Spatiotemporal variations in fine-scale habitat use of the world’s second largest population of humpback dolphins

We assessed habitat use by the population of Indo-Pacific humpback dolphin, Sousa chinensis, in the waters off Zhanjiang, China, by performing boat-based surveys between 2013 and 2018. In total, we gathered 253 sightings of humpback dolphins. To assess habitat use of humpback dolphins within the study area, we measured two spatial metrics at each sighting site: WDT (tide-calibrated water-depth) and DS (distance to the nearest shore). Results showed that dolphins mainly were observed in shallow (WDT: 8.46 ± 5.13 m, mean ± SD) and inshore (DS: 2.17 ± 2.14 km) waters. Their preference of inhabiting shallow waters was more apparent during the wet season (April–September) than during the dry season (October–March); however, they were encountered in waters much closer to shore during the dry season than the wet season. By weighting survey effort, our sighting density maps further confirmed that humpback dolphins changed their habitat use between wet and dry seasons. Such spatiotemporal variations in dolphin habitat use might be associated with spatiotemporal movements of their prey. Our findings provide insights into variations in fine-scale habitat use of inshore apex predators, and also assist to designate proper conservation measures for these vulnerable animals.

The biogeography of group sizes in humpback dolphins (Sousa spp.)

Humpback dolphins (Sousa spp.) are obligate shallow-water and resident species, and they typically live in fission-fusion societies composed of small-sized groups with changeable membership. However, we have scant knowledge of their behavioral ecology, starting with potential factors influencing inter-population variability of their group sizes. Here, we compiled a new global dataset of humpback dolphin group sizes based on 150 published records. Our data indicated an inter-specific consistency of group-living strategy among the 4 species in the Sousa genus, as these species preferred living in small-sized groups with a mean size of mostly no more than 10, a minimum size of single individual or small pairs, and a maximum size of several tens or ≈100. In addition, we clearly showed the geographic variations in group sizes of humpback dolphins at a global scale. We found that the geographic variations in humpback dolphin group sizes were primarily associated with the latitude, sea surface temperature, and abundance. To conclude, our findings provide insights into social dynamics and socioecological trade-offs of humpback dolphins, and help better understand how these resident animals adapted to their shallow-water habitats from the perspectives of biogeography and socioecology.