Abundance And Distribution Pattern Of Chinese White Dolphin (Sousa Chinensis Osbeck,1765) In Sanniang Bay,Guangxi Province

The Indo-Pacific humpback dolphin (Sousa chinensis), also known as the Chinese white dolphin (CWD), is a small dolphin which distributes in estuarine and inshore waters of the Northeastern Indian, the coastal waters of Southeastern China and Southeast Asia. This species was listed as a Order I National Key Protected Animal in China in 1988. It is classified as Vulnerable in the Red List of the International Union for Conservation of Nature and Natural Resources (IUCN) in 2016. Five known humpback dolphin populations have been identified in Chinese waters, including the one inhabiting Sanniang Bay (SNB) in Chinese waters. However, the survey efforts on the distribution and abundance of this species are not enough in recent years in SNB. In this study, field survey was conducted in SNB adjacent waters from Jul,2010 to Feb,2015. Through our study we provide the latest data and trend of this species living in SNB as well as provide scientific basis for better management and conservation of the population and habitat of CWD. The map of distribution sites is drawn via Surfer 10.0 with the GPS data collected from the photos of CWD. Photo-identification studies are conducted to get the number of population and age composition data. The capture-recapture method is used to estimate the abundance of this population. Then, the database of individual identification of CWD in SNB is established. After that, the annual and quarterly variation of the number of identified individuals and the age composition is analyzed. At last, the difference and correlation in Minimum Recapture Interval (MinRI), Maximum Single Recapture Interval (Maxsingle) and First-identified-to-last-recapture Interval (MaxFI to Lc) among spotted sub-adult (SS), spotted adult (SA) and unspotted adult (UA) is detected respectively using the data from those individuals which can be recaptured over 5 times during the whole survey periods. The results are listed below:1. The survey suggests that dolphins in SNB are relatively concentrated in distribution in SNB waters, Guangxi Province, China. Compared with former survey results, we find that the main areas of CWD inhabited have moved southward and eastward obviously. Dolphins in SNB mainly inhabit the waters between SNB and the Dafengjiang River Estuary (DFRE). The distribution areas range from the outer edge of the mouth of SNB to the southern end of Leigongsha in the south as well as from Damiaodun Island which is on the western edge of SNB to the southern waters of Xichang Town which is on the east coast of the DFRE, with an east-west span of about 21.2 km, north-south span of about 9.6 km, and an area of about 220 km2. The core distribution areas tend to move southward to the outer waters from the central region of SNB and eastward to the southern and eastern waters the DFRE. The habitat change results from the land reclamation and industrial construction in Qinzhou Bay, which is located on the west of Sanniang Bay, may be the reason of core distribution areas moving eastward and southward.2. The seasonal differences on the number of newly identified individuals have no significant correlation with survey time (r=0.527, p=0.473) and the number of photos (r=0.436, p=0.564). It is easier to find new individuals in spring and autumn, while more difficult to recognize new ones in summer and winter. A possible explanation is that there is seasonal variation of the range pattern of CWD in SNB adjacent waters.3. During the survey periods, a total of 783 times of CWD are sighted, numbering 158 individuals, including 3 UA (1.90%),39 SA (24.68%),53 SS (33.54%),47 UJ (Unspotted juvenile,29.75%) and 16 UC (Unspotted calf,10.13%). It means that younger individuals (including UC, UJ and SS) equal 73.42%of the total number of CWD population with a mean percentage of 67.99% ± 4.58%. Abundance of the CWD population in SNB is estimated via Seber estimation model. The results indicate N=97 (95%C/=94-100).The discovery curve also indicates that the total number of dolphins identified reaches stabilized after the cumulative number of survey days being over 41. The estimated results of Seber estimation model support that the population of CWD in Sanniang Bay may be a closed one. At last, the database of individual identification of S. chinensis in SNB is established according to the results of photo-identification.4. The analyses of MinRI and Residence Time (RST) conclude that there are at least 40%to 50%of the dolphins showing a high loyalty to SNB waters. All of the 43 dolphins stay in SNB waters for over a year. The RST of 28 (65.12%),18 (41.86%) and 8 (18.60%) individuals is more than 2 year,3 year and 4 year respectively.5. The results of correlation test show that the number of CWD identified has a significant correlation with the number of Photos (r=0.684, p=0.002, N=18) and the number of survey days (r=0.501, p=0.034, N=18), while has no significant correlation with survey time (r=0.440, p=0.067, N=18). The cumulative numbers of CWD identified each year in SNB reflect that the amount of survey efforts could have a great impact on the results of field survey on population abundance.6. Compared with younger dolphins, dolphins in the age of SA and UA could be more accessible when performing photo-identification and capture-recapture work as their marks on surface being more reliable and stable. Thus, the results of photo-identification will be more reliable. Results of Independent-Samples T Test find that SA individuals have a significant correlation with SS on the index of MaxFI to Lc (t=3.636, p=0.001, df=40). The majority of dolphins in the age of SA can be recaptured 3 years later after the first sight, while dolphins in SS age may be recaptured successfully just over 2 years.All of the results of this study will be a basic data for further research of CWD in SNB and be helpful to those cooperating researchers.