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sentative of the number of vocal species present in an area (Figure 4C). Soundscape analyses have provided a means for better understanding the influences of environmental parameters such as sea ice presence and lunar cycles on local acoustic processes (Miksis-Olds et al., 2013a; Staaterman et al., 2014), assessing habitat quality and health on coral reefs (McWilliam and Hawkins, 2013; Staaterman et al., 2014), measuring biodiversity (Parks et al., 2014; Harris et al., 2016), and for better understanding the impacts and risks of human contributions to the soundscape have on marine life.
Utilization of Underwater Soundscapes
Over the past decade, the costs of collecting and analyzing passive acoustic-monitoring data have been steadily decreas- ing, leading to an increasing number of studies that explore how animals use information from their environmental soundscape for communication, orientation, and navigation (Slabbekoorn and Bouton, 2008; Pijanowski et al., 2011; also see article by Slabekoorn in this issue of Acoustics Today). The concept of using ambient or reflected sounds (as opposed to specific communication signals) as cues to direct movement or identify appropriate habitats has recently been identified as a new field of study referred to as soundscape orientation, and the concept is also included within the broader field of sound- scape ecology in the scientific literature (Slabbekoorn and Bouton, 2008; Pijanowski et al., 2011). It has been speculated that large baleen whales use ambient acoustic cues or acoustic landmarks to guide their migration (Able, 1980; Kenney et al., 2001). Similarly, it has been proposed that soundscape cues could provide ice seals in the water, a salient acoustic gradi- ent between open water and solid ice conditions by which the seals can orient to maintain access to open water for breathing (Miksis-Olds and Madden, 2014).
Laboratory and field studies have demonstrated that both invertebrates and fishes use soundscape cues for orientation and localization of appropriate settlement habitat. Stanley et al. (2011) measured the sound intensity level required to elicit settlement and metamorphosis in several species of crab lar- vae, and Simpson et al. (2008) showed that coral reef fish seem to respond more strongly to the higher frequency components (>570 Hz) of the reef soundscape. Habitats with greater biodi- versity are often associated with richer acoustic soundscapes compared to low-diversity habitats, which in itself may be an important cue for animal orientation in water and air (Sueur et al., 2008; Pijanowski et al., 2011; Stanley et al., 2012).
An example of the utility of long-term soundscape analysis is the survey of low-frequency underwater ocean sound over
the past 50 years off the West Coast of the United States. Us- ing a combination of declassified US Navy recordings and scientific datasets, a steady increase in low-frequency sound (10-200 Hz) has been documented and mainly attributed to an increase in commercial shipping (Ross, 2005). Sound lev- els have increased at approximately 3 dB/decade (0.55 dB/ year) up until the 1980s (McDonald et al., 2006) and then slowed to 0.2 dB/year (Chapman and Price, 2011). The most recent measurements in this region show a leveling or slight decrease in the sound levels since the late 1990s despite in- creases in the number and size of ships (Andrew et al., 2011).
Blue, fin, sei, Brydes, right, and humpback whales all com- municate in the 10- to 200-Hz frequency band; infrasound from waves crashing onshore (that marine animals likely use for orientation) is also in this band. Understanding how ma- rine life uses this frequency band and the effects of human contributions in this same frequency band is the subject of many soundscape studies. Shipping increases alone do not fully account for the observed 10- to 12-dB increase in the 20- to 40-Hz band from 1965 to 2003 (Ross, 1993, 2005). Activities from oil and gas exploration and production as well as from renewable energy sources have also increased the total sound levels in this band (Boyd et al., 2011). Bi- otic sound levels have likely also increased due to recovering whale populations and the “Lombard effect,” which is the in- crease in call amplitude to compensate for higher noise lev- els. The Lombard effect has been demonstrated in humans and many animal populations and may contribute to rising low-frequency levels as animals vocalize louder to be heard above the noise (Tyack, 2008).
Climate change is increasing the amount of glacial ice en- tering the oceans, and as glaciers disintegrate, they gener- ate low-frequency noise with large source levels that con- tributes to the regional noise budget for extended periods (Dziak et al., 2013). The regional limits of soundscapes, even for low frequencies that propagate long distances, is under- scored by the differences in long-term sound level increas- es. Although studies have reported a significant increase in ambient-noise levels in the North Pacific, current studies in the Indian, South Atlantic, and equatorial Pacific Oceans have not observed a uniform increase in ocean sound levels (Miksis-Olds et al., 2013b; Miksis-Olds and Nichols, 2016). Very little is known about the global soundscape as a whole, and this is an active area of ocean exploration. Theory and observations suggest that human-generated noise could be approaching levels at which negative effects on marine life may be occurring (Boyd et al., 2011).
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