Change Detection of Hydro-Acoustic Environment for Freshwater Cetaceans Using Remote Sensing in Poyang Lake

Recently, the population of Yangtze finless porpoise has been declining. Dredging in Poyang Lake might possibly influence the acoustic environment of the porpoise which relies on echolocation for communication and detection of their prey. A better insight in the spatial distribution of dredging related noise would be useful to assess possible impacts. Traditional in situ measurements of hydro acoustic pollution are however costly to carry out. The aim of this study was to detect change of the hydro-acoustic environment induced by noise from dredging platforms and vessels transporting sand. In situ measurement of the SPL of platforms and vessels and remotely sensed localization of this infrastructure was used to develop a model predicting SPL. The near-infrared bands of ASTER were used to classify active platforms, mobile vessels and other dredging infrastructures. The time interval between the stereo pair (band 3N and 3B) was used to separate mobile vessels from static infrastructure, while the size of objects was used to distinguish active platforms composed of a platform with associated vessels from smaller inactive platforms and immobile vessels. Remotely sensing based noise maps of 2000, 2005 and 2006 revealed an increase in broad bandwidth noise which corresponded to the levels recorded in situ in 2007. The high frequency noise level measured in 2007 differed considerably from the remote sensing based sound predictions. Spectral overlap between the noise generated by the dredging platform and the sound used by porpoise suggests that dredging might possibly influence the echolocation of porpoise. The highly increased SPL in the northern part of Poyang Lake might disturb the behavior of the porpoise, and might isolate the population in Poyang Lake from others. Since there is a high possibility of threatening the survival of the Yangtze finless porpoise by dredging, some measures should be taken to eliminate or alleviate the impacts, such as relocating the dredging activities or replacing the noisy by more silent dredging and transport equipment.