Three-decade distribution shifts of the Yangtze finless porpoise in the lower reaches of the Yangtze River

Freshwater cetaceans are among the most endangered animals globally, facing severe threats from habitat loss, fragmentation, and degradation. Quantitative understanding of their long-term distribution shifts under these threats remains limited, hindering effective conservation strategy development. Here, we analyzed three decades (1994–2022) of spatiotemporal distribution patterns for the critically endangered Yangtze finless porpoise ( Neophocaena asiaeorientalis ) within a 270 km high-density section downstream of the Poyang Lake estuary. Despite significant population decline, core and key distribution areas showed no net contraction. However, spatial redistribution occurred: in sections adjacent to the estuary, core distribution patches increased in number and area (from 64.9% to 84.7%), while marginal sections experienced severe reductions (from 35.1% to 15.3%). Fragmentation (decreased patch size, reduced largest patch index, increased inter-patch distance) dominated marginal sections. High site fidelity was observed, with 21.5% of core areas unchanged since 1994 and 70.2% unchanged since 2006, primarily in sections adjacent to the estuary. Group sizes declined significantly (1994: 3.92 ± 2.69; 2006: 1.66 ± 1.03; 2022: 2.08 ± 1.56), indicating smaller social units. Our results suggested that strategies must be section-specific, prioritizing protection in stable core habitats and restoration in fragmented zones. In situ efforts should stabilize high-fidelity core areas while rebuilding degraded habitats, using group size as a key recovery indicator. • Core distribution areas did not notably contract despite population decline, but spatial redistribution occurred. • Habitat fragmentation was only detected in marginal sections far from the Poyang Lake estuary. • Core distribution areas showed high fidelity: 21.5% unchanged since 1994, 70.2% since 2006. • In situ conservation must prioritize protecting stable cores and restoring fragmented zones, adapting to each section.