Seasonal Trends and Diel Patterns of Downsweep and SEP Calls in Chilean Blue Whales

Abstract

To learn more about the occurrence and behaviour of a recently discovered population of blue whales, passive acoustic data were collected between January 2012 and April 2013 in the Chiloense ecoregion of southern Chile. Automatic detectors and manual auditing were used to detect blue whale songs (SEP calls) and D calls, which were then analysed to gain insights into temporal calling patterns. We found that D call rates were extremely low during winter (June–August) but gradually increased in spring and summer, decreasing again later during fall. SEP calls were absent for most winter and spring months (July–November) but increased in summer and fall, peaking between March and April. Thus, our results support previous studies documenting the austral summer residency of blue whales in this region, while suggesting that some individuals stay longer, highlighting the importance of this area as a blue whale habitat. We also investigated the daily occurrence of each call type and found that D calls occurred more frequently during dusk and night hours compared to dawn and day periods, whereas SEP calls did not show any significant diel patterns. Overall, these findings help to understand the occurrence and behaviour of endangered Chilean blue whales, enhancing our ability to develop conservation strategies in this important Southern Hemisphere habitat.

Temporal Trends and Effects of Noise on Upsweep Calls of Eastern South Pacific Southern Right Whales

“…During this active foraging time they do not vocalize (Parks et al, 2012). Diel trends in ESPSRW upsweep calling rate were also consistent with diel patterns of 'D' call production by sympatric Chilean blue whales (Balaenoptera musculus; Redaelli et al, 2022). Redaelli et al (2022) found that D calls are more numerous during dusk and night than dawn or daytime hours, suggesting that the whales might be foraging at depth for denser aggregations of prey during daylight hours and producing D calls to maintain contact with conspecifics more at dusk and night.…”

“…Increased levels of ship noise during day and dusk could be due to fishing activity during those light phases, with ships traveling through the area where the recording device was located during the day and potentially returning to shore at dusk. In an analysis of a larger data set from the Chiloense ecoregion that included the data analyzed for this study, Redaelli et al (2022) found that blue whale D calls were produced primarily in dusk and night hours while songs (SEP calls) did not exhibit diel patterns. In our dataset, SEP calls were extremely common in minutes classified as high noise.…”

“…Prior to this study, it was unknown whether ESPSRW calls exhibited diel patterning. Some cetacean species are known to call more at night, such as those who call to maintain contact with conspecifics when visual cues are not available (common dolphins, Delphinus delphis: Goold, 2000; blue whales, Balaenoptera musculus: Redaelli et al, 2022;Wiggins et al, 2005), while others are known to call more during daylight hours, such as those who socialize during the day and feed during night hours (sei whales, Balaenoptera borealis: Baumgartner and Fratantoni, 2008).…”

Preliminary characterization and diel variation of fin whale (Balaenoptera physalus) downsweep calls off Isla Chañaral, northern Chile

“…Širović (2016) found that the final result of detections from the same data set can be affected by the analyst's subjectivity in identifying vocalizations due to each analyst selecting signals according to their personal criteria and experience. The presence of blue ( B. musculus ) and sei ( B. borealis ) whales (which have occasionally been seen in the north of Chile) around Isla Chañaral makes it difficult to identify fin whale calls, but an adjustment in window size and careful checking of signals allows for better identification (Acevedo et al, 2017; Ou et al, 2015; Redaelli et al, 2022; Širović et al, 2016).…”

“…Manual detection of whale vocalizations represents a challenge for researchers in the field of bioacoustics because the selection of each signal is a slow process and influenced by analyst subjectivity. Širovi c (2016) found Chañaral makes it difficult to identify fin whale calls, but an adjustment in window size and careful checking of signals allows for better identification (Acevedo et al, 2017;Ou et al, 2015;Redaelli et al, 2022;Širovi c et al, 2016).…”