IWN Daily Digest — Tuesday, 13 October 2026
Ganges river dolphins in a tidal West Bengal river are detected at noticeably different rates depending on the hour and the height of the tide, and researchers say that should change how the species is counted.
As reported by Mongabay India:
“A new study in the river found that tide, time of day and lunar cycle influences when the dolphins were detected.” “Tidal state should be standardised across abundance surveys.”
The month-long study, published in the journal Aquatic Mammals, was run on the Rupnarayan, an 85-km-long tributary of the Hooghly in West Bengal that is influenced by tides from the Bay of Bengal. Researchers from the Wildlife Institute of India anchored a single F-POD, an underwater recorder that listens for echolocation clicks, in the river from 4 to 31 January 2024 and collected 651 hours of recordings.
Detections of Ganges river dolphins peaked between about 4 and 6 a.m. and again between 6 and 8 p.m., and were lowest between about 10 a.m. and 3 p.m. The clearest link was with tidal height. Detections were highest at roughly 1.5 to 2 metres and fell as the water rose, and above about 4 metres the predicted detections were substantially lower.
Detections also dropped towards the new moon and climbed during the waxing phase. The authors call that finding preliminary because the recorder covered only one lunar cycle.
Counting Dolphins
Gargi Roy Chowdhury, a principal project associate at the Wildlife Institute of India and the study’s first author, said tidal state should be standardised across abundance surveys. Co-author Vishnupriya Kolipakam, a scientist at the institute, said the 24-hour pattern in the Rupnarayan did not match the published literature.
The International Union for Conservation of Nature’s latest Red List assessment, published online in July 2022, categorises the Ganges river dolphin as Endangered.
The authors did not measure prey availability, and acoustic detection records only whether a dolphin was within range of the recorder. Chowdhury also said the overlap between dolphin detections and vessel traffic was substantial and differed by size class, in a river Chowdhury described as a human-modified landscape with intense motorised fishing.
Sources: Mongabay India · Wildlife Conservation Trust
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