Issues and an optimal application of acoustic survey for freshwater cetaceans

Freshwater cetaceans are among the data deficient and now recognized as the most threatened group of marine mammals. Large scale studies are required to be done on an urgent basis to ensure reasonable chance of survival for these species. A detailed analysis of the freshwater habitats globally, indicate tropical shallow environment (important for the survival of the freshwater cetaceans). Visual surveys being employed for abundance estimates have significant limitations and acoustic surveys have recently been identified as effective alternative and could complement the visual surveys for conservation efforts. However, two major issues can be prominent such as shallow water sound propagation and noise pollutions. It is well known that shallow waters result in multiple interaction of the acoustic signal with the surface and the bottom prior to being received at the hydrophone receiver. The tropical waters further cause random surface and bottom behaviour resulting in substantial site specific underwater channel fluctuations being coupled to the desired marine mammal signal. Thus, the acoustic survey efforts in the tropical shallow habitats are highly sensitive to the local conditions and demand efforts to mitigate the underwater channel fluctuations while pursuing analysis efforts on the received signal. Increasing population in these regions has resulted in unregulated waterborne activities linked to economic growth. Such activities have caused significant increase in the ambient noise in these habitats. The effectiveness of acoustic sensors is further limited by the high ambient noise in the habitat and could also impact survival chances of these critically endangered species. This work attempts to put in perspective the challenges of acoustic surveys in tropical shallow habitats and provide a way ahead for effective passive acoustic monitoring of the freshwater cetaceans globally.