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Inter-annual decrease in pulse rate and peak frequency of Southeast Pacific blue whale song types

Abstract : A decrease in the frequency of two southeast Pacific blue whale song types was examined over decades, using acoustic data from several different sources in the eastern Pacific Ocean ranging between the Equator and Chilean Patagonia. The pulse rate of the song units as well as their peak frequency were measured using two different methods (summed auto-correlation and Fourier transform). The sources of error associated with each measurement were assessed. There was a linear decline in both parameters for the more common song type (southeast Pacific song type n.2) between 1997 to 2017. An abbreviated analysis, also showed a frequency decline in the scarcer southeast Pacific song type n.1 between 1970 to 2014, revealing that both song types are declining at similar rates. We discussed the use of measuring both pulse rate and peak frequency to examine the frequency decline. Finally, a comparison of the rates of frequency decline with other song types reported in the literature and a discussion on the reasons of the frequency shift are presented. Blue whale (Balaenoptera musculus) songs are the repetition of several highly stereotyped low-frequency, high energy units that compose song phrases, first described in 1971 1. Song units and phrases have been qualified as'remarkably consistent' within a song, but also between individuals 1. Song in blue whales has been attributed to reproductive display by males 2. Numerous, distinct songs have been identified worldwide 3 , each displaying stability in the temporal and frequency characteristics of units and phrases and intervals between units or phrases. However, this global pattern has been shown to be affected by a general decreasing trend in frequency over dec-adal timescales 4. This linear decline in tonal frequencies of blue whale song types is a recently described unexplained phenomenon. It appears to occur worldwide, based on analyses of different regional song types, spanning five decades 4. New studies have recently confirmed these results for Antarctic blue whale song type 5-7 for the southwestern Pacific Ocean song type 8 , or for different song types in the Indian Ocean 7,9,10. So far, no studies have examined frequency shift in southeastern Pacific Ocean blue whale songs, even though these were the first blue whale songs to be identified as such 1. A similar inter-annual frequency decrease has been recently measured for bowhead whale (Balaena mysticetus) 11 and fin whale (Balaenoptera physalus) populations 7,12 and in other low frequency sounds attributed to unidentified baleen whales 6,13. There are two blue whale song types in the southeast Pacific Ocean known as SEP1 and SEP2. SEP1 was first described almost fifty years ago 1 , while SEP2 was first recorded in 1996 14 near the Equator and described in detail as a new song type in 2014 15,16. In recent times, SEP2 has been found to be the dominant song type in the Chilean
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Julie Patris, Susannah Buchan, Kathleen Stafford, Ken Findlay, Rodrigo Hucke-Gaete, et al.. Inter-annual decrease in pulse rate and peak frequency of Southeast Pacific blue whale song types. Scientific Reports, Nature Publishing Group, 2020, ⟨10.1038/s41598-020-64613-0⟩. ⟨hal-02586669⟩



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