Effects of Human-made Sound on the Behavior of Whales

by Pat Leonard last modified 2007-03-23 09:59

Marine Mammals and Low-Frequency Sound

Decades ago, the myth of the "Silent Sea" was widely believed. This impression was reinforced by the inability of humans to hear well under water, and the general difficulty of building and using equipment to record underwater sounds. In reality, the world's oceans are noisy places, and becoming noisier due to human activities. Elevated noise levels are known to have harmful effects on terrestrial animals, including humans, so it has become a national priority to measure and understand the effects of underwater noise on marine animals, especially whales.

What is the importance of sound to marine mammals?

Many marine mammals rely on sound for communication, navigation, or detection of predators and prey. Whales may use sound to attract mates, repel rivals, communicate within a social group or between groups, navigate, or find food. Presumably, disruption of any of these biologically important functions could interfere with normal activities and behavior, thereby affecting the reproductive success of individuals and the sizes of a populations. Sound, particularly low-frequency sound, propagates very efficiently underwater; some human activities could affect quite large areas of the ocean. The effects of loud sound on whales might range from subtle changes in behavior (shortening or lengthening an activity) to physiological damage (permanent hearing loss). Behavioral effects could have serious consequences for populations, if they involved large-scale effects disruption of migration, feeding, breeding, or other critical activities.

What are the sources of human-generated undersea sound?

Most undersea noise from human activities is accidental, the byproduct of shipping, oil drilling and production, and the operation of undersea machinery. In these cases, sound is not a critical component of the activity. The ocean could be made much more quiet if ships and other marine equipment were designed and operated to minimize noise output. Trucks, cars, and motorcycles all have mufflers, because airborne sounds irritate humans. Similar attention to underwater noise abatement could significantly improve the undersea acoustic environment.

Many other undersea sounds are an essential part of the human activity.

Geologists probe the ocean's seafloor for oil using loud sounds (air guns, sparkers). Navy ships probe the undersea for submarines using loud sounds (sonar). Scientific researchers using loud sounds to study the structure of ocean water masses, the seafloor and underlying rock. Fisherman and scientists use loud sounds to survey fish and plankton populations. Often, there are no practical alternatives to these intentional uses of sound, so the question becomes how to achieve the goals of this work with minimal biological impact.

Whales have evolved in the presence of loud natural noise sources

The natural acoustic environment of the ocean, in which marine mammals evolved, is characterized by significant levels of ambient noise, and sporadic events that produce very loud sounds. Examples of natural events that produce extremely loud sounds are earthquakes and lightning strikes. During their lifetimes, most marine mammals are exposed to many such events, so it is very likely that their ears have evolved the ability to tolerate occasional exposure to very loud sounds.

Marine mammal ears also have to be able to tolerate the drastic pressure changes associated with breaching; the rapid transition from water to air and back again produces pressure changes at the animal's ear that dwarf any acoustic signals. With regard to behavioral effects, the ability of marine mammals to cope with the natural changes in undersea sound environments may afford them some tolerance of human-generated noise, but this conclusion must be tempered by concerns about the novel characteristics of human noise sources. There is abundant evidence that marine mammals tolerate loud natural sounds. US Navy undersea surveillance systems (IUSS) have recorded numerous examples of whales continuing their vocal activity with no change throughout noisy natural events such as undersea earthquakes, which produce sounds much louder than those that humans contribute. Click here to see an example of such an event. This does not, however, guarantee that they can tolerate the loud, novel stimuli that humans introduce. We also do not know the extent to which whale communication systems have been disrupted by the general increase in ocean noise due to shipping.

Some loud undersea sounds are made by animals. Many whale species produce sounds at levels between 170 and 180 dB re 1 uPa (decibels relative to the underwater reference sound level, one microPascal). The sounds produced by a nearby whale may be among the loudest that a marine animal experiences, and in many places they will be the most numerous loud sounds.

Conclusions are difficult

Although researchers from this laboratory and others have studied this problem for a few decades, the data are relatively sparse. There are clear indications of changed behavior, including some cases in which animals have abandoned critical habitats. However, most studies have revealed behavioral changes whose long-term sigificance is difficult to assess. One challenge for this research is the difficulty of obtaining detailed behavioral observations of animals that spend 10-20% of their time submerged, out of sight. Another is the diverse behavioral repertoire of marine mammals, and the difficulty of predicting what animals will do under undisturbed conditions