Studies On The Spatial Use,Community Dynamics,and Reproductive Ecology Of Indo-Pacific Humpback Dolphins(Sousa Chinensis) In Jiangmen Waters

The Indo-Pacific humpback dolphin(Sousa chinensis;hereafter referred as "humpback dolphin")is a species of small toothed whales distribute in estuarian and coastal waters from eastern Indian Ocean to western Pacific Ocean.Since its habitat overlaps with the area of frequent human activities,this species is greatly influenced by anthropogenic factors,and it is now listed as "Vulnerable" species in the International Union for Conservation of Nature(IUCN)Red List of Threatened Species.The humpback dolphin was distributed continuously along the southeastern coast of China several decades ago,however,the current distribution of this species is fragmented.The largest humpback dolphin population of the world is in the Pearl River Estuary(PRE)-Moyang River Estuary(MRE)waters.Its eastern.boundary is in Hong Kong waters,and its westernmost boundary extends to the waters of Hailing Island,Yangjiang,Guangdong Province.Due to the integrated influence of political,economic,cultural,scientific,and other factors,the humpback dolphins living in the east PRE are being more concerned by researchers,the government,and the public than the humpback dolphins living in the west of the PRE,and the scientific researches focusing on the latter are few.Jiangmen waters is in the west of the PRE,and it is the joint of the Pearl River system and the Moyang River system.The humpback dolphins living in this area may play an important role in connecting the geographic populations of the two estuaries.Unfortunately,the current situation of spatial use,population dynamics and reproductive ecology of humpback dolphins in this area has not been studied clearly.In addition,the Guangdong Jiangmen Chinese White Dolphin Provincial Nature Reserve(GJCWDPNR)is right in the east of Jiangmen waters and the west of Huangmao Sea(WHMS).Only by fully understanding the population ecology characteristics of the humpback dolphins living in and around the nature reserve,can we better formulate region-targeted conservation and management strategies for the humpback dolphins.In order to fill the above research gaps,the present study re-analyzes the data collected during boat-based photographic sampling surveys from 2013 to 2019,and establish the individual photo-identification(photo-ID)database of the Jiangmen humpback dolphins by using the image data management software DISCOVERY.Based on the database,the spatial use,population dynamics and reproductive ecology of the Jiangmen humpback dolphins are studied.Firstly,the study analyzes the spatial distribution characteristics of the Jiangmen humpback dolphins and their preference for specific environmental factors by using various analysis methods based on geographic information technology,and then,the study focuses on identifying the habitat and home range of the humpback dolphin community living in the WHMS;Secondly,the study continues to focus on studying the WHMS community,and estimates its size and survival rate in different seasons using the mark-recapture modeling;Then,the study estimates the main reproductive parameters of the WHMS community;Finally,the study discusses the anthropogenic activities and their negative impacts on the WHMS humpback dolphins during the study period.Based on the main results of the present study,a series of region-targeted conservation and management strategies are suggested.The main results and conclusions of the present study are as follows:1.A new individual photo-ID database of the Jiangmen humpback dolphins is established based on the photo-ID data collected from 2013 to 2019 by using the image data management software.The database contains 573 identifiable(D?1)humpback dolphins,including 477 subadults and adults,and 96 juvenile individuals.In the database,there are 471 individuals belonging to the WHMS community,and 394 are highly recognizable(D?3)subadults and adults.The most frequently sighted individual is resighted 42 times,and 3 17 individuals are sighted more than once.The cumulative discovery curve has not reached the platform stage,which reflects that the Jiangmen humpback dolphins belong to an open population,and individual recruitment and immigration will affect the curve reaching the platform stage.2.In Jiangmen waters,humpback dolphins are mainly found in the area with water depth of 5-15 m,and their offshore distance are no more than 5 km.Their preference for tide is not obvious,but their offshore distance will decrease when the tide rises.The spatial heterogeneity of humpback dolphins in Jiangmen waters may be caused by the heterogeneity of food resources and human activities.The 95%Fixed Kernel(FK)habitat area(235.00 km2)of the WHMS community is large,but its 50%FK habitat area(31.67 km2)is small,which indicates that the WHMS community distribute widely,but has a strong site fidelity.The nursing habitat is smaller than that of the community's total habitat,indicating that female individuals have even higher site fidelity,and it is very likely to be influenced by the nursing behavior.The habitat area of the WHMS community is very small in the wet season(95%FK:186.24 km2,50%FK:18.88 km2),and it is close to the harbor basin of the Taishan Thermal Power Plant In the dry season,the habitat area is relatively large(95%FK:310.51 km2,50%FK:79.83 km2),and it spreads outside to the surrounding waters,as well as to the estuary.The