Hearing test of a Melon-headed whale stranded on July, 2021. (Image by IHB)  

A cetacean stranding event occurs when an animal, such as a whale, dolphin, or porpoise is unable to return to the water and when an animal in the water is unable to return to its natural habitat without assistance, or when an animal strands on the beach or in the water dead. The term “mass stranding” refers to two or more individuals stranded in the same area at the same time.    

In a study published in Ecotoxicology and Environmental Safety, a research group led by Prof. WANG Kexiong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences found the severe hearing loss in the stranded melon-headed whale (Peponocephala electra) was probably caused by transient intense anthropogenic sonar or chronic shipping noise exposures.   

Between June and October 2021, highly concentrated live mass stranding events of dolphins and whales happened on the eastern coast of China. To find the reason for the stranding, the researchers used non-invasive auditory evoked-potential techniques to investigate the hearing threshold of a stranded melon-headed whale at a frequency range between 9.5 and 181 kHz.   

They found that at the frequency range from 10 to 100 kHz, hearing thresholds for the whale were between 20 and 65 dB higher than those of its phylogenetically closest species, Pygmy killer whale.    

Using a Gompertz growth model, the researchers calculated the age of the stranded whale to be 19, which implies age-related hearing loss may not have a big impact on this animal. They further excluded the possible natural factors such as the lunar cycle and violent storms that can cause whale stranding.   

The researchers then focused on anthropogenic activity, such as fishing, shipping and seismic surveys, which are intense in the places where the mass stranding events occurred. Seismic surveys can generate transient intense noise, while fishing and shipping can cause chronic noise, both of which can be risk factors for hearing loss in cetaceans.    

They thus proposed that the severe hearing loss in the stranded melon-headed whale was probably caused by transient intense anthropogenic sonar or chronic shipping noise exposures.   

Based on the results, Wang’s team concluded that the hearing loss could have been the main factor leading to the observed live mass stranding events. They recommended that noise monitoring and relevant noise mitigation measures be implemented to reduce anthropogenic noise pollution exposure levels for marine mammals in the coastal areas of China, and that specialized marine mammal health personnel, stranding response teams and networks be established and specially trained.   

There should also be standardized protocols for marine mammal stranding responses, so appropriate human interventions such as medical attention, guidance seaward, or rehabilitation at an appropriate facility can be carried out promptly to enhance animal survival. 

(Editor: MA Yun)