893 / 2024-09-19 17:43:40

The World’s Largest Known Population of Eden’s Whales in the Beibu Gulf, southern China

Bryde's whale, Beibu Gulf, Populaiton size, age structure, whale-watching

Bryde’s whale (Balaenoptera edeni) can be seen along the coastlines of the Indian and western Pacific Oceans. After much taxonomic debate, Eden’s whale (B. e. edeni) and the offshore Bryde’s whale (B. e. brydei) are currently recognized as two provisional subspecies. In 2018, Bryde’s whales appeared seasonally around Weizhou Island and Xieyang Island in Guangxi, Beibu Gulf, China, mainly during November-April after then. We identified these whales as Eden’s whale by using molecular technology. Eden’s whale subspecies is the least known baleen whale, which is comprised of a number of small inshore populations, currently only two Eden’s whale populations, those inhabiting the Gulf of Thailand (63 ± 8) and off Japan (53), have been systematically estimated.

From 2018 to 2024, six years of dedicated study have revealed a plethora of information about the behavior and ecology of the species such as population size, age structure, and distribution pattern. The vessel-based surveys were conducted for 292 days, Eden’s whales were found for 353 times.

Abundant feeding behaviors were identified, i.e. two non-lunging (tread-water and/or trap-feeding, 7.7%, and a novel behavior we call “pirouette feeding”, 15.3%) and six lunging (right lateral lunging, RLL, 10.9%; left lateral lunging, 0.2%; upright lunging, 6.3%, vertical lunging, 18.5%; inverted lunging, 0.3%; cooperative lateral lunging, COL, 40.9%). COL is the first cooperative feeding behavior recorded for Genus Balaenoptera. Five lateralized feeding behaviors accounted for 65.8% of individual feeding events, with a strong right-side bias (99.7%) compared to the left (0.3%). Additionally, bubble trail were found for eight times, four events were performed by a solitary individual, whereas the other four were by pairs of whales. This is the first direct evidence of bubble trails produced by Eden’s whales worldwide.

Using characteristics of the dorsal fin left and right side, 83 and 88 whales were identified, respectively. The CJS open models in MARK resulted in low apparent survival of 0.817 and 0.822 for left and right dorsal fin data set, respectively. The POPAN model estimated 83-123 identifiable whales, indicating the largest reported population worldwide. The low survival, capture probability, and annual small population size, and increasing discovery curve indicates that the population dynamics are complex, possibly reflecting wider movement beyond the study area. We speculate that some individuals were residents in the Beibu Gulf, while most whales were transients that migrate yearly between the Beibu Gulf and the East China Sea, South China Sea or/and Southeast Asia.