seasonal change of the habitat range is probably related to the seasonal fishery resources distribution.The community's habitat area fluctuates annually(96.02-291.21 km2).Since 2017,its annual habitat area has been increasing,which may be related to the impact of human activities,but may also be related to the use of other areas by new immigrants in the community.Different individuals in the community have different home range areas,but most individuals have small home range areas.Only 17.5%of individuals have a home range area of more than 60 km2,and the average individual home range area is 42.99±SD 22.05 km2.In addition,even for individuals with severe dorsal fin injuries,their home range can be very wide,indicating that dorsal fin injuries do not necessarily hinder the individual's ability to move,thus affecting their distribution.At present,the overlap area between the GJCWDPNR and the humpback dolphin's habitat is relatively small,and the dolphin sighting density in the south of the nature reserve is very low.3.The WHMS humpback dolphin community is an open and mobile community.A large number of individuals in the community have strong site fidelity.In the wet season,the number of transients will increase,accounting for 12.8%in the community;but in the dry season,the proportion of transients in the community is only 3.4%,and so the WHMS community in the dry season is basically composed of residents.The apparent survival rate of the community is 0.991±SE 0.001 during the dry-wet season interval,and 0.981±SE 0.005 during the wet-dry season interval.Considering that the emigration of transients will influence more on the apparent survival rate of the community during the wet-dry season interval,the apparent survival rate during the dry-wet season interval is closer to the real survival rate.However,the survival rate estimate only represents the survival rate of subadults and adults in the community.As the survival rate of calves is much lower than that of subadults and adults,the real survival rate of the whole community should be lower than the current result.A total of 496 individuals(CV=0.130,95%CI=385-640)entered the WHMS "super-population" during the survey period.From 2016 to 2018,the community size estimates are listed below:2016:336(CV=0.161,95%CI=245-460),2017:363(CV=0.095,95%CI=301-438),2018:409(CV=0.102,95%CI=335-500).4.The reproduction of humpback dolphins in the WHMS is obviously seasonal.Nursing event occurs all year around,but the calves are mainly born from April to September.The inter-birth interval(IBI)of dolphin mother is 3.50±SD 0.96.Mother who is old,or mother who spends long time to nurse will have a long IBI,while the death of calf will shorten the IBI.The crude birth rate is 0.057±SD 0.010,and the calf survival rate of 1 year old is 0.879±SD 0.070.Mother fecundity is 0.162±SD 0.022,and it remains stable among years.The main factors affecting the survival rate of humpback dolphin calves in the WHMS are the age of mother,the nursing investment of mother,intraspecific relationship,environmental factors,and the negative influence from human activities.Among them,the cooperative nursing as a positive intraspecific relationship,is very common in the WHMS community.Cooperative nursing can strengthen the relationship between female individuals,improve the nursing efficiency of mothers,and then improve the calf survival rate in the community.5.During the survey period,the humpback dolphins are suffering from anthropogenic disturbance which is mainly from the following aspects:(1)The disturbance and occupation of dolphin habitat caused by coastal development,especially land reclamation,and port and bridge construction projects;(2)The disturbance on dolphin activities caused by heavy traffic;(3)The occupation of dolphin habitat caused by oyster aquaculture activities;(4)The disturbance and potential harm on dolphins caused by frequent fishing activities.Based on the above study results,the following conservation and management strategies are suggested:(1)The development and construction of sea-related projects should be cautiously carried out in the WHMS,especially in the humpback dolphin habitat;(2)The current GJCWDPNR should be appropriately optimized and adjusted based on the habitat use characteristics of the WHMS humpback dolphins,all the development activities in the reserve should be prohibited,and supervision and management should be enhanced.(3)An ecological corridor should be set up between the east and the west of the PRE as a migration channel for humpback dolphins to avoid habitat fragmentation;(4)To reduce fishery and aquaculture activities in the WHMS,dolphin-watching ecotourism can be carried out;(5)Science popularization and education activities should be continued,and on the premise of maintaining high-quality knowledge output,increase the influence.The WHMS humpback dolphin community in Jiangmen waters is an open and mobile community.The survival rate of subadults and adults in this community is high,the fecundity rate of the females is stable,and the community size may have an increasing trend in the future.However,it should be noted that the overlap between the GJCWDPNR and the WHMS humpback dolphin's habitat is too small,and some anthropogenic activities in the WHMS are influencing the community.Therefore,the optimization and adjustment of the GJCWDPNR should be concerned,and the conservation and management of the GJCWDPNR and its surrounding areas should be enhanced.