Life at the borderline: Responses of Ganges river dolphins to dry‐season flow regulation of river and canal habitats by the Farakka barrage

Abstract

Modification of river flows by dams and barrages has fragmented riverine habitats and threatened the survival of Indian subcontinental river dolphins. In this study, habitat use by endangered Ganges river dolphins in response to dry‐season operations (December‐March) of the Farakka barrage on the Ganga River, located about 20 km upstream of the India–Bangladesh border, was assessed. Ten surveys were conducted in: (i) the Ganga River below the barrage, where rapid changes in water levels were expected owing to barrage operations; (ii) the Farakka feeder canal with stable water levels; and (iii) the Hooghly River downstream of the canal. Dolphin distribution, water depth, fishing boat count and temperature were recorded in all surveys and the data were analysed in a dynamic occupancy modelling framework. Temporal trends in river discharge, primary productivity and fish availability were also estimated to explain river dolphin responses. Occupancy and colonization by dolphins increased in all stretches by spring/March, correlated with rising water temperature and increased fishing intensity, which was related to upstream migration of hilsa fish. In the Ganga River, dolphin occupancy in deep river pools remained stable, but resulted in increased overlap with fisheries as river flow declined. The feeder canal, with stable flows, witnessed increased colonization by dolphins, but also greater risk of bycatch in fishing nets. The canal that appeared as a hydrological refuge thus became an ‘ecological trap’. The findings highlight the need for integration of flow maintenance and fishery regulations in the monitoring and management of canals as effective refuges for riverine species. We advocate the inclusion of ecological flow criteria and fishery regulations to reduce dolphin bycatch mortality, in the context of establishing river dolphin conservation plans and transboundary water‐sharing guidelines. The revision of the India–Bangladesh water‐sharing treaty in 2026 could incorporate the above considerations.

The effect of physiographic and hydrologic complexities and their alterations on the distribution of obligate freshwater dolphins

“…Positive impacts of hydrologic alterations on dolphin distribution relate to the creation of suitable habitat conditions due to increase in countercurrent habitats, maintenance of minimum depth, increase in the number of deep pools, increase in river sinuosity, and increase in aggregate habitat (Bashir et al, 2010 ; Momblanch et al, 2022 ; Prajapati, 2021 ; Samad et al, 2022 ; Sinha, 2000 ; Smith, Aminul Haque, et al, 1998 , Smith, Sinha, et al, 2000 ; Sonkar & Gaurav, 2020 ). Positive impacts on the Ganges dolphin and the Indus dolphin occurrence as a result of such alterations may be partly attributed to their ecological preference for greater depths.…”

A review of the status, threats and management priorities of a remnant population of Indus River dolphins in the Beas River, India

“…Fisheries bycatch was one of the main factors involved in the extinction of the Yangtze River dolphin ( Lipotes vexillifer ) and is also pushing many other cetacean populations towards extinction (Brownell et al, 2019; Northridge et al, 2017; Read, Drinker & Northridge, 2006). River dolphins and fishers tend to utilize the same areas of rivers, and it was shown in the Ganga River that as flow declined dolphins remained in their preferred deep pool habitat but there was an increased overlap with fisheries (Kelkar et al, 2010; Samad, Kelkar & Krishnaswamy, 2022). It is very likely that bycatch was a significant threat to dolphins in the Beas River before the fishing ban, when fishing was more widespread and intense and often involved migrant labourers from other parts of India.…”

“…Freshwater cetaceans are habitat specialists with restricted distributions, and the rivers in which they occur are extensively used, modified and degraded by humans. Water infrastructure, including dams, diversions, embankments and dredging are affecting most freshwater cetaceans, with the result that populations are becoming increasingly fragmented and, being generally small, fragmented populations are at increased danger of extinction (Araújo & Wang, 2015; Braulik et al, 2014; Pavanato et al, 2016; Samad, Kelkar & Krishnaswamy, 2022; Smith & Reeves, 2000a). Irrigation dams and water diversions have been most extensively developed in the Indus River system in Pakistan, and this has had large negative impacts on the Indus River dolphin ( Platanista minor ), an obligate freshwater cetacean endemic to that river system (Braulik et al, 2014; Reeves, Chaudhry & Khalid, 1991).…”