Low-frequency sound level in the Southern Indian Ocean

Abstract

This study presents long-term statistics on the ambient sound in the Southern Indian Ocean basin based on 2 years of data collected on six widely distributed autonomous hydrophones from 47°S to 4°S and 53°E to 83°E. Daily mean power spectra (10-100 Hz) were analyzed in order to identify the main sound sources and their space and time variability. Periodic signals are principally associated with the seasonal presence of three types of blue whales and fin whales whose signatures are easily identified at specific frequencies. In the low frequencies, occurrence of winter lows and summer highs in the ambient noise levels are well correlated with iceberg volume variations at the southern latitudes, suggesting that icebergs are a major sound source, seasonally contributing to the ambient noise, even at tropical latitudes (26°S). The anthropogenic contribution to the noise spectrum is limited. Shipping sounds are only present north and west of the study area in the vicinity of major traffic lanes. Acoustic recordings from the southern sites may thus be representative of the pristine ambient noise in the Indian Ocean.

Long‐Term and Seasonal Changes of Large Whale Call Frequency in the Southern Indian Ocean

“…As a proxy for the ice‐related noise, Figure displays the number per week of free icebergs, smaller than 8 km 2 , detected from satellite altimetry (Tournadre et al, ) in the Indian sector of the Southern Ocean (30° E–110° E, up to 40° S). In a wider geographic window (30° E–150° E) at a monthly time scale, Matsumoto et al () measured a correlation coefficient of 0.84 between the 30‐ and 36‐Hz noise at Cape Leeuwin and the iceberg volume (see also Tsang‐Hin‐Sun et al, ). For the whole time series in Figure , the Pearson correlation coefficient between the number of icebergs and the 30‐ to 33‐Hz noise level equals 0.6.…”

“…The advent of passive acoustic monitoring of the ocean in the last decades has improved our knowledge of the oceanic acoustic environment. Along with the abiotic (e.g., sea surface process, earthquakes, and volcanic activity) and anthropogenic (e.g., shipping and seismic exploration) sources, the biotic sources, and especially the marine mammal sounds, greatly contribute to this oceanic acoustic environment (Menze et al, ; Miksis‐Olds et al, ; Tsang‐Hin‐Sun et al, ; Wenz, ; Wilcock et al, ). Vocalizations of large baleen whales dominate the low‐frequency range of many recordings in various areas (Dziak et al, ; Haver et al, ; McDonald, Hildebrand, et al, ; Menze et al, ; Širović et al, ; Tsang‐Hin‐Sun et al, ).…”

“…Along with the abiotic (e.g., sea surface process, earthquakes, and volcanic activity) and anthropogenic (e.g., shipping and seismic exploration) sources, the biotic sources, and especially the marine mammal sounds, greatly contribute to this oceanic acoustic environment (Menze et al, ; Miksis‐Olds et al, ; Tsang‐Hin‐Sun et al, ; Wenz, ; Wilcock et al, ). Vocalizations of large baleen whales dominate the low‐frequency range of many recordings in various areas (Dziak et al, ; Haver et al, ; McDonald, Hildebrand, et al, ; Menze et al, ; Širović et al, ; Tsang‐Hin‐Sun et al, ). Blue and fin whales indeed emit stereotyped calls of low frequency (<100 Hz) and high intensity (∼180 dB re 1 μPa 2 /Hz; McDonald et al, ; Samaran, Guinet, et al, , Širović et al, ) that propagate over long distances (up to several hundreds of kilometers; Gavrilov & McCauley, ; Samaran et al, ; Širović et al, ).…”

Seasonal and Diel Vocalization Patterns of Antarctic Blue Whale (Balaenoptera musculus intermedia) in the Southern Indian Ocean: A Multi-Year and Multi-Site Study

“…in April 2012 and October-November 2014 at site NCRO, or April 2010 at SWAMS), the levels of noise are fairly constant throughout the year at each site, which ensures that variations in Z-call detection are not artifacts of the ambient noise level. A further analysis of the ambient noise level can be found in [39]. …”

“…A clear higher level in the ambient noise is observed at the RAMA station, in the Central Indian Basin, than in the rest of the OHASISBIO sites; it is likely due to a greater contribution of shipping noise at these latitudes [39]. Contrary to what is expected and generally observed [42], the deep water ambient noise level measured at our stations in the 40–60 Hz frequency band is decreasing from 2010 to 2015, especially at MAD and NEAMS sites.…”