Annual decrease in pulse rate and peak frequency of Southeast Pacific blue whale song types since 1970

We examined the decrease in the frequency of the dominant southeast Pacific blue whale song type (SEP2) from 1996 to 2017, using acoustic data from five different sources 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). We assess the sources of error associated with each measurement. There was a linear decline in both parameters for SEP2. A shorter comparative analysis also showed frequency decline in the scarcer southeast Pacific song type (SEP1) between 1970 to 2014, revealing that both SEP1 and SEP2 are declining at similar rates. We discuss the use of measuring both pulse rate and peak frequency to examine frequency decline. We also compare the rates of frequency decline with other song types reported in the literature.