Monitoring over two decades shows declining population size and distribution range for a Critically Endangered population of Irrawaddy dolphins in the Mahakam River, Indonesia

The Critically Endangered Irrawaddy dolphin Orcaella brevirostris population in the Mahakam River, Indonesia, experienced substantial declines in both abundance and distribution between 1997 and 2023. Until 2007, the population ranged across 523 km of river, including 243 km of the Mahakam mainstem and 280 km of large tributaries. From 2010 until 2023, following major habitat alterations, the range declined by 180 km (34%), comprising a 72 km upstream- and an 8 km downstream retraction, and the loss of 100 km of tributary habitat. Population size estimated through mark-recapture models of photo-identified individuals declined from 84 individuals (CV = 12%) in 2005 to 63 (CV = 4%) in 2023. The mean population birth interval, a first record for wild O. brevirostris , was 3.5 yr, with an annual birth rate of 0.29 per mature female. Mean annual birth and mortality rates between 2017 and 2023 were 0.08 and 0.07, respectively, with 71% of recorded deaths of known cause attributed to gillnet entanglement. Mean annual population recruitment rate in this period was 0.05. Acoustic deterrents introduced in 2020 have reduced bycatch mortality, but current removal rates still exceed sustainable thresholds. Widespread habitat degradation and prey depletion—driven by palm oil plantation expansion, pollution, and destructive fishing practices—pose further significant threats. Urgent conservation action is required to reduce mortality to sustainable levels, including promoting dolphin-safe fishing gear, expanding community patrols, protecting riparian habitats, and enforcing laws against poisoning, electrofishing, and pollution.