Anthropogenic noise effects to CI beluga prey are discussed in the “Threat Type: Reduction in Prey” section (III.A.6); and cumulative effects involving noise are considered in the “Threat Type: Cumulative Effects of Multiple Stressors” section (III.A.2).
Sources of Noise in Cook Inlet
The acoustic environment of Cook Inlet is naturally noisy, complex, and dynamic. Natural sources of noise are particularly abundant in the CI beluga hearing range and include: bottom substrate being transported by high currents; sand and mud bars generating breaking waves during low tide/high current periods; river mouths becoming rapids at low tide periods; and fast and pancake ice being formed during winter months and under continuous mechanical stress by high tide oscillations and currents. Furthermore, the inflow of cold freshwater of glacial origin can vary considerably near major river mouths and arms in the upper Inlet, creating a complex sound propagation environment due to changes in both salinity and temperature as a result of sharp water mass fronts. These differences in water density and temperature act as sound barriers, reflecting and refracting sound energy. In addition, the large volume of fresh water from glacial areas surrounding Cook Inlet introduces suspended glacial silt and sediments into beluga habitat. Silt and other fine sediments suspended in the water column create acoustic clutter (a volume of scattered sound reflection) that can further impede echolocation performance. The presence of all of these natural sources of noise varies over time and space, as does their contribution to the overall ambient noise of Cook Inlet. Their contribution is important as a wide range of frequencies overlap with beluga signals, including both lower frequency ranges used for social communication and higher frequency ranges used for echolocation. The effects of these natural conditions, while difficult to quantify, may compromise CI beluga acoustic communication and echolocation, particularly as the sound transmission distance increases. Consequently, the natural acoustic space for CI belugas may be more limited than for belugas found elsewhere. This particular condition enhances the potential for negative effects when anthropogenic sources of noise are introduced into CI beluga habitat.
Due to the co-occurrence of Alaska’s urban center and the current range of CI belugas, a wide variety of anthropogenic noises that could affect recovery exists, especially in the upper Inlet. Most sources of anthropogenic noise in Cook Inlet are seasonal and occur during the ice- free months, although some sources are present year-round. Sources of anthropogenic noise in Cook Inlet include: propeller cavitation, engines, and depth sounders associated with vessels; dredging activities; pile driving activities; military detonations; aircraft; airguns used for seismic surveys; drilling associated with oil and gas exploration; hydraulic/mechanical noise; and sounds associated with other noise-producing activities. Although there are several technical reports documenting specific Cook Inlet noise sources and their signal characteristics, a comprehensive survey of anthropogenic noise sources in Cook Inlet and beluga exposure to these sources has not been conducted. Most of the identified sources in the Inlet are not well documented, and many are not controlled, monitored, or regulated.
Due to industrial activity and development in the current range of CI beluga, a wide variety of anthropogenic noise sources that could potentially interfere with recovery are present in CI beluga habitat. Sources are listed below by order of importance, based on signal characteristics and the spatio-temporal (space and time) acoustic footprint. The order was determined by considering the following factors: intensity (loudness), frequency (range of tones), and duration of acoustic signal; area affected by the sound source; and duration of sounds in both seasonal terms (e.g., happening all summer) and frequency of occurrence (e.g., happening once per week throughout the summer; M. Castellote, NMFS, unpub. data).
- Tug boat noise: propeller cavitation (the formation of bubbles in a liquid) and engine noise including azimuth/bow thruster noise;
- Cargo/tanker noise: propeller cavitation and engine noise including bow thruster noise;
- Small vessel noise: outboard and inboard engine noise and propeller cavitation;
- Dredging: suction and/or grabbing operations;
- Pile driving noise: hammering or vibratory noise (rotatory or oscillatory to a lesser
- Military detonations of high explosives: demolition and projectile explosions in military firing ranges;
- Oil/gas exploration: airgun sources for seismic survey and high power active transducers (multibeam echosounders, sub-bottom profilers, etc.);
- Shore construction noise: other than pile driving;
- Oil/gas exploitation: platform noise (in-air noise radiated into the water), drilling noise (in water and/or bottom substrate), air/water vessels during operations;
- Commercial jet aircraft: overflights, take offs, and landing approaches;
- Military jet aircraft: overflights, take offs, and landing approaches;
- Propeller aircraft: overflights, take offs, and landing approaches;
- Depth sounders: from vessels;
- Fishing related noise (other than engine noise): hydraulic/mechanical operations;
- Research related noise: sonars such as acoustic Doppler current profilers and dual- frequency imaging sonars; scientific echo sounders and other active transducers, boat transit for photo-identification surveys, and instrument deployment/retrievals, etc.; and
- Pipe and cable laying operations.
Climate change is having an indirect effect on ocean noise pollution (Reeder and Chiu 2010). As levels of carbon dioxide rise in the atmosphere, ocean waters are becoming more acidic. Ocean acidification reduces concentrations of seawater salts that absorb sound, particularly low- frequency sound. This ocean pH change is predicted to be greatest in higher latitudes, allowing lower frequency sound to carry farther and to be stronger at a given distance. Shallow sound channeling exists in Cook Inlet, which allows potential noise impacts to be concentrated in shallow waters and become more spatially extensive (i.e., sound channels can trap noise and allow it to travel farther). At the same time, climate change may directly result in either an increase or decrease of in-water noise. For example, warming temperatures may reduce the prevalence of ice cover, and thus reduce ice-associated noise, but warmer temperatures may also result in higher wind speeds resulting in higher noise levels at the waters’ surface.
Potential Effects of Noise on CI Belugas
There is an extensive body of literature regarding the effect of anthropogenic noise on marine mammal behavior. Most of the studies addressing this problem have used behavioral attributes such as changes in site fidelity, dive patterns, swimming speed, orientation of travel, herd cohesiveness, and dive synchrony to indicate possible disturbance or stress caused by noise (Richardson et al. 1995). A review and summary of available information regarding effects from anthropogenic noise to beluga hearing and behavior is presented in Appendix IX.E – CI Beluga Hearing, Vocalization, and Noise Supplement.
Studies on belugas have revealed that anthropogenic noises have the possibility to cause threshold shifts in beluga hearing capabilities (e.g., Finneran et al. 2000, 2002a; Schlundt et al. 2000); to mask the ability of animals to hear and decipher specific sounds (e.g., Erbe et al. 1999; Erbe 2000); to result in belugas altering their vocal behaviors (e.g., Lesage et al. 1999; Sheifele et al. 2005); or to result in displacement of animals from habitats (e.g., Finley et al. 1990; Richardson et al. 1997; Harris et al. 2007).
Anthropogenic noise, particularly the combined effect of different sound sources occurring simultaneously or consecutively, has the potential to affect beluga acoustic perception, communication, echolocation, and behavior (such as foraging and movement patterns). Behavioral effects include processes of sensitization (increased response following repeated exposure) or habituation (decreased response following repeated exposure) and physiological processes related to hearing and stress. In the long term, anthropogenic noise may induce chronic effects altering the health of individual CI belugas, which in turn have consequences at the population level (i.e., decreased survival and reproduction). Although the effects on CI belugas of the diverse types of anthropogenic noises occurring in their habitat have not been analyzed and are currently unknown, there is enough evidence from other odontocete species (and for some effects in other beluga populations) to conclude that the potential for a negative impact to CI beluga recovery is of high relative concern.