Single-hydrophone method for reconstructing dynamic behavior of endangered Chinese river dolphin

Chinese river dolphin (Lipotes vexillifer), usually called 'baiji', is one of the most endangered mammals in the world. The preservation of these mammals directly ensures the health of the ecosystem. However, not much behavioral information on baiji is available so far, that could effectively assist in undertaking their preservation. This research is an attempt to reveal baiji's hitherto unknown underwater dynamic behavior. For this work we have analyzed and extracted motion parameters from clicks of a free-swimming baiji recorded in 1996 at a single hydrophone. The absence of detailed bottom information made it difficult to localize baiji using the multiple reflections and forced us to rely completely on surface reflections only. From measurements of time delay between the direct path and the surface echo, not only large-scale behavior but also fine-scale behavior has been revealed, such as: periods of diving and surfacing, circling behavior underwater and a characteristic nodding motion. These studies have contributed towards a broad understanding of baiji's foraging exercises from the standpoint of formulating an acoustical survey for the search for this endangered marine mammal.