The largest known population of Eden's whale aggregates in the Beibu Gulf, southern China

Basic information on the distribution, survival, and abundance of Eden's whale (Balaenoptera edeni edeni) is lacking. During April 2018 to April 2024, we monitored Eden's whales in the Beibu Gulf, southern China. Using characteristics of the dorsal fin left and right side, 83 and 88 whales were identified, respectively (mean 3.5 times). The POPAN and Cormack–Jolly–Seber (CJS) open models in MARK resulted in low apparent survival of .824 and .817 for left dorsal fin data set, .816 and .822 for right dorsal fin data set and lower capture probability of .522 and .427 for left, .468 and .461 for right. The POPAN model estimated 82–123 identifiable whales and 100–160 whales adjusted by mark ratio, indicating the largest reported population worldwide. The mean annual population size ranged from 26 to 58, with a yearly increasing–decreasing fluctuation. The low survival, capture probability, and annual population size, and increasing discovery curve indicate 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.