Low Survivals and Rapid Demographic Decline of a Threatened Estuarine Delphinid

Abstract

Beibu Gulf’s (BBG) Indo-Pacific humpback dolphins present both a genetic differentiation and phenotypical differences from conspecifics from other areas of the South China Sea. Given the recent urbanization and industrialization in southern China, humpback dolphins from the BBG warrant conservation attention. However, this population’s demographic trend is unclear, making it hard to take conservation measures. To assess the population status of humpback dolphins in the BBG, photo-identification surveys were conducted between 2015 and 2019 in the inshore region surrounding the Dafeng River Estuary, which represents the most urbanized and industrialized coastal area of the BBG region. Robust design modeling suggested a constant survival for the female adults (0.89, 95% CI: 0.83–0.94). In comparison, the survival of the juvenile and sex-undetermined adults dropped from 0.92 (95% CI: 0.75–0.98) in 2015 to 0.86 (95% CI: 0.71–0.94) in 2016 and bounced back to 0.89 (95% CI: 0.80–0.94) in 2018. The low level of survival may justify the rapid decline in the annual population size from 156 (95% CI: 133–184) in 2015 to 102 (95% CI: 98–107) in 2019. We found little impact of emigration on the dolphin demographic process. Instead, the low and fluctuating survivals, although with overlapping confidence intervals, seemingly suggested a presence of strong marine stressor(s). Our study highlighted that obtaining high-resolution data is essential to improving our understanding of the demographic dynamics. Moreover, the anthropogenic stress in the BBG region should be quantitatively studied in both temporal and spatial perspectives, to help depict the ecological response of the dolphins to anthropogenic activities.

First live sighting of an Indo-Pacific humpback dolphin (Sousa chinensis) in the Yellow Sea, the northern-most record of the species range

“…In addition, we failed to match this individual to the existing photo-identification catalogues of populations from the Xiamen Bay, Lingding Bay, Leizhou Peninsula, Sanniang Cove and west Hainan populations (Wang et al ., 2016; Tang et al ., 2021). Therefore, this individual was unlikely a migrant from one of the currently known populations from the south China waters (Lin et al ., 2022). On the other hand, the presently reported sighting was very close (<50 km) to the location of the by-catch reported in 2003 (Figure 1B, Han et al ., 2004).…”

“…In regard to the information gathered on these animals' dispersing distance, we suggest that this individual may not come from one of the known populations of the East China Sea or the South China Sea, but from a previously undescribed population in the Yellow Sea. The putative population may be one of the relict populations of species surviving the Last Glacial period (Lin et al, 2010), which were generally very small in their population sizes (as the one in the Ningde waters which is <20 individuals) and highly susceptible to human development (Lin et al, 2016(Lin et al, , 2022). An area, the Yalu River mouth, which is the largest river mouth and most likely the least disturbed area of the region, could be the most suitable niche of the species.…”

“…In Chinese waters, the spatial distribution of Indo-Pacific humpback dolphins is patchy. Several distinct populations have been described in the South- and East China Sea, including one in the Dafeng River estuary off Guangxi province (Chen et al ., 2016; Lin et al ., 2022), one large population extending from the Pearl River Estuary (Lingding Bay, Jefferson, 2000) to Jiangmen waters (Li et al ., 2019), with a linear distance of over 180 km (which was called Pearl River Delta region according to Karczmarski et al ., 2016) off mid-Guangdong province, one population off the east coast of Leizhou Peninsula (including both Leizhou Bay, Guangzhou Bay and the connecting waters, Xu et al ., 2015), two populations at both sides of Taiwan Strait (Xiamen Bay and the western Taiwan Coast, Wang et al ., 2012 a ; Zeng et al ., 2020), and one off the west side of Hainan Island (Li et al ., 2016). Small populations (<20 individuals) were also reported off Shantou city (Wang et al ., 2012 b ) and Ningde city (Chen et al ., 2012).…”

Population distribution, connectivity and differentiation of Indo‐Pacific humpback dolphins in Chinese waters: Key baselines for improving conservation management

“…Cross‐comparisons found no matching individuals in any pair of regions (Figure 2b). The survey effort in the PRE and SNB (2,458 and 158 identified individuals, respectively) was sufficient to capture the majority of the population in each of these two regions (Chen et al, 2010a; Chen et al, 2011b; Chen et al, 2016; Peng et al, 2020; Lin et al, 2022b). In contrast, the numbers of identified individuals in the SWH (169) and EZJ (373) were not considered fully representative of the population sizes (Xu et al, 2012; Xu et al, 2015; Liu et al, 2022) because the discovery curves depicting the cumulative number of identified individuals were still rising (Tang et al, 2021).…”

Season, age, sex, and location impact the density of tooth rake mark and dorsal fin notch of Indo‐Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

“…Since the skin color pattern of IPHDs is associated with age (Jefferson, 2000;Jefferson, et al, 2012), photo-identification analyses provide information on population age-class structure. In contrast, the sex of the dolphins is difficult to obtain, and several studies that attempted to describe social structure, and estimate reproductive success or survival of IPHDs highlighted the need to determine the sex of more individuals (e.g., Dungan et al, 2012Dungan et al, , 2016Guo et al, 2020;Lin et al, 2022;Zeng et al, 2021). The possibility to determine the sex of these animals, using photographs, has been tested by Wright et al (2017), with the study being focused on healing time and sex differences in two populations.…”