A pioneering survey of deep-diving and off-shore cetaceans in the northern South China Sea

A pioneering boat-based survey was conducted in 2019,to gather baseline information regarding the presence,composition,relative abundance,and spatial distribution of deep-diving and off-shore cetaceans in the northern South China Sea(SCS).A total of 27 sightings comprising at least 8 cetacean species were recorded during the 13-day survey,including 5 deep-diving species(i.e.Risso’s dolphin[Grampus griseus],short-finned pilot whale[Globicephala macrorhynchus],sperm whale[Physeter macrocephalus],Cuvier’s beaked whale[Ziphius cavirostris],and an unidentified beaked whale[either the ginkgo-toothed beaked whale,Mesoplodon ginkgodens,or Deraniyagala’s beaked whale,Mesoplodon hotaula]),as well as 3 off-shore dolphins(i.e.pantropical spotted dolphin[Stenella attenuate],striped dolphin[Stenella coeruleoalba],and Fraser’s dolphin[Lagenodelphis hosei]).With the exception of pantropical spotted dolphins,all other species were sighted and recorded at sea in the northern SCS for the first time.The pantropical spotted dolphin was the most frequently sighted species,comprising 30%of the total sightings.Deep-diving cetaceans were mainly sighted in the northern Xisha Archipelago,whereas off-shore dolphins were distributed across the survey area.The pantropical spotted dolphin was observed in aggregations of more than 100 individuals and nearly all encountered species included calves;these findings suggested that the survey area functions as an important feeding and calving ground for various cetacean species.This pioneering survey provides fundamental information regarding cetacean fauna in the northern SCS and highlights the need to strengthen research and conservation efforts concerning these species.

First live sighting of Deraniyagala’s beaked whale(Mesoplodon hotaula)or ginkgo-toothed beaked whale(Mesoplodon ginkgodens)in the western Pacific(South China Sea)with preliminary data on coloration,natural markings,and surfacing patterns

Beaked whales represent around 25%of known extant cetacean species,yet they are the least known of all marine mammals.Identification of many Mesoplodon species has relied on examination of a few stranded individuals.Particularly,the ginkgo-toothed beaked whale(Mesoplodon ginkgodens)and Deraniyagala’s beaked whale(Mesoplodon hotaula)are among the least-known of beaked whale species,without confirmed sightings of living individuals to date.We present a sighting of 3 free-ranging individuals of M.ginkgodens/hotaula whale from a dedicated marine mammal vessel survey carried out in the South China Sea in April and May 2019.Photographic data(301 photographs)from the sighting were compared to photos of fresh stranded ginkgo-toothed beaked whale and Deraniyagala’s beaked whale from both historical and unpublished records.We found that free-ranging M.ginkgodens and M.hotaula individuals can be easily distinguished from other Mesoplodon species due to differences in melon and gape shapes and coloration patterns.However,accurate at-sea differentiation of M.ginkgodens and M.hotaula may not be possible due to high similarity in both coloration and scarring patterns.In addition to our photo-identification data,we collected what we believe to be the first preliminary descriptions of surfacing behavior and diving patterns of one of these species.Finally,the presence of scars possibly caused by fishing gear or marine litter raises concerns about anthropogenic impacts and conservation of these poorly known species.

Diel differences in blue whale(Balaenoptera musculus)dive behavior increase nighttime risk of ship strikes in northern Chilean Patagonia

The northern Chilean Patagonia region is a key feeding ground and a nursing habitat in the southern hemisphere for blue whales(Balaenoptera musculus).From 2014 to 2019,during 6 separate research cruises,the dive behavior of 28 individual blue whales was investigated using bio-logging tags(DTAGs),generating≈190 h of data.Whales dove to significantly greater depths during the day compared to nighttime(day:32.6±18.7 m;night:6.2±2.7 m;P<0.01).During the night,most time was spent close to the surface(86%±9.4%;P<0.01)and at depths of less than 12 m.From 2016 to 2019,active acoustics(scientific echosounders)were used to record prey(euphausiids)density and distribution simultaneously with whale diving data.Tagged whales appeared to perform dives relative to the vertical migration of prey during the day.The association between diurnal prey migration and shallow nighttime dive behavior suggests that blue whales are at increased risk of ship collisions during periods of darkness since the estimated maximum ship draft of vessels operating in the region is also≈12 m.In recent decades,northern Chilean Patagonia has seen a large increase in marine traffic due to a boom in salmon aquaculture and the passenger ship industry.Vessel strike risks for large whales are likely underestimated in this region.Results reported in this study may be valuable for policy and mitigation decisions regarding conservation of the endangered blue whale.

Temporal variation of the underwater soundscape in Jiaotou Bay,an Indo-Pacific humpback dolphin(Sousa chinensis)habitat off Hainan Island,China

The underwater soundscape is an important ecological element affecting numerous aquatic animals,in particular dolphins,which must identify salient cues from ambient ocean noise.In this study,temporal variations in the soundscape of Jiaotou Bay were monitored from February 2016 to January 2017,where a population of Indo-Pacific humpback dolphins(Sousa chinensis)has recently been a regular sighting.An autonomous acoustic recorder was deployed in shallow waters,and 1/3-octave band sound pressure levels(SPLs)were calculated with central frequencies ranging from 25 Hz to 40 kHz,then were grouped into 3 subdivided bands via cluster analysis.SPLs at each major band showed significant differences on a diel,fishing-related period,seasonal,and tidal phase scale.Anthropogenic noise generated by passing ships and underwater explosions were recorded in the study area.The fish and dolphin acoustic activities both exhibited diel and seasonal variations,but no tidal cycle patterns.A negative significant relationship between anthropogenic sound detection rates and dolphin detection rates were observed,and fish detection rates showed no effect on dolphin detection rates,indicating anthropogenic activity avoidance and no forced foraging in dolphins in the study area.The results provide fundamental insight into the acoustic dynamics of an important Indo-Pacific humpback dolphin habitat within a coastal area affected by a rapid increase in human activity,and demonstrate the need to protect animal habitat from anthropogenic noises.