Behavioural ecology of Indo-Pacific bottlenose and humpback dolphins

University dissertation from Stockholm : Zoologiska institutionen

Abstract: Dolphins in East Africa face various threats, such as direct hunt, bycatch and habitat degradation. It is therefore important to undertake research that can facilitate necessary conservation and management actions to minimize these threats. This thesis investigates the behavioural ecology of the populations of Indo-Pacific bottlenose (Tursiops aduncus) and humpback (Sousa chinensis) dolphins inhabiting the waters off the south coast of Zanzibar. Dolphins were identified using photo-identification techniques and information on behaviour, location and depths were collected during boat surveys conducted between 1999 and 2002. Mark-recapture methods were used to estimate population sizes of 161 (95% CI 144-177) Indo-Pacific bottlenose dolphins and 71 (95% CI 48-94) humpback dolphins in the 26 km2 study area in 2001. High frequency of re-sightings indicated that both species were resident in the area. Analyses of spatial distribution, corrected for survey effort, showed that both bottlenose and humpback dolphins primarily utilized only a small part of the study area. These areas may be regarded as critical areas for respective species, given the relatively high density of groups encountered. The impact of dolphin tourism on female bottlenose dolphins was investigated both on individual and group level using follows of focal females and scan-sampling of focal groups. Increased level of tourism activities increased the proportion of travel, non-directional movement patterns and active dives. These changes can have adverse effects on both individual and population level by giving dolphins less time for nursing and causing shifts in habitat use. Further, this could also lead to reduced dolphin tourism potential. The explanations for mixed species groups in mammals are reviewed and an approach for investigating such groups is proposed. Mixed species groups of Indo-Pacific bottlenose and humpback dolphins frequently occur off the south coast of Zanzibar. Anti-predator coupled with social advantages, may offer a likely explanation for the formation of these groups. Mixed-species groups may have different functions depending on the individuals that participate. The findings in this thesis will be essential when planning future conservation and management actions to protect the dolphins as a socio-economic important resource.

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