Severe Hearing Loss in the World's First Successfully Captive‐Born Yangtze Finless Porpoise: Impact of High Underwater Sound Exposure and Congenital Hearing Disorders

Aquariums globally have seen significant growth in recent decades. However, persistent negligence exists concerning underwater sound pollution in aquariums and its impact on cetaceans. Here, the audiogram of Taotao, the world's first successfully captive-born and bred Yangtze finless porpoise, and the underwater sound levels in the Baiji Aquarium at the Institute of Hydrobiology, Chinese Academy of Sciences were examined. In contrast to the previously published U-shaped audiograms of the Yangtze finless porpoise, Taotao's audiogram exhibited a W-shaped pattern. Additionally, the audiogram of Taotao was, on average, 42 ± 15 dB (mean ± SD) higher than that of other non-aquarium-born Yangtze finless porpoises in the Baiji Aquarium and 43 ± 11 dB higher than that of wild Yangtze finless porpoises, particularly in the 20-60 kHz range and at 90 kHz. The underwater sound spectra in the Baiji Aquarium do not account for the notches observed in the porpoise's audiogram below 60 kHz, suggesting that congenital hearing disorders may be the cause of Taotao's hearing loss in these frequency bands. In contrast, the cumulative weighted broadband sound exposure levels of underwater sound within the aquarium (mean: 162 dB) exceeded the temporary threshold shift onset threshold for non-impulsive noise (153 dB) and the permanent threshold shift onset threshold for impulsive noise (155 dB) in finless porpoises. The high levels of underwater sound exposure, particularly from conspecific porpoises, highlight the need for increased focus on the welfare of captive animals.