Recordings of Blue Whales sound with single hydrophone and Ecological Acoustic Recorders (EARs)
In courtesy of Iversen, M., Rasmussen, M. H., Magnúsdóttir, E. E. and Lammers, M. O.
Blue whale calls were recorded from the 21st to 23rd of June 2009 with a single hydrophone and Ecological Acoustical Recorders (EARs). Fifteen (144 min.) out of 94 recordings made with the single hydrophone and 21 recordings (31.2 min.) with EARs contained blue whale sound. These recordings were analyzed both aurally and with BatSound Pro software and all files were amplified and filtered. Spectrograms of blue whale calls were done using a 1024-point FFT, 85% overlap, and Hanning window. A total of 163 blue whale sounds were analysed and revealed 11 different sounds types, most in the audible range. Both the single hydrophone and the EARs captured four of the more common sound types; however, the single hydrophone recorded two other common sounds and one only with the EARs. Unfortunately, we had no recordings were the two recording methods were used simultaneously. All blue whale sound frequencies were comparable to previously reported frequencies on the species and several sound types were directly comparable to sounds recorded in the St. Lawrence estuary by Berchok et al. (2006).
Blue whales have a large geographical distribution and previous studies have found that at least 10 distinct regional calls remain stable over decades. As a result, sound has been suggested to be a good indicator of subdivisions of the species(5). Blue whales have been observed feeding in June and July in Skjálfandí Bay, Iceland since 2004(2, 4). We aim to compare blue whale sounds recorded with two different methods, single hydrophone and Ecological Acoustic Recorders, to see if those two methods could be combined in future analysis on blue whale sound production and occurrence in Skjálfandi Bay.
Materials & Methods
The recordings were done with a single hydrophone and with Ecological Acoustic Recorders (EARs) in Skjálfandí Bay (65°57’18”N, 017°25’31”W) in Iceland during the 21st to 23rd of June 2009. The single hydrophone recordings were supported by visual observations and photo-identification. Table 1 summarizes the methods.
All files were amplified, low-pas and analyzed both aurally and using BatSound Pro software. Spectrograms of blue whale calls were done using a 1024-point FFT, 85% overlap, and Hanning window. Spectrogram correlation algorithm was used for finding blue whale downswept sounds on the EARs whereas the sound files from the single hydrophone were all manually scanned.
A total of 163 blue whale sounds were analysed and revealed 11 different sounds types, most in the audible range (table 2). Nine of these sound types were recorded by the single hydrophone and six by the EARs.
To analyze the occurrence and sound production of blue whales in a new area, it is very practical to combine these two methods. The recordings from a single hydrophone can reveal the variety of sound types produced by the whales as well as collecting greater quality of recordings due to closer proximity to the animals. This information creates basis for the construction of detection algorithms used for finding sound of interest in a long-term recordings within the same area. The blue whale sounds recorded in the Bay contained mainly frequencies between 19 to 261 Hz. These frequency ranges are comparable to blue whale frequencies worldwide (e.g. 9) and the moderate downswept calls, grunts, bubbling and blurps observed in this study corresponds roughly to the previously reported calls in the Northeast Atlantic by Berchok et al. (2006). However, the downswept calls are seen more or less worldwide (e.g. 6; 8 & 10).
1) Húsavík Research Center, University of Iceland, Hafnarstrett 3 , 640 Húsavík, Iceland, 2) Danish Institute for Study Abroad, Science & Health department, Vestergade 4.7, 1456 Copenhagen K, Denmark,) Hawaii Institute of Marine Biology, University of Hawaii at Manao, P. O. Box 1106, Kailua, HI 96834, US
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Dansk-Islands fond, DIS, Oticon, Peter Teglberg Madsen, Gísli A. Víkingsson, Tryggvi Finnsson, Viola Palvona , Lee Millerand and Line Reinstrup,