Spatially explicit estimation of the Bawean deer population using antler‐based male identification and sex and age ratios

Abstract The Bawean deer ( Axis kuhlii ), endemic to Bawean Island, Indonesia, is one of the world's most isolated ungulates. This study aimed to 1) identify individual males using antler morphology, 2) estimate the male population using capture–recapture modeling, and 3) estimate the number of females and fawns based on their relative frequencies in independent video detections. We deployed 30 camera traps in protected areas (45.57 km 2 ) over 276 days (January–October 2023), accumulating 8,280 trap nights. Using the Jolly–Seber–Schwarz–Arnason model with recruitment (JSSAsecrf) in the openCR framework, we evaluated 3 detection functions and selected the model with the lowest Akaike's Information Criterion corrected for small sample size. To incorporate uncertainty from both spatially explicit capture‐recapture (SECR)–based male estimates and demographic ratios, we calculated confidence intervals for female, fawn, and total population sizes using nonparametric bootstrapping with 5,000 iterations. We identified 45 individual males at 12 sites. The most parsimonious model estimated male density at 2.44 ± 0.47 deer/km 2 , corresponding to 111.19 (95% CI = 76.45–161.72) males. The total population was estimated at 250.39 deer (95% CI = 156.46–352.97), including 118.97 (95% CI = 72.92–173.67) females and 20.23 (95% CI = 11.50–32.11) fawns. While male survival was high (0.93 ± 0.02), recruitment was low (0.06 ± 0.02), as was the fawn‐to‐doe ratio, with ~17 fawns per 100 does. These findings suggested population stability, with estimates slightly higher than those from previous assessments, but persistently low recruitment raises concerns about the long‐term viability of the Bawean deer. Management strategies should prioritize enhancing fecundity and fawn survival to support the population's future persistence.