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Failure in the Colonization of a New Area by Indo-Pacific Bottlenose Dolphins (Tursiops aduncus), Japan
Abstract: Coastal dolphins inhabiting areas influenced by human activities can face anthropogenic threats, so monitoring surveys to detect changes in abundance are required. Photo-identification surveys of Indo-Pacific bottlenose dolphins (Tursiops aduncus) were conducted from 2001 to 2013 in the waters south of Amakusa-Shimoshima Island, Japan, where a few dozen dolphins emigrated from the waters north of the island. We report the range, the group composition, the number of individuals, and the breeding of the dolphins settled in the new area. The dolphins were found only in narrow straits between islands. One strait was the core habitat area. Two types of groups were observed: groups with females (range of group size = 15 to 29) and those without females (range of group size = 1 to 3). The total number of dolphins gradually decreased from 31 individuals in 2001 to 18 in 2011, which was mainly attributed to the reduction in the male number from 17 to 8. Few new immigrants to the area were found. In 2009, a temporary invasion by large groups of dolphins inhabiting the northern original area occurred. After the temporary invasion started to occur, we did not observe new calves nor calves that survived to the next year. The breeding had been successful except for the first two years after the settlement. The presence of the much larger group might have negatively impacted the maternal feeding environment. It was possible that the environmental deterioration due to the red tide, which can affect fish survival, was progressing. Two females disappeared in 2011, and all the remaining dolphins disappeared in 2012. Certain dolphins were found in other areas. In the present study, the process behind the dolphins’ population range expansion is examined along with factors that may have hindered the colonization process.
Key Words: new habitat, colonization, small community, invasion, photo-identification survey, Indo-Pacific bottlenose dolphins, Tursiops aduncus
Page Numbers: 311-320