Fin whale 40‐Hz calling behavior studied with an acoustic tracking array

Fin whales (Balaenoptera physalus), the second largest baleen whale and an endangered species, occur worldwide and are found in all major oceans (Reilly et al., 2013). Fin whales produce two main types of stereotypical, frequency downswept, high-amplitude, short duration (<1 s) calls, that are known by their primary frequencies as “20-Hz” and “40-Hz” calls (Širovi c, Williams, Kerosky, Wiggins, & Hildebrand, 2013; Watkins, 1981). The 20-Hz calls may have reduced bandwidth of ~1 Hz and also may include higher frequency components at 135–140 Hz (Castellote, Clark, & Lammers, 2012). The 20-Hz calls are the most commonly reported fin whale sound, and may serve a social purpose for establishing and maintaining contact when produced in irregular sequences (Edds-Walton, 1997) or as a reproductive function when produced by males in regular sequences forming song (Croll et al., 2002). The 40-Hz call occurs in irregular sequences and is more common during the summer, but its social context is not well understood (Širovi c et al., 2013; Watkins, 1981). Fin whale dive behavior has been described as two types: short dives of 2–6 min and longer dives of 6–14 min or longer (Goldbogen et al., 2006; Stimpert et al., 2015; Watkins, 1981). Both the 40-Hz call, and single 20-Hz calls, are produced more often during long dives (Stimpert et al., 2015; Watkins, 1981), and at times when several whales are diving near each other. During these dives, sequences of 5–10 calls are produced, apparently by more than one animal, based on the character of the calls. Typically, calls are not produced when fin whales are at the surface, leading to longer time gaps between successive calls in a call sequence. The 20-Hz call source sound pressure level (SL) has been reported as high as 189 dB re 1 μPa at 1 m (rms) (Širovi c, Hildebrand, & Wiggins, 2007; Weirathmueller, Wilcock, & Soule, 2013), allowing the call to be detected at long distances (10s of km) and to be localized and tracked using passive acoustic monitoring (PAM) techniques (McDonald, Hildebrand, & Webb, 1995; Soule & Wilcock, 2013; Varga, Wiggins, & Hildebrand, 2018; Weirathmueller et al., 2013; Wilcock, 2012). Call localization allows SLs to be estimated, an important parameter for estimating detection probability and population densities from PAM using distance sampling techniques (Hildebrand et al., 2015; Marques, Thomas, Ward, DiMarzio, & Tyack, 2009), and successive localizations result in tracks, providing Received: 11 March 2019 Accepted: 10 January 2020