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.
Highlights
Groups are a fundamental unit of gregarious animal species (Casari and Tagliapietra, 2018)
Three areas along the northern coast of the South China Sea were selected as sampling regions: the waters southwest off Hainan Island (SWH; Li et al, 2016; Liu et al, 2020b), Sanniang Bay
This study clearly illustrated that traditional estimation methods, i.e., observer-based counts and photo-identification could generate variable group size estimates for humpback dolphins
Summary
Groups are a fundamental unit of gregarious animal species (Casari and Tagliapietra, 2018). It is difficult to generate accurate group size estimates for wild animals, since the estimation process may be affected by diverse factors (Walsh et al, 2009; Clement et al, 2017). A fundamental approach to estimate group size of freeranging dolphins is on-site counts by observers from vessels (Mann, 1999). Social dynamics may differ among dolphin species (Gowans et al, 2007), which can greatly affect the estimation process of group size. Observers’ experience can be improved through training and practice, it is still hard to remove potential bias from observer-based counts (Gerrodette and Perrin, 1991; Clement et al, 2017), and the bias may increase with the group size (Gerrodette et al, 2019)
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