macrocephalus) in the northern South China Sea: Evidence of a nursing ground?

Sperm whales (Physeter macrocephalus) are widely distributed in deep waters worldwide, yet relatively little is known about this species in the South China Sea (SCS). Here, an integrated approach was used to investigate sperm whales in this region. First, we compiled records of 28 strandings, two bycatch events, and 63 opportunistic sightings of sperm whales in the SCS, and these historical records indicated that sperm whales have occurred occasionally in the SCS. Second, we conducted four ship-based surveys from 2019 to 2021 to investigate cetaceans in the northern SCS. During these surveys, we recorded 125 cetacean sightings, including nine sperm whale sightings, with a group size of 5.1 ± 2.3 individuals (mean ± SD). Of nine sperm whale sightings, seven were determined to be nursery groups, with 1–2 calves/juveniles per group. This finding suggests that the northern SCS might be a nursing ground for sperm whales. Twenty-two individuals were photographically identified from six groups with no resightings, indicating that there are probably more whales within the investigated area that have not been identified. Third, we tagged three adult sperm whales from a nursery group with LIMPET satellite tags on July 24, 2020. Satellite-tracking data showed that these whales occupied a relatively small area (≤2200 km2) within <10 days, with an estimated travel distance of ≤400 km for each individual. To conclude, this is the first study to document the frequent existence and probable residence of sperm whales in the northern SCS. Considering that sperm whales in this area may represent only a small portion of the entire SCS population, additional field surveys and biologging efforts are required to investigate sperm whales in the SCS and help reveal the broad distribution, movement patterns, and habitat preferences of this species in the region.