Roaring and repetition: How bowhead whales adjust their call density and source level (Lombard effect) in the presence of natural and seismic airgun survey noise

Abstract

Over 500 000 automated and manual acoustic localizations, measured over seven years between 2008 and 2014, were used to examine how natural wind-driven noise and anthropogenic seismic airgun survey noise influence bowhead whale call densities (calls/km2/min) and source levels during their fall migration in the Alaskan Beaufort Sea. Noise masking effects, which confound measurements of behavioral changes, were removed using a modified point transect theory. The authors found that mean call densities generally rose with increasing continuous wind-driven noise levels. The occurrence of weak airgun pulse sounds also prompted an increase in call density equivalent to a 10–15 dB change in natural noise level, but call density then dropped substantially with increasing cumulative sound exposure level (cSEL) from received airgun pulses. At low in-band noise levels the mean source level of the acoustically-active population changed to nearly perfectly compensate for noise increases, but as noise levels increased further the mean source level failed to keep pace, reducing the population's communication space. An increase of >40 dB cSEL from seismic airgun activity led to an increase in source levels of just a few decibels. These results have implications for bowhead acoustic density estimation, and evaluations of the masking impacts of anthropogenic noise.

A comparison of three methods for estimating call densities of migrating bowhead whales using passive acoustic monitoring

“…6). For S307 and S309, the pattern was different in that the SECR detection functions were unreasonably flat-unreasonable because we do not expect to detect bowhead whale calls at 30 km with probability ~ 0.9 (Thode et al 2020)-and estimates of SECR were extremely large. The proportion plot for S107 (Fig.…”

“…For PS and DS, data were truncated at w PS = 4 km and w DS = 30 km, respectively. Previous work (Blackwell et al 2015;Thode et al 2020) concluded that 80% of all whale calls are detected within 3.5 km radius of a sensor, regardless of their source level (how loud they were). We assumed that no calls could be detected from beyond 200 km and, hence, set w SECR = 200 km.…”

“…1). Pomerleau et al (2011) showed that the mean dive depth of bowhead whales does not exceed 100 m. As the difference between this and the sensor depth is much smaller than the distance that bowhead whales can be detected from (e.g., Thode et al 2020), we ignore depth and use horizontal space in the following (Barlow and Taylor 2005). For these localized calls, the distance to the detecting sensors can be determined (Table 2).…”

Limited vocal compensation for elevated ambient noise in bearded seals: implications for an industrializing Arctic Ocean

“…In order to understand the risks associated with changing Arctic noise conditions, it is essential to identify biologically relevant noise thresholds that have the potential to influence critical behaviours [30,31]. In the context of this study, we define a biologically relevant noise threshold as one that has the demonstrated potential to impair acoustically and contextually mediated biological activities (e.g.…”

The effects of noise on the vocal behaviour of singing humpback whales (Megaptera novaeangliae)

“…This was done by first excluding periods of time when vessels were observed in the study area, then excluding recordings where boat noise was audible or could be seen in the spectrograms, and finally checking that the power spectral density of the noise fitted the wind noise curves described by Wenz (1962) and Cato (2012). Similar results were also obtained by Thode et al (2020) who found a Lombard effect in bowhead whales (Balaena mysticetus) in response to wind-dominated ambient noise. The study was conducted in the Beaufort Sea, where the authors were able to exclude the presence of anthropogenic noise due to the scarcity of human activities in the area.…”