In the Rupnarayan River in West Bengal, Ganges river dolphins show distinct activity at dawn and dusk, and their presence depends most strongly on the height of the tide. A new study conducted with underwater acoustic recorders reveals how these rare mammals synchronise their behaviour with predictable natural cycles.
Scientists from the Wildlife Institute of India recorded sounds in the river's tidal zone over the course of a month. Peaks in detections fell in the intervals from 4 to 6 in the morning and from 6 to 8 in the evening, while activity dropped noticeably in the middle of the day. Water level had the strongest influence of all: the greatest number of signals was recorded at a tide height of 1,5–2 metres, after which the number of detections declined.
This dependence appears to be linked to the movement of prey. Tidal currents bring rich concentrations of small fish into the estuary, and the dolphins probably use these windows for efficient hunting. The lunar cycle also plays a part: the greatest activity was noted during the waxing moon. The study covered only a single lunar cycle and did not directly measure the availability of food, so the conclusions require cautious interpretation.
These data are important for monitoring the population. Instead of random daytime outings, scientists will now be able to plan observations at the optimal time, taking tides and lunar phases into account. This will improve the accuracy of population estimates and help to better understand how changes in the river's hydrology affect the survival of the species.
The Ganges river dolphin (Platanista gangetica) inhabits freshwater and estuarine waters, where tides create a dynamic environment. Understanding its daily and tidal rhythms shows how finely these animals are adapted to the natural fluctuations of the ecosystem, and underscores the need to preserve these rhythms when managing rivers.
Taking temporal factors into account in monitoring programmes will make it possible to track the state of populations more accurately and respond to threats in good time.
