Recent bioacoustics researches on biosonar, hearing, and noise effect on the Indo-Pacific humpback dolphins (Sousa chinensis)

Abstract

The Indo-Pacific humpback dolphins (Sousa chinensis) are widely distributed in the coastal waters of the tropical and sub-tropical ocean in Asia, including southeast China. Conservation of the humpback dolphins in Chinese waters has been on the agenda of local scientific and conservation communities since the 1980s, despite little research including bioacoustics research has been conducted. Our recent bioacoustics studies indicated that the biosonar clicks of the wild humpback dolphins were short-duration, broadband, ultrasonic pulses, similar to those produced by other whistling dolphins of similar body size. However, their click source levels with an average of around 190 dB re: 1μ Pa in peak-to-peak, appear to be much lower than those of other whistling dolphins. Hearing sensitive frequency range of the humpback dolphins is generally higher than 5 kHz and lower than 120 kHz with possible age-related hearing loss for old dolphins. The humpback dolphin could therefore be characterized as a mid-frequency cetacean, which operates sounds in mid- to high-frequency range. Any sufficiently intense sounds with mid- to high-frequency components could have deleterious effects on the humpback dolphins through interference on animals’ behavior and with the animals’ ability to detect signals from conspecifics, and echoes of echolocation clicks.