Estimating occupancy and population size from repeated counts and detection history of camera traps for saltwater crocodiles in Myanmar

Saltwater crocodiles Crocodylus porosus are regarded as critically endangered in Myanmar because they are limited to the Meinmahlakyun Wildlife Sanctuary (MKWS) in the Ayeyarwady delta region. Little contemporary data exists on their distribution and population size which hinders effective conservation and management. We conducted standardized spotlight surveys and camera trap surveys along the rivers inside the MKWS, and two nearby reserved forests. By addressing biases in detectability, we used maximum-likelihood and Bayesian approaches (1) to find out occupancy and population parameters of saltwater crocodiles, and (2) to assay abiotic and anthropogenic factors affecting it. We used single-season occupancy models, Hierarchical N-mixture models, a Spatial Count model, and the relative abundance index to estimate population sizes. We applied two-phase adaptive sampling which is designed to focus on occupancy surveys over a wide area of interest in the first season (phase I during February to May) and abundance surveys sticking to the areas of high predicted occupancy in the second season (phase II during October to February). The dataset includes the counts of adult, sub-adult, juvenile and Eye shine only (EO) crocodiles detected during the repeated spotlight surveys, detection history from camera traps and environmental variables collected during surveys. Population size estimates inside the MKWS were 75, 58 and 68 respectively from spotlight index, N-mixture, and SC models. Saltwater crocodiles were more likely to occupy waterways inside MKWS than the reserved forests, and preferred narrow waterways with low salinity, and no human settlements. Our result based on spotlight relative abundance index indicates a stable population size of c.106 when compared with the estimate of Thorbarnarson et al. (2000), who reported that less than 100 individuals were present in MKWS, and therefore, future surveys are recommended to monitor this trend.