Mingyue Ouyang

Post‐release monitoring is critical for evaluating the success of rehabilitating stranded cetaceans, yet such data are scarce for many species in the South China Sea (SCS). We satellite‐tracked a rehabilitated subadult male short‐finned pilot whale ( Globicephala macrorhynchus ; named “ Haitang ”), following a live stranding on Hainan Island, China. We collected tracking locations and behavioral data over a 52‐day period and conducted a dedicated 3‐day expedition to resight Haitang at sea in the sixth week post‐release. Results confirm Haitang' s survival beyond the critical 6‐week benchmark. Its daily movement distance (6.2–145.9 km) and speed (0.6–5.9 km/h) were consistent with those of healthy, free‐ranging conspecifics. Furthermore, diving behavior including time‐at‐depth budgets, maximum dive depth (621 m), diel patterns, and thermal‐depth profiles reflected natural foraging activity and environmental adaptation. Notably, high density and spatial proximity of local conspecifics during the post‐release expedition indicate favorable conditions for social integration and long‐term survival. Movement trajectories suggest that the Qiongdongnan slope represents a critical habitat for this species in the northern SCS. This study provides the first empirical evidence of successful rehabilitation and release of short‐finned pilot whale in the SCS, supporting future stranding response and conservation initiatives.

Deraniyagala's beaked whale, Mesoplodon hotaula, was previously known from eight published strandings and no confirmed live sightings, making it one of the least‐known cetaceans. We conducted six cetacean survey cruises in the South China Sea between 2019 and 2023, and during 120 sea‐days we recorded 12 sightings of unidentified Mesoplodon. From biopsy samples and acoustic recordings, five sightings were subsequently confirmed as M. hotaula. Using photographs, we provide the first field descriptions and observations of this species. The calf/juvenile has a distinctive facial coloration and dorsal striping pattern that appear to be diagnostic among Indo‐Pacific Mesoplodon; among adult males, the dark rostrum of M. hotaula and the distinctively white‐tipped beak of ginkgo‐toothed beaked whale, M. ginkgodens, distinguish these similar‐looking, partially sympatric species. Mean group size was small (3.0, n = 5), with a preference for deep (2,156 m, n = 5), warm (30.3°C, n = 3) water. We update the known distribution of this species, present evidence of fishery interactions, and document the first reported mixed‐species Mesoplodon group: a juvenile Blainville's beaked whale M. densirostris associated with four M. hotaula. These sightings identify the South China Sea as a potentially important area for this almost completely unknown species.