Vorticity preference of the Yangtze finless porpoise (Neophocaena asiaeorientalis) during the dry season at the Wanhe River Estuary confluence

•The Yangtze finless porpoise (Neophocaena asiaeorientalis) is critically endangered.•Porpoise clustering was observed in a confluence of the Wanhe River Estuary, China.•A hydrodynamic model was created to investigate the driving mechanism of clustering.•We showed a preference for vorticity in the range 0.001 s−1–0.0015 s−1•This has implications for habitat protection and feeding behavior of freshwater cetaceans.