Abstract
Understanding population structure and habitat use of poorly known cetacean species is a first step toward scientifically informed management decisions. In the southern range of New Caledonia (South Pacific), a long-term dataset of Indo-Pacific bottlenose dolphins (Tursiops aduncus) encounters primarily during winter seasons 1997 to 2019 (473 group observations) was used to assess social structure, habitat use and potential threats. A total of 338 individuals were photographically identified, forming three distinct communities in the south-west lagoon, the south lagoon and the Isle of Pines. Mark-recapture histories revealed that the three communities were weakly connected and might be considered as independent management units. Suitable habitats were estimated with presence-only distribution models relative to topographic and seabed substrate predictors. Habitat suitability increased with proximity to coasts or reefs, at shallow depth, and over muddy bottom. These habitats had various levels of protection and were used by humans, mostly in the south-west lagoon. External injuries were interpreted to determine natural interactions and potential anthropogenic threats. The prevalence in injuries did not vary among the three areas. A substantial proportion of injuries related to propeller hits was reported, representing a total of 16.7% (34 of 204) of all injuries observed on dolphins. The three communities of Indo-Pacific bottlenose dolphins revealed in the southern part of New Caledonia are particularly vulnerable due to their insularity, their coastal habitat use and the low levels of connectivity found among them. In a context of increasing maritime traffic, fishing and recreational activities, this study provides a useful baseline to the urgent assessment of the conservation status of dolphins in New Caledonia.
Highlights
Connectivity is known to positively influence populations by favoring gene flow (Wright, 1943)
The isolation of Isle of Pines community (IoPc) could be influenced by the topography of the region: the Sarcelle Pass that separates the Merlet Integral Marine Protected Area (IMPA) from the north-west reefs of the Isle of Pines is quite shallow, it appears to be deep and unsuitable to Indo-Pacific bottlenose dolphin presence (Kiszka et al, 2012)
The social structure and spatial distribution of Indo-Pacific bottlenose dolphins in the southern range of New Caledonia during the winter season has highlighted three distinct communities that should be considered as three independent management units, a suggestion supported by an extremely low genetic diversity which alters the resilience of this species in New Caledonia
Summary
Connectivity is known to positively influence populations by favoring gene flow (Wright, 1943). In the absence of connectivity, insularity may result in higher susceptibility of small populations to inbreeding (Charlesworth and Charlesworth, 1987) and increase the risk of local species extirpation, especially when these populations face threats from human activities Nonmigratory insular species such as lagoon-living cetaceans are at risk as their gene flow may only be ensured by punctual and rare immigration events (Oremus et al, 2015). Three phylogenetic units specific to African waters, to the Pacific Ocean and to the Arabian Sea have been highlighted within the species (Hale et al, 2000; Oremus et al, 2015; Amaral et al, 2017; Gray et al, 2018) They are characterized by local ecological adaptations, as well as reproductive isolation and minimal or absent gene flow. Regional adaptations to environmental parameters can in turn lead to morphological differences between populations (van Aswegen et al, 2019)
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