Habitat impact on acoustic bio-sonar characteristics of Irrawaddy dolphins across two distinct habitats

Abstract

Acoustic bio-sonar characteristics of non-marine toothed whales are very critical for any conservation efforts using passive acoustic monitoring. The acoustic surveys have now been recognized as effective methods for freshwater cetaceans monitoring and abundance estimation compared to the traditional visual surveys. These species have been declared as data deficient by the IUCN and acoustic surveys present themselves as the relatively cost-effective approach. Irrawaddy dolphins Orcaella brevirostris referred to as facultative freshwater cetaceans because they occupy both fresh- and near-shore marine waters are among the cetaceans at greatest risk to population extirpation and perhaps extinction. Their vulnerability stems from habitat requirements that are coupled to marine and freshwater habitats subjected to intense human intervention. The bio-sonar characteristics of these species are highly sensitive to their habitat and the Irrawaddy dolphins are spread across varied environments over distant geographic locations. A good understanding of their acoustic bio-sonar characteristics across habitats can potentially provide significant conservation inputs. The two habitats considered in this work are contrasting in their origin and evolution. The first is the rivers systems of the Sundarbans mangrove forest with a deltaic habitat and second is the Chilika Lagoon with rain water and high tide fed habitat. Detailed analysis of the Sundarbans Irrawaddy dolphins were reported in 2013 with well-defined acoustic signal parameters. The authors have applied the same parameters for a comprehensive comparison of the species in the two habitats with a critical review on the correlation of the habitat features and the acoustic bio-sonar characteristics. Long term recording of the dolphin clicks in the Chilika lagoon have been analyzed and compared with the Sunderbans results available in the open literature. The acoustic parameters and their physical correlation with the animal behaviour and the habitat status have been presented. Multiple parameters such as centroid frequency, peak frequency, signal source level, bandwidth, duration of clicks, inter-click interval etc have been considered for the study.