New study shows tides and moon cycles shape Ganges river dolphin activity in West Bengal

Ganges river dolphins in West Bengal's Rupnarayan river time their activity based on the tides, the time of day, and the moon cycle. A new month-long study published in the journal Aquatic Mammals tracked the animals using an underwater acoustic recorder that listens for dolphin clicks. The Rupnarayan river is an 85-kilometre-long tributary of the Hooghly river, influenced by tides from the Bay of Bengal located 220 kilometres away. The study site features intense motorized fishing and the Kolaghat thermal power plant downstream.

Findings And Tides

Researchers recorded 651 hours of underwater sound from January 4 to 31, 2024, using an F-POD device anchored against strong tidal currents. Sound detections peaked twice a day, around 4 to 6 a.m. and 6 to 8 p.m., while dropping to their lowest points between 10 a.m. and 3 p.m. Detections also peaked when water levels were between 1.5 and 2 metres, dropping sharply when water levels rose above four metres. Gargi Roy Chowdhury from the Wildlife Institute of India explains that moderate water levels likely make foraging easier because prey becomes concentrated. Detections also rose as the moon moved from a new moon into its waxing phase, though researchers note this lunar pattern is preliminary.

Context

Passive acoustic monitoring relies on underwater sound recorders to track animal presence by picking up natural echolocation clicks rather than direct visual sightings. Ganges river scientists use these audio tools because river dolphins spend most of their time underwater in murky freshwater environments where visual spotting is difficult. Understanding these natural activity patterns helps researchers design better conservation surveys that account for daily and tidal changes.