Acoustic compensation in Indo-Pacific humpback dolphins ( Sousa chinensis ) fails to overcome vessel noise-induced reduction in biosonar detection range

The increasing anthropogenic noise in the oceans has affected odontocetes at multiple levels. Odontocetes are known to employ acoustic mechanisms to compensate for increasing noise, but our understanding of the effectiveness and limitations of these adaptations remains limited. A specific case study was conducted on Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting Xiamen Bay, where the population has declined by 37.5% over the past decade. Concurrently, vessel traffic density increased by 427.7%, exposing the dolphins to substantial acoustic pressure. Field measurements showed that the dolphins increased their echolocation click amplitudes by an average of 8.0 dB in response to vessel-induced noise elevations averaging 14.6 dB. However, these acoustic adjustments proved inadequate to fully overcome vessel noise impacts, leading to a 38.0% reduction in detection range and a 69.8% decrease in effective detection space. Although the dolphins demonstrated adaptive control over their biosonar clicks to cope with vessel noise, this study provided new evidence that vessel noise impaired the dolphins' detection capabilities, raising attention to address the extent of noise tolerance for odontocetes in coastal waters.