Spatio-behavioural habitat partitioning by two sympatric island-associated dolphins in Tañon Strait, central Philippines

Abstract

Very little is known of the spatio-behavioural dynamics of island-associated cetaceans, and even less so about multi-species communities such as those seen in Tañon Strait, central Philippines. Declared as a protected seascape in 1998, Tañon Strait is a semi-enclosed body of water characterized by a narrow coastal zone and considerable depths relatively close to shore. The spinner dolphin Stenella longirostris and the Indo-Pacific bottlenose dolphin Tursiops aduncus are the two most frequently seen species of the diverse cetacean fauna in the strait; and both were historically victims to by-catch. With the application of geographically-referenced data and focal behaviour sampling collected from 2014 to 2016, we investigated the patterns of sympatric co-occurrence of these two dolphin species in Tañon Strait and documented their pattern of habitat partitioning. While the core distribution areas of bottlenose dolphins are primarily in shallower inshore waters, spinner dolphins use a wide range of habitats in the strait. Their predominant day-time behaviour also differs; while bottlenose dolphins spend a large proportion of time foraging, spinner dolphins use the area mainly for resting, with some core resting areas in deep waters away from costal bays and coves. In overall, there seem to be a considerable spatio-behavioural dissimilarity between these two species, with foraging and resting representing the primary determinants of the range use pattern of bottlenose and spinner dolphins, respectively. This pattern of habitat partitioning and behavioural dissimilarity may be essential to their sympatric co-occurrence; a process that is likely an important facilitator of sympatricity in general. The identification of species-specific core areas and their pattern of range use may serve for a valuable tool in the management of recreational and maritime activities in waters of the Tañon Strait Protected Seascape, and should be considered in a management plan that is currently under development.