From March 2001 to December 2003 eight direct count surveys in an upstream direction were conducted for Ganges river dolphins Platanista gangetica gangetica in the Vikramshila Gangetic Dolphin Sanctuary, a c. 60 km long segment of the middle Ganges River in Bihar, India. The mean number of dolphins recorded during upstream surveys was 119.4±SD 31.8 (range 88–174), with an encounter rate of 1.8 dolphins km−1 (range 1.4–2.8). During these surveys a rich diversity of other threatened aquatic wildlife was also documented, including the Indian smooth-coated otter Lutrogale perspicillata, gharial Gavialis gangeticus, a variety of freshwater turtles, and 135 water bird species. An assessment of fisheries documented 76 fish species of which 43% were caught exclusively in monofilament gill nets, a gear known to kill dolphins by entanglement. Eight new records of fishes preyed upon by Ganges river dolphins were identified from the stomach contents of two dolphin carcasses (Setipinna brevifilis, Osteobrama cotio cotio, Puntius sophore, Crosochelius latius, Mystus cavasius, Heteropneustus fossilis, Macrognathus pancalus, Sperata seenghala). These fishes and other species previously recorded in the diet of the dolphins composed 33.3% of the total catch sampled in 2001–2003. Interviews of 108 fishing households revealed that literacy rates were low (29.9%) and almost 50% earned less than USD 411 per year. The most important conservation actions that could be taken are for national and state governments to establish civil control and promote the development of community-based fishing cooperatives. These cooperatives could enjoy ownership rights to certain river segments in exchange for employing sustainable fishing techniques that are less injurious to dolphins.

ABSTRACT1. River flow regulation and fragmentation is a global threat to freshwater biodiversity, ecosystem processes, and associated human activities. Large dams in the Ganges river basin of the Indian subcontinent have severely altered natural flow regimes, particularly in the low-flow dry season. Altered flows could have negative impacts on endangered species such as the Ganges river dolphin Platanista gangetica.2. Habitat use by river dolphins was investigated in relation to river channel depth and morphology, over 332 km of the flow-regulated Gandak River in India. Dolphin distribution patterns were compared across multiple spatial scales in the Gandak, Kosi, Chambal, Sone Rivers and the upper and lower sections of the Ganges main stem.3. Dolphin presence was recorded in 40% of segments in the Gandak river, with a best count of 257 (range 250-267) and average individual encounter rates at 0.75 dolphins km -1 (SD 0.89). Bayesian zero-inflated spatial models showed that river dolphin abundance was positively influenced by river depth, presence of meanders and corresponded closely with gillnet fishing. Minimum mid-channel depth requirements were estimated at 5.2 m for dolphin adults and between 2.2 and 2.4 m for mother-calf pairs. 4. Adult dolphins showed highly similar habitat preferences across regulated or unregulated rivers, for depths >5 m, and meandering channels. Dry-season habitat availability was reduced as the degree of flow regulation increased across rivers, mainly owing to loss of lateral and longitudinal channel connectivity.5. Overall encounter rates were reduced from >3 km -1 in less regulated stretches, to <0.3 km -1 in regulated rivers. Clustering of dolphins in deep pools increased along the gradient of river flow reduction, with dolphins almost absent from intervening segments because of low flow rates. These results indicate the importance of maintaining adequate dry-season flows to ensure river habitat availability and connectivity for dolphins.

1.Limited visual perception in aquatic environments has driven the evolution of diverse sensory modalities in aquatic mammals. Dolphins largely use echolocation for prey capture in the face of limited visual and olfactory cues. Multiple foraging modes exist, although an understanding of how sensory systems are adapted to environmental and prey characteristics is limited. This is especially true for animals with extreme sensory specialisation, such as South Asian river dolphins of the genus Platanista. This taxon is effectively blind and retains plesiomorphic traits from its once-diverse ancestors. Distributed in murky rivers of the Indus-Ganga-Brahmaputra basins, it is thought to use mainly echolocation for feeding on fish and shrimp. 2. We hypothesised that foraging modes used by Platanista differ according to prey position in the water column (at the river surface, mid-column, and bottom) and are mediated by ecomorphology, acoustics, prey characteristics, and habitat features. 3. To test this, we combined a detailed review of the literature (anatomy-physiology-morphology studies, observations in captivity) with preliminary field observations and acoustics studies to investigate foraging mode selection. 4. Platanista displays peculiar foraging and feeding behaviours, including side swimming, rotational feeding, and grasp-suction feeding. Feeding is restricted to small prey with low body depth. At the river surface and bottom, echolocation-based foraging may not be efficient due to acoustic reflection or reverberation effects. 5. Platanista uses echolocation clicks to scan and detect prey at distances of about 20 m across the river mid-column, possibly switching to passive listening at the surface, and electroreception at the bottom, to maximise prey capture rate and feeding success. 6. Platanista is remarkable for its ability to persist in some of the most intensively used and abused river basins of the world. Using echolocation, electroreception, and passive listening might reduce overall foraging costs and contribute to the adaptability of the taxon.

Local ecological knowledge (LEK) can offer insights into fisheries management by describing long‐term changes that are difficult to unravel in data‐poor river‐floodplain fisheries. LEK is derived from complex interactions between fishers’ observations of environmental change and their institutional capacities to manage fisheries. Hence, it is important to understand where and how LEK and formal scientific studies on fish species’ decline could complement each other. In this paper, the causes of decline of 58 fish and two shrimp taxa were identified from LEK data (1999–2019) obtained from river–floodplain fisheries of the Gangetic plains (Bihar, India). Qualitative analyses of LEK were used to generate species‐specific hypotheses and historical insights on their declines. Destructive fishing, overfishing and the Farakka barrage were cited by fishers as the major causes of declines. Potential reasons for these perceptions were explored in relation to fishers’ experiences of conflicts in the region over fishing rights and access.

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