Rigorous gharial population estimation in the Chambal: implications for conservation and management of a globally threatened crocodilian.
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. 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=-1.48, P=0.069), and closed-population models were used to estimate abundances. 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. The best model estimated absolute densities of 3.08±0.43, 1.11±0.3 and 1.19±0.25 adult, subadult and juvenile gharials km-1, respectively, while the model-averaged estimate was 2.93±0.37, 1.01±0.21 and 1.24±0.21 adult, subadult and juvenile gharials km-1, respectively, compared with relative densities of 0.94, 0.45 and 0.30 adult, subadult and juvenile gharials km-1, respectively, from boat-based daylight surveys. On the basis of our best model, we suggest a detection probability based correction factor of 3.27, 2.47 and 3.97 to boat-based daylight survey estimates of adult, subadult and juvenile gharials, respectively. 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.