Site Use and Environmental Thresholds of the Asian Small-Clawed Otter (Aonyx cinereus) in the Polluted Ciliwung River, Jakarta

Urban rivers in Southeast Asian megacities are severely degraded, yet some still support unexpected wildlife persistence. This study examined site-use patterns and environmental thresholds of the Asian small-clawed otter (Aonyx cinereus) in the Ciliwung River, Jakarta, Indonesia, one of the region’s most polluted urban waterways. We conducted systematic sign surveys at 60 sites across three river segments representing an urban pollution gradient from Bogor to Jakarta between May and November 2025. Environmental variables, including water quality and substrate heterogeneity, were measured at each site. Otter signs were detected at 33 of 60 sites (55%). Site use declined slightly along the gradient, from 60.0% in the upstream segment to 50.0% in the urban core, although differences among segments were not statistically significant. Total Dissolved Solids (TDS) emerged as the most important predictor of otter presence, with site use declining markedly beyond approximately 194 ppm. Substrate heterogeneity also showed a strong positive association with site use: sites with mixed substrates had 75.0% site use, compared with 41.7% at predominantly concrete sites. Site use was higher in the wet season (68.0%) than in the dry season (45.7%), suggesting seasonal variation in habitat suitability. These findings indicate that A. cinereus can persist in heavily modified urban river systems, but continued persistence likely depends on maintaining structural habitat complexity and improving water quality. The study provides quantitative benchmarks for urban river restoration and supports the relevance of reconciliation ecology in tropical megacities.