Soundscape characteristics of the Eastern Taiwan Strait Indo-Pacific humpback dolphin habitat

Continuous underwater acoustic recordings were made at two locations of the Eastern Taiwan Strait humpback dolphin habitat: Yunlin (YL, near a cargo shipping lane and with water depth 8-12 m) and Waisanding (WS, relatively pristine with water depth 1.5-4.5 m), off western Taiwan between June and December 2012. One-minute averaged sound pressure levels (SPLs) in both locations were computed for data collected in July 13-30 and September 2-20 in the following four frequency bands: 150-300 Hz, 1.2-2.4 kHz, 3-6 kHz, and >10 kHz. These frequency bands correspond to sounds in frequency bands from cargo vessels, croaker fish chorus, dolphin whistles, and dolphin echolocation clicks, respectively. Comparisons of median SPLs in these frequency bands between WS and YL were made among four discrete periods of a day: morning (04:00-09:59), daytime (10:00-15:59), evening (16:00-21:59), and nighttime (22:00-03:59). The results show notably elevated sound levels in the 1.2-2.4 kHz frequency band in both locations between sunset and sunrise. This indicates that croaker fish chorus are the dominant sources of the soundscape during the evening and nighttime periods. Although the SPLs in low-frequency band of 150-300 Hz were also high at times, there is no temporal pattern on the occurrence of these high levels. Closer examination of the occurrence of high SPLs in the 150-300 Hz frequency band shows that they did not always correspond to cargo vessel passage. It is hypothesized that flow noise and other hydrodynamic actions may contribute to the high SPL in this frequency band. This study highlights complexity of the soundscape in a coastal marine ecosystem. The soundscape characteristics depend on the SPLs of different contributing sources, and are also highly influenced by the local bathymetry and the dynamics of the physical environment. In order to conduct a comprehensive assessment on the potential anthropogenic noise effects on marine life, it is important to have a good understanding of all sound sources and the soundscape characteristics of the marine environment.