Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis): An Examination of Methodological and Biogeographical Variances

Abstract

Observer-based counts and photo-identification are two well-established methods with an extensive use in cetacean studies. Using these two methods, group size has been widely reported, especially for small dolphins. Both methods may come with potential errors in estimating the group size, yet there is still a lack of comparison between both methods over a broad range of group size. Particularly, biogeographical variances in group size estimates were often mixed with methodological variances, making it difficult to compare estimates from different geographic regions. Here, group size estimates of a small, shallow-water, and near-shore delphinid species, Indo-Pacific humpback dolphins (Sousa chinensis), were simultaneously sampled using observer-based counts and photo-identification at three regions in the northern South China Sea. Data showed that dolphin group size from two methods were highly variable and associated with sampling regions. Generalized linear mixed models (GLMMs) indicated that dolphin group size significantly differed among regions. Statistical examinations further demonstrated dolphin group size could be affected by a complex combination of methodological and biogeographical variances. A common hurdle to examine potential factors influencing the estimation process is the inability to know the true group size at each sample. Therefore, other methods that could generate comparable estimates to represent true group size are warranted in future studies. To conclude, our findings present a better understanding of methodological and biogeographical variances in group size estimates of humpback dolphins, and help yield more robust abundance and density estimation for these vulnerable animals.

Short‐finned pilot whales in the South China Sea: Insights into regional distribution, movement pattern, and habitat characteristics

“…Due to the limitation of sample size, we applied a Mann-Whitney U test to examine (1) whether group size of pilot whales varied between field sighting data set (2019)(2020)(2021)(2022) in the SCS and opportunistic sighting data set in adjacent waters (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015); (2) the effect of mixedspecies association on group size, i.e., the difference in group size between groups of single-species (i.e., only pilot whales present) and mixed-species (i.e., pilot whales observed with other cetacean species); (3) whether spatial measurement data i.e., WD, Dis_200, and Dis_1000, varied between single-and mixed-species groups; and (4) whether spatial measurement data varied between sighting-and tracking-based locations (de Boer et al, 2016;Frantzis & Herzing, 2002;Liu et al, 2019a). The method for handling group size data followed Liu et al (2021b). Boxplots were used to illustrate statistical test results.…”

Population distribution, connectivity and differentiation of Indo‐Pacific humpback dolphins in Chinese waters: Key baselines for improving conservation management

“…Dong et al (2021) Evidence of the population differentiation of humpback dolphins in Chinese waters is also available from other behavioural aspects, such as sociality and habitat use strategy (Table 1). Liu et al (2021a) compared the group size data collected from three humpback dolphin populations in SWH, EZJ and SNB. Their results indicated that humpback dolphins in SWH formed larger groups than those in SNB and EZJ.…”