Small-scale towing survey combined acoustical and visual observation for finless porpoise in the Yangtze River

Abstract

The population of Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis) has been decreasing rapidly in the last couple of decades. One of the large groups of this species is found near the convergent area of Poyang Lake and the Yangtze River. In this study, to monitor and assess the spatial distribution of this local population, the feasibility of towing acoustic survey method combined with visual observation using small boat was examined. During 11-16 May 2007, we conducted acoustic visual combined observations about 20 km per day in the convergent area. A time window of 100 s was employed to define simultaneous detections by both methods. Using the ratio of simultaneous detections, detection probabilities were calculated by the Petersen method as 56 % for acoustics and 24 % for visual detections, respectively. The detection probabilities of this study were higher than previous work. A survey using small boat and short cable to tow the acoustic system was effective to use to monitor local population.