Habitat decline of the largest known Indo-Pacific humpback dolphin (Sousa chinensis) population in poorly protected areas associated with the hypoxic zone

Abstract

Reserve planning based on the investigation of the core habitat distribution of wild cetaceans is a powerful conservation strategy for protecting target species. However, studies on core habitats and their variations at the large-scale distribution of cetaceans are limited. In this study, we conducted a seven years (2015-2021) boat-based field observation surveys with highly applicable and generalized methods to analyze the habitat changes and how these changes influenced the largest known Indo-Pacific humpback dolphin (Sousa chinensis) population in the Pearl River Estuary (PRE), China. Our findings revealed that there is 3204 km2 of dolphin habitat in the PRE, and dolphin distribution preferences are related to nutrients and biological resources. The decreasing trend of dolphin sighting rates in most areas (73.2%) indicated that the dolphin habitat had declined, and the annual habitat decline rate (2.83%; 95% confidence interval: 0.58%-5.08%) was consistent with the population decline rate. The annual habitat decline rate in the hypoxic zone revealed that low dolphin sighting rates from March to October in the hypoxic zone was 0.43% faster than that in the non-hypoxic zone, indicating long-term sustained effects of summer hypoxia on the dolphin population. Existing reserves have partially prevented habitat decline. However, more than 82.3% of the habitat has not been effectively protected and encounters the threats of pollution and hypoxic stress. This study offers new scientific evidence for developing effective monitoring strategies for humpback dolphin populations in the PRE, and also help establish spatial planning and management measures for reserve habitats.

Endocrine-Disrupting Chemical Exposure Induces Adverse Effects on the Population Dynamics of the Indo-Pacific Humpback Dolphin

“…However, it is imperative to acknowledge that this trend indirectly signifies the decline in the population and recruitment rate, suggesting an escalating decline rate . Annual sighting rates during 2015–2021 continue to decrease at a rate of 2.83%, roughly in line with the suggested rate of population decline . Collectively, these pieces of evidence strongly support the deduction that the PRE population is declining and may be at the risk of extinction.…”

“…28 Annual sighting rates during 2015−2021 continue to decrease at a rate of 2.83%, roughly in line with the suggested rate of population decline. 14 Collectively, these pieces of evidence strongly support the deduction that the PRE population is declining and may be at the risk of extinction.…”

“…The Indo-Pacific humpback dolphins (Sousa chinensis) hold a crucial position among the coastal cetaceans in the South China Sea. Their habitat preference for these coastal habitats distinguishes them from other cetacean species and underscores their ecological significance, predominantly in shallow coastal waters and estuaries . Empirical studies indicate that the Pearl River Estuary (PRE) population, which constitutes the largest population in the world, may meet the criteria for being classified as Endangered or Critically Endangered, − despite its current designation as Vulnerable on the IUCN Red List.…”

Population parameters and heterogeneity in survival rates of Indo-Pacific humpback dolphins in a heavily urbanized coastal region of southeast China: implications for conservation

“…With time, however, it became increasingly evident that further studies of population processes of the PRD dolphins were much needed and at spatio-analytical scales that could provide a considerably greater data resolution than the early work could offer (Karczmarski et al, 2016). Some such studies have recently been completed and indicate a distinct socio-spatial and demographic structure of the dolphin population across the PRD (Chan, 2019;Guo et al, 2020;Chan et al, 2022a), and further fine-scale socio-spatial structure within LB where multiple clusters interact socially but remain considerably dissimilar spatially (Karczmarski et al unpublished 2 ). Various environmental factors also vary spatially in LB.…”

“…Various environmental factors also vary spatially in LB. Numerous manmade stressors differ in their type (e.g., habitat loss, pollutant load, maritime traffic, underwater noise) and intensity (e.g., extent of habitat destruction, intensity of fisheries pressure) across the anthropogenic seascape of LB (Chen et al, 2013;Wang et al, 2016;Wu et al, 2016;Yu et al, 2016;Gui et al, 2017;Pine et al, 2017;Gui et al, 2019;Guo et al, 2022;Li et al, 2022). Correspondingly, there is a considerable spatial heterogeneity in dolphin prey resources, an apparent result of heavy fishing pressure in the region (e.g., Duan et al, 2009;Lin et al, 2019;Zhou et al, 2019).…”