Low flows trap river dolphins

Posted on December 28, 2013

Large-scale flow regulation, irrigation and power-generation projects pose serious threats to river biodiversity.  In India, the widespread construction of dams and barrages has reduced dry-season flows to the point where animal populations are becoming severely fragmented or are already locally extinct.  Threatened forms include charismatic species such as the Ganges river dolphin  ( Platanista gangetica gangetica ) and crocodile (Gavialis gangeticus), which are both listed as endangered species.  To establish the minimum flow requirements for river dolphin populations, Indian researchers compared data from dry season surveys carried out in a range of rivers in the Ganges basin.  The rivers were biophysically similar but some were highly regulated, with 3-4 upstream dams or barrages, while others were less regulated, with only a single upstream dam or barrage.  Dolphins were found to be associated with deep river sections, meanders, confluences, and centres of gillnet fishing where fish densities were high.  Dolphins were less abundant in highly regulated rivers (< 0.3 sightings / km) than less regulated rivers (> 3.0 sightings / km).  They were extremely clustered in deep pools but largely absent from the intervening areas, a finding consistent with concerns that low flows in the dry season are restricting dolphin movements and isolating populations.  The sustainable management of both river biodiversity and fisheries depends on maintaining minimum depths of water; in the case of river dolphins it seems that adults require depths of at least 5.2 m and subadults need at least 3.8 m.    Unfortunately, average water levels in regulated rivers in the Ganges basin are currently much lower (< 1.5 m).  In India the need to safeguard environmental flows has been largely ignored in favour of the water needs of agriculture and industry, but a more sustainable rebalancing needs to occur if ecological impoverishment and mass extinctions are to be avoided.

Reference:  Choudhary, S. et al.  2012.   River dolphin distribution in regulated river systems: implications for dry-season flow regimes in the Gangetic basin.  Aquatic Conservation: Marine and Freshwater Ecosystems 22, 11–25.  http://onlinelibrary.wiley.com/doi/10.1002/aqc.1240/pdf