Population status, structure and distribution of the greater one-horned rhinoceros Rhinoceros unicornis in Nepal

We assessed the abundance and distribution of the greater one-horned or Indian rhinoceros Rhinoceros unicornis in all its potential habitats in Nepal, using block counts. In April 2011 5,497 km were searched in 3,548 elephant-hours over 23 days. The validity of the block count was assessed by comparing it with counts obtained from long-term monitoring using photographic identification of individual rhinoceroses (ID-based), and estimates obtained by closed population sighting-mark-resighting in the 214 km(2) of Chitwan National Park. A total of 534 rhinoceroses were found during the census, with 503 in Chitwan National Park (density 1 km(-2)), 24 in Bardia National Park (0.28 km(-2)) and seven in Suklaphanta Wildlife Reserve (0.1 km(-2)). In Chitwan 66% were adults, 12% subadults and 22% calves, with a female : male ratio of 1.24. The population estimate from sighting-mark-resighting was 72 (95% CI 71-78). The model with different detection probabilities for males and females had better support than the null model. In the Sauraha area of Chitwan estimates of the population obtained by block count (77) and ID-based monitoring (72) were within the 95% confidence interval of the estimate from sighting-mark-resighting. We recommend a country-wide block count for rhinoceroses every 3 years and annual ID-based monitoring in a sighting-mark-resighting framework within selected subpopulations. The sighting-mark-resighting technique provides the statistical rigour required for population estimates of the rhinoceros in Nepal and elsewhere.