Abstract
Passive acoustic monitoring (PAM) with autonomous bottom-moored recorders is widely used to study cetacean occurrence, distribution and behaviors, as it is less affected by factors that limit other observation methods (e.g., vessel, land and aerial-based surveys) such as inclement weather, sighting conditions, or remoteness of study sites. During the winter months in Hawai‘i, humpback whale male song chorusing becomes the predominant contributor to the local soundscape and previous studies showed a strong seasonal pattern, suggesting a correlation with relative whale abundance. However, the relationship between chorusing levels and abundance, including non-singing whales, is still poorly understood. To investigate how accurately acoustic monitoring of singing humpback whales tracks their abundance, and therefore is a viable tool for studying whale ecology and population trends, we collected long-term PAM data from three bottom-moored Ecological Acoustic Recorders off west Maui, Hawaii during the winter and spring months of 2016–2021. We calculated daily medians of root-mean-square sound pressure levels (RMS SPL) of the low frequency acoustic energy (0–1.5 kHz) as a measure of cumulative chorusing intensity. In addition, between December and April we conducted a total of 26 vessel-based line-transect surveys during the 2018/19 through 2020/21 seasons and weekly visual surveys (n= 74) from a land-based station between 2016 and 2020, in which the location of sighted whale pods was determined with a theodolite. Combining the visual and acoustic data, we found a strong positive second-order polynomial correlation between SPLs and abundance (land: 0.72 ≤ R2≤ 0.75, vessel: 0.81 ≤ R2≤ 0.85 for three different PAM locations; Generalized Linear Model:pland≪ 0.001,pvessel≪ 0.001) that was independent from recording location (pland= 0.23,pvessel= 0.9880). Our findings demonstrate that PAM is a relatively low-cost, robust complement and alternative for studying and monitoring humpback whales in their breeding grounds that is able to capture small-scale fluctuations during the season and can inform managers about population trends in a timely manner. It also has the potential to be adapted for use in other regions that have previously presented challenges due to their remoteness or other limitations for conducting traditional surveys.
Highlights
Many animals across numerous taxa produce sounds, that can be recorded and analyzed to study their behaviors and ecology
Acoustic Data Acoustic data were collected with two deep-water and one shallow-water bottom-moored, autonomous Ecological Acoustic Recorders (EARs) using a Sensor Technology SQ2601 hydrophone with a sensitivity of −193.5 dB (Lammers et al, 2008) during the humpback whale breeding season
Adult and some juvenile male humpback whales sing (Herman et al, 2013), but their increasing and decreasing contribution to the marine soundscape in Hawai‘i during the breeding season mirrors the bell-shaped abundance curve resulting from the species’ staggered, age-class and sex-segregated migration pattern (Baker and Herman, 1981; Mobley et al, 1999; Au et al, 2000; Craig et al, 2001, 2003; Chen, 2017; Kügler et al, 2020)
Summary
Many animals across numerous taxa produce sounds (e.g., invertebrates: Henry and Wells, 2010; Bohnenstiehl et al, 2016; amphibians: Tobias et al, 2010; Dapper et al, 2011; reptiles: Hibbitts et al, 2006; Sicuro et al, 2013; birds: Catchpole and Slater, 2008; Phongkangsananan et al, 2014; fish: Maruska et al, 2012; Tricas and Boyle, 2014; terrestrial mammals: Boughman, 1997; De La Torre and Snowdon, 2009; Van Belle et al, 2014; Pitcher et al, 2015; marine mammals: Smolker and Pepper, 1999; Van Parijs et al, 2000; Rogers and Cato, 2002; Belikov and Bel’kovich, 2007; Sanvito et al, 2007), that can be recorded and analyzed to study their behaviors and ecology. PAM projects on various whale and dolphin species range widely from simple detection (e.g., presence/absence in an area) and distribution studies (Van Parijs et al, 2002; Oswald et al, 2011; Soldevilla et al, 2011; Schaffeld et al, 2016) to addressing questions about exposure to potentially disturbing anthropogenic sounds (Lucke et al, 2009; Dunlop et al, 2018). Using individual’s vocalization rates to estimate densities becomes challenging when an abundance of animals vocalize so often that individual sounds overlap and cannot be distinguished in recordings, as is often the case in large aggregations including on the breeding grounds of certain marine mammals, such as the humpback whale (Megaptera novaeangliae) (Au et al, 2000; Seger et al, 2016)
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