Diving behaviour of freshwater finless porpoises (Neophocaena phocaenoides) in an oxbow of the Yangtze River, China

Dive-depth and swim-speed of a juvenile and an adult free-ranging, Yangtze finless porpoises ( Neophocaena phocaenoides ) were observed using velocity-time-depth recorders in an oxbow of the Yangtze River. In total, 8222 individual dives were recorded over 59 hours. Two dive types, deep-dive (≥2.7 m) and shallow-dive, were recognized. Horizontal travel distances of two finless porpoises in a day were 94.4 km and 90.3 km, which were longer than those of oceanic relative species (harbor porpoises, Phocoena phocoena ). Although the shallow water limited the maximum dive-depth, dive-duration, and bottom-time of finless porpoises were similar to the harbor porpoises. A sudden drop of swim-speed below 0.25 m s −1 was frequently observed nearby the maximum dive-depth. This seemed to indicate “turning around” behaviour, possibly during prey pursuit.