UC San Diego

Blue whales, Balaenoptera musculus, are the largest animals on Earth and yet there is much unknown about their life history. Blue whales produce low frequency sounds including short (1-4 s) down-sweeping D calls, commonly associated with foraging. This study used long term passive acoustic monitoring to investigate a new sequence of D calls observed in data collected from 2007 to 2020 in Southern California and localized sequences on July 22nd 2019. On that day, 28 sequences occurred in a two hour and twenty-minute period. The D calls during this time had an average 7.3 s +/- 1.7 s inter-call interval within a sequence and 7 calls +/- 1 call per sequence. In the data from 2007 to 2020, sequences were present in all the years with a peak in sequences per day in 2019 with the maximum number of sequences per year also occurring in 2019. To understand how sequences were produced spatially, the D calls on July 22nd 2019 were localized. This resulted in 194 locations for all D calls and an average location for each sequence. Average locations were scattered within a 250 m by 350 m area, indicating that an individual whale may be producing the sequences. Using the location for each call, source level was computed using received level and transmission loss. The average root mean square source level calculated over 30 to 80 Hz was 162.3 +/- 2.2 dB re 1µPa at 1 m. By combining both observations of calling behavior and localizations, we can gather insight into the temporal and spatial behavior of blue whales when emitting D calls.