Zigzag transect survey by towed passive acoustic method for finless porpoise in the Yangtze River

Accurate estimation of abundance is one of the most important tasks for conservation and management of the endangered species. However, the population estimation of cetaceans in riverine systems is difficult due to the limited cruise course restricted by shallow water and ship lanes. In the present study, we conducted a zigzag survey trial with towed acoustic system in the main stem of the Yangtze River and appended Poyang Lake on the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), which has been decreasing rapidly in the last couple of decades. The porpoise detected were 4 and 28 individuals per 14 km (by straight-line distance) of the river and lake respectively on the same day. This corresponded to the seasonal site preference of the porpoises reported previously. Eighty-four percent of the porpoises were detected between 100 and 400 m from the bank. We also counted the number of navigating ships, which was 52 and 29 in the same sections of the river and lake respectively. Most of the ships were cargo vessel and navigated in the middle of the river and lake. Although the survey boat had to carefully across the channel, the zigzag transect seemed to be feasible. In the future work, we will examine the persistence to the bank of the porpoises with a bigger data set and should be able to estimate the accurate abundance of the local population.