Underwater noise in the Xinluo reserve: A potential stressor for the critically endangered Yangtze finless porpoise

Nature reserves in the Yangtze River, China, are crucial for protecting the Yangtze finless porpoise ( Neophocaena asiaeorientalis asiaeorientalis ) and their habitats. Despite a 10-year fishing ban along the middle and lower reaches of the Yangtze River instated on January 2020 to alleviate pressures on the porpoise's prey, other stressors, particularly underwater noise, continue to impact these rare cetaceans, which rely heavily on vocalization for prey detection, communication and navigation. Between September 2020 and September 2021, underwater noise monitoring was carried out in the Xinluo Baiji National Nature Reserve, located in Hubei province, China, with the aim of assessing the potential impact of underwater noise pollution on the Yangtze finless porpoise. The reserve was divided into 30 sub-sections to systematically monitor underwater noise across the reserve. Passive acoustic monitoring was conducted at each site within these sub-sections during four sampling periods, and these data were subjected to comparative analysis with data collected in 2012 and 2017. The results showed that the root mean square sound pressure levels at each site ranged from 117 to 162 ​dB. Power spectral levels (11.2–152 ​kHz) exceeded the average audiogram levels for the porpoise at most sites. 94.2% and 93.3% of the sites exceeded underwater acoustic thresholds known to induce a hearing threshold shift for the porpoise in terms of both the averaged cumulative unweighted sound exposure level, and cumulative weighted sound exposure level respectively. Noise levels in this section during 2020–2021 were approximately 25 and 30 ​dB higher compared to the levels recorded in 2012 and 2017, and likely due to increased shipping activity and the proximity of sampling sites to the shipping channel. The rise in noise levels might contribute to the observed declines in the porpoise population within this area. To mitigate these impacts, implementing noise control measures is essential, such as regulating navigation speed and limiting the number of vessels in the reserve.