Introduction

Chinese white dolphins (also Indo-Pacific hump-back dolphins, Sousa chinensis) are widely distributed from western Africa to Australia. The Taiwanese population inhabits the shallow western coastal waters (Yeh 2011) and is believed to be isolated from populations in adjacent waters such as Xiamen and Hong Kong (Wang et al. 2008).

The population is closed and estimated to contain less than 80 individuals (Chang 2011;Wang et al. 2012; Yu et al. 2010). The conservation status of this regional population is so critical that it was listed in the IUCN Red List of Threatened Species as

“critically endangered” (Reeves et al. 2008).

Population abundance is one of the most important baseline data for ecological studies. Not only does it play an important role in planning conservation actions, but studying this parameter annually allows a better understanding of the population trend of a species, especially when the species faces a wide range of anthropogenic threats such as industrial and fishing activities, as is the case of the Chinese white dolphin population in Taiwan (Ross et al. 2010).

Two major methods, the transect line survey and mark-recapture method, have been widely used to estimate the population abundance of Chinese white dolphins off the western coasts of Taiwan (Wang et al. 2007; Wang et al. 2012; Yu et al. 2010) as well as other cetaceans around the world (regarding the transect line survey: Forcada &

perpendicular distances from the sighting points to the transect lines over the course of the survey period (Buckland et al. 2001). Transect line survey researches have been conducted on Chinese white dolphins in Hong Kong, Xiamen, Dafengjang, and Hepu (Jefferson and Hung 2004; Chen et al. 2008; Chen et al. 2009). With regard to the Taiwanese population, previous studies using transect line surveys estimated its population size to be between 99 (CV=51.6%, Wang et al. 2007) and 64 (CV=42.0%, Kuo 2013 Chapter 2). However, these results may contain a high bias due to limited sampling data. On the other hand, for a small population such as that in Taiwan, while forming an accurate estimate of abundance is crucial to its conservation planning, conducting transect line surveys that cover all possible habitats can involve high costs for a large amount of survey efforts on extremely low sightings (Kuo 2013 Chapter 2).

The mark-recapture method provides an alternate approach to abundance estimation. It uses the markings on individual animals (actively induced or passively observed by researchers) to identify some members of an encountered group. When another group is encountered subsequently, as the “recapture” process, the percentage of the marked individuals within the group, can be used to estimate the abundance of the entire population. There are different mark-recapture models tailored to different population characteristics. For example, when the study population is closed, meaning that there is no change in population size due to reproductive or social affects, the classic Lincoln-Petersen method is often recommended. For most situations where the population is open and fluctuating with survivorship, mortality, or migration from other subpopulations, the Jolly-Seber model is widely applied (Amstrup et al. 2010). Still, as some populations are not completely open or closed, application of either model could cause estimation bias. In response, Pollock in 1982 proposed a variant closed population

model, i.e., the robust design model, that seeks to combine the advantages of both the open and closed population models. This method defines the longer time periods for sampling that contain temporal sampling gaps as “primary periods,” during which the population is considered to be open. On the other hand, the temporal occasions for dense sampling are defined as “secondary periods,” during which the population is considered to be closed. As such, the method allows population changes during the primary periods, while assuming a stable population within the secondary periods.

Unlike the transect line survey that uses an indirect approach by providing estimates for population density in a given study area to estimate abundance, the mark-recapture method estimates the abundance of the population directly, with various mathematical models provided other parameter estimations such as the population’s reproductive, migratory, and mortality rates. It is worth noting that when a population is closed or the effect of migration is negligible and the transect line survey covers the entire habitat, both the transect line survey and the mark-recapture method should give similar estimates.

In cetacean research, mark-recapture method based on photo-identification (hereafter, photo-ID) technique has been proven to be an effective way for estimating abundance as long as other population parameters, including recruitment and survival rates are properly considered (Hammond et al. 1990). In the case of Chinese white dolphins, this method has been used in many locations, including Algoa Bay in South

For the Taiwanese population, the mark-recapture method was first applied by Yu et al. (2010). The study used Bailey’s modified Petersen method, which is one variation of closed population models, and reported the population abundances to be around 75 and 80 individuals in the year of 2008 and 2009, respectively. Then Wang et al. (2012), using the robust design method, reported the abundances to be 54 to 74 individuals between 2007 and 2010, but the abundance estimates varied greatly from year to year due to uneven and limited sampling. Since a large quantity of high quality photo-ID data has been assembled by Lien-Siang Chou’s team through systematic and sustained surveys up to 2012, it seems advisable to conduct a thorough mark-recapture study on the Chinese white dolphins in Taiwan.

In this study, a photo-ID based robust design mark-recapture method was adopted to estimate the annual abundance and other relevant reproductive parameters, including survivorship and migration, of the Chinese white dolphins in Taiwan in order to track more precisely on the population trend. Additionally, two kinds of relative birth indicators, the unmarked ratio and crude birth rate, were estimated from the photo-ID database. The results should be useful for making conservation policies for this small closed population of the Chinese white dolphin in western Taiwanese waters.

3.2 Material and methods