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.

Summary1. India's Chambal River hosts the largest population of the critically endangered gharial. Boat-based daylight surveys to date only provide indices of relative abundance, without measures of survey bias or error. No attempt to quantify detection probabilities in these surveys has yet been made, and thus, absolute density estimates of this population remain unknown. 2. We surveyed 75 km of the River Chambal and photographed individual gharials for capture-recapture analysis. The total sampling effort yielded 400 captures. Population closure was supported (z = À1Á48, P = 0Á069), and closed-population models were used to estimate abundances. 3. Models were selected using the Akaike Information Criterion (AIC) index of model fit. The best model estimated 231 ± 32 adult, 83 ± 23 subadult and 89 ± 19 juvenile gharials (Mean ± SE), respectively, while the model-averaged estimate was 220 ± 28 adult, 76 ± 16 subadults and 93 ± 16 juvenile gharials, respectively. 4. The best model estimated absolute densities of 3Á08 ± 0Á43, 1Á11 ± 0Á3 and 1Á19 ± 0Á25 adult, subadult and juvenile gharials km À1 , respectively, while the model-averaged estimate was 2Á93 ± 0Á37, 1Á01 ± 0Á21 and 1Á24 ± 0Á21 adult, subadult and juvenile gharials km À1 , respectively, compared with relative densities of 0Á94, 0Á45 and 0Á30 adult, subadult and juvenile gharials km À1 , respectively, from boat-based daylight surveys. On the basis of our best model, we suggest a detection probability based correction factor of 3Á27, 2Á47 and 3Á97 to boat-based daylight survey estimates of adult, subadult and juvenile gharials, respectively. 5. Synthesis and applications. Used within the framework of capture-recapture analysis, photoidentification provides a reliable and noninvasive method of estimating population size and structure in crocodilians. We also opine that without determining the current status of gharials, highly intensive strategies, such as the egg-collection and rear-and-release programmes being implemented currently, initiated on the basis of underestimates of population sizes, are unwarranted and divert valuable conservation resources away from field-based protection measures, which are essential in the face of threats like hydrologic diversions, sand mining, fishing and bankside cultivation.