Characterization of fin whale song off the Western Antarctic Peninsula

Abstract

Song is produced by a variety of terrestrial and marine animals and is particularly common among baleen whales. Fin whale (Balaenoptera physalus) song is comprised of relatively simple 20 Hz pulses produced at regular intervals. The timing of these intervals, in addition to the presence and frequency of overtones, appears to be unique to each population. The purpose of this study was to characterize Western Antarctic Peninsula fin whale song and describe temporal pattern variations in song type and occurrence. Recordings were collected in the area from 2001-2004 and again 2014-2016. One song type was identified with a primary inter-pulse interval (IPI) of approximately 14 s and secondary IPI of 12.5 s. This song occurred in three pattern variants: singlet, doublet, and long triplet. The interval between pulses increased by 1.5 s between recording periods while the frequency of the overtones decreased from 89 Hz to 86 Hz. Song was never recorded in August and while it was recorded at other times in some years, it was consistently present in recordings from April through June across all years. While multiple pattern variants were present each year, singlets were generally the most prevalent variant. Doublets and triplets occurred from February through June, with highest levels of variants in February. In later years the triplet variant presence increased and in 2016 it comprised 53% of recorded song bouts. Further research is needed to understand the reasons why song changes over time and to examine the feasibility of using song to delineate and identify populations.

Fig 1. Study area map.

Acoustic recorder deployment sites: Site 1 (S1, red) was used 2001–2003, Elephant Island (EI, purple) in 2003 & 2014, South Shetland Island (SSI, green) in 2015, and Elephant Island East (EIE, blue) in 2016. Inset map in top left corner shows study area in red square. Map images were obtained from USGS National Map Viewer.

Fig 2. WAP fin whale song.

Two-minute spectrograms recorded in the vicinity of South Shetland Island on April 2, 2015, depicting patterned fin whale 20 Hz pulses with accompanying overtones. The spectrograms display all three recorded song variants: singlet (top), doublet (middle), and triplet (bottom). The spectrograms were calculated using 2000-point FFT, 90% overlap and Hanning window.

Fig 3. Daily median fin whale song inter-pulse interval (IPI).

Plot of daily median song IPI, with error bars representing first & third quartiles. Each datapoint summarizes the song from a randomly selected date from each half-month. Singlets are in pink, doublet variants in green, and triplets in blue. Gray-shaded areas indicate breaks in recording effort. Dates on the x-axis are reported in mm/dd/yy format.

Fig 4. Annual mean overtone frequency.

Mean overtone frequency (circles; in Hertz) and standard deviation (bars) of fin whale pulses for each year analyzed. Red line represents linear trend. Gray-shaded area indicates a break in recording effort.

Fig 5. Monthly song variant proportions.

Proportion of pulses by month over the course of a year (September to August) belonging to each call variant. Singlets are in pink, doublets in green, and triplets in blue in each stacked bar.

Fig 6. Annual song variant proportions.

Proportion of pulses per annual recording effort belonging to each call variant. Singlets are in pink, doublets in green, and triplets in blue.