Habitat use dynamics of Yangtze finless porpoises in highly developed river reaches under a fishing ban

Reconciling economic development with aquatic wildlife conservation remains difficult in heavily urbanized rivers. The Yangtze finless porpoise ( Neophocaena asiaeorientalis ) is endemic to the Yangtze River basin. The Nanjing Yangtze Finless Porpoise Provincial Nature Reserve lies in an intensively developed reach. We analyzed 20 standardized vessel surveys conducted from 2018 to 2025. We compared population and spatial use patterns among the pre-ban phase from 2018 to 2020, the early-ban phase from 2021 to 2023, and the mid-ban phase from 2024 to 2025. We also used MaxEnt to examine associations between environmental conditions and habitat suitability at three water levels in 2025. The results showed that: (1) Mean sighting rates were 0.128, 0.170, and 0.222 individuals km⁻¹ in the three phases. The rate during the mid-ban phase was 73.4% higher than before the ban and 30.6% higher than during the early-ban phase. However, sighting rate and group size did not differ significantly among phases. (2) Core areas were generally larger after the ban. Compared with the pre-ban phase, core areas during the mid-ban phase increased by 180.6%, 192.0%, and 55.4% under low, mid, and high water level. Core area size differed little between the two phases after the ban, but core locations changed. They shifted from the central bridge reach toward several centers around Qianzhou, Meizizhou, and Zimuzhou. (3) Environmental associations varied with water level. Under low water, fish density, noise, and zooplankton had permutation importances of 28.3%, 26.9%, and 20.9%. Noise ranked first under mid and high water, with values of 54.7% and 49.3%. These findings show that porpoise spatial use changed among fishing ban phases. They also show that present habitat associations varied with water level in a highly developed river reach.