Several published studies (e.g., Awbrey et al. 1988; Klishin et al. 2000; Mooney et al. 2008) and one unpublished study (White et al. 1978) have assessed the hearing sensitivity of captive belugas using behavioral or electrophysiological (i.e., auditory evoked potential [AEP]) methods. In addition, one published study investigated hearing sensitivity (from 4 to 150 kilohertz 11 [kHz]) in seven wild Bristol Bay belugas using AEPs (Castellote et al. 2014). Hearing abilities in these belugas were generally similar to those measured in captive belugas. All seven belugas heard well, up to at least 128 kHz, and two heard up to 150 kHz. Lowest auditory thresholds (35–45 dB 12) were identified in the range 45 to 80 kHz (Figure 12).
Greatest differences in hearing abilities among individuals occurred at both the high end of the auditory range and at frequencies of maximum sensitivity. Collectively, these studies indicate belugas have an overall auditory bandwidth of approximately 40 Hz to 150 kHz, roughly eight times that of humans (Au 1993).
Beluga echolocation (sonar) has been well studied and described (Au et al. 1985, 1987). Studies show that belugas have highly developed and sophisticated echolocation capabilities, with the capacity to adapt their click energy distribution as a function of the ambient noise in order to maximize the echo reception (Au et al. 1985). The echolocation capabilities of belugas, when compared to bottlenose dolphins, appear to be superior in the ability to detect targets (e.g., short steel cylinders) in the presence of masking noise (Turl et al. 1987) and in the ability to detect targets in clutter (reverberation composed of echoes scattered back to a sonar from objects and heterogeneity in the water and on its boundaries) (Turl et al. 1991). In an effort to detect a target in the midst of masking noise, belugas were shown to gain signal-to-noise ratio by projecting and receiving signals off the surface of the water, a technique not observed in the bottlenose dolphin (Penner et al. 1986). Hypothetically, this may be a similar strategy to using the underside of ice cover to reflect signals, possibly an adaptation to living in an Arctic environment. Turl and Penner (1989) suggest: “[T]he beluga lives in a high-noise and reverberant environment. It might be expected that the beluga’s sonar system has developed optimal adaptations to minimize the effects of interference found in the Arctic.”
Beluga Acoustic Social Signals
Belugas are among the most vocal cetaceans, making a wide variety of sounds that fall into two acoustic categories: whistles or narrow band frequency modulated vocalizations, and pulsed sounds or trains of broad band pulses. The latter can be divided into two functional categories: click trains, used largely for echolocation, and burst pulse sounds (bursts of pulses with rapid pulse repetition rates), believed to be social signals, which may sound to the human ear like grunts, squawks, screams, whines, and whistles.
These varied sounds earned belugas the nickname “Sea Canaries” by early Arctic whalers. There have been a number of attempts to classify the vocal repertoire of belugas (Fish and Mowbray 1962; Morgan 1979; Sjare and Smith 1986a, 1986b; Faucher 1988; Bel’kovich and Sh’ekotov 1993; Recchia 1994; Angiel 1997; Belikov and Bel’kovich 2001, 2003, 2006, 2007, 2008; Karlsen et al. 2002). This body of data provides some indication that sounds vary with behavioral and group context, and suggests geographic variation in signal use among populations. It is thought that these calls, both in captivity and in the wild, function to maintain group cohesion, and the variants shared by related animals are used for mother-calf recognition (Vergara et al. 2010). For example, belugas show an increase in the rate of vocalizations during social gatherings in the Canadian high Arctic, in Svalbard, Norway, and in the White Sea, Russia (Sjare and Smith 1986b’ Karlsen et al. 2002; Belikov and Bel’kovich 2003). They become much quieter when disturbed by humans or frightened (Finley 1990, Karlsen et al. 2002; Sjare and Smith 1986b; Belikov and Bel’kovich 2003). There is evidence of a decrease or even a cessation in acoustic activity of belugas in the presence of natural predators (e.g., killer whales) or engine noise.
Belikov and Bel’kovich (2003) attempted to correlate specific beluga call types with four behavioral states: quiet swimming, social interactions, sexual behavior, and disturbance caused by humans. While all call types were heard during all four behavioral states, there was a significant increase in “chirps” heard during sexual behavior and social interactions, and a decrease in whistles during sexual behavior. The conclusion was that “chirping” was the best acoustic indicator of beluga behaviors, marking both social and sexual interactions (Belikov and Bel’kovich 2003).
Acoustics of CI Belugas
Limited work has been done regarding acoustics of CI belugas. Castellote et al. (2011) recorded the acoustic behavior of CI belugas concurrently with visual observations using both boat-based and land-based methods in open waters as well as inside river mouths (Eagle and Little Susitna Rivers). The authors described how the acoustic behavior of CI belugas is modified when feeding. During presumed feeding or prey search, social calls were absent and echolocation clicks often occurred in train packets. Burst pulses were also found more often, although the authors indicated that few of these were conclusively assigned as “terminal buzzes” related to prey capture since most of the events were partially incomplete, probably due to the highly directional nature of these sounds. The authors concluded that echolocation train packets ending with a terminal buzz were produced by feeding belugas, that this behavior was commonly recorded in river mouths, and that it could be acoustically monitored with the potential to be used as an indirect indicator of foraging behavior. Garner et al. (2014) also used echolocation data to assess the seasonal use of Eagle River by CI belugas.
As noted above (see Section II.B.4. Use of Critical Habitat by Belugas), Castellote et al. (2016a) obtained information on the seasonal distribution and foraging behavior of belugas in Cook Inlet through passive acoustic monitoring of beluga social calls and echolocation activity at 13 locations in lower Cook Inlet and upper Cook Inlet. Belugas were detected at 12 of the 13 locations, with no detections in lower Cook Inlet at Homer Spit (the most southern site monitored). In general, the seasonal distribution of acoustic detections was consistent with descriptions based on aerial surveys and satellite telemetry. Echolocation data were also used to explore when and where presumed foraging buzzes occurred.
Passive acoustic recordings of CI belugas have also been collected in conjunction with a construction project at the Port of Anchorage. Širović and Kendall (2009) deployed a passive acoustic array of sonobuoys during 20 days in summer 2009 to acoustically detect the presence of belugas in the vicinity of in-water pile driving at the Port of Anchorage. Belugas were detected 55% of the monitoring time, with virtually all detection based on echolocation clicks (one whistle and over 65,000 clicks). Kendall et al. (2013) suggested that during the monitoring period, other lower frequency beluga whale vocalizations (e.g., whistles) were potentially masked, there may be have been an overall reduction in beluga vocalizations, or it is possible belugas were avoiding the area during construction activity.
A review of available information reveals four main gaps regarding our acoustic knowledge on CI belugas:
- Hearing sensitivity data collected for seven wild belugas indicated that hearing abilities in these belugas were generally similar to those reported for captive belugas (Castellote et al. 2014). Thus based on this one study, it appears that hearing measurements in a laboratory setting may be reasonable substitutes for data from wild belugas. However, larger sample sizes are needed to fully assess maximum hearing sensitivity and variability within the species (especially age and sex based differences).
- Very specific noise types (e.g., simulated underwater explosions, pure tones, seismic water gun, white noise, icebreaker noises) have been used in hearing experiments with belugas. Even if these studies set the baseline information on the effect of noise in the beluga auditory system, their results might not be applicable to CI belugas because most of the noise sources tested are foreign to Cook Inlet. Castellote et al. (2016b) evaluated the sources, acoustic characteristics, and frequency of occurrence of anthropogenic noise in Cook Inlet and concluded that the temporal prevalence and levels of anthropogenic noise measured “indicate that beluga communication and hearing is largely masked by anthropogenic noise in most of the locations and periods sampled.” Future research should broaden the geographic extent and months sampled, and improve the classification of unknown noise sources.
- Little is currently known regarding chronic effects of noise exposure on belugas. CI belugas are exposed to anthropogenic noise sources of notable prevalence (e.g., tug boats, pile driving, dredging), but most of the studies to date have been focused on short-term and acute exposure to noise. Similarly, most of the current studies on the effects of anthropogenic noise on belugas have been focused at the physiological level (e.g., masked temporary threshold shifts, TTS, PTS), but the effects of anthropogenic noise at the behavioral level (e.g., geographical displacements, changes in acoustic communication) have barely been considered.
- The current understanding of social communication in different populations of belugas highlights an important lack of standardized methods. Considering that the repertoire of beluga vocalizations has been suggested to vary geographically, the standardization of acoustic analysis methods is needed to better understand the population structure and seasonal distribution of this species. Research efforts in this direction will probably be beneficial in a broader scale, not just towards the Cook Inlet population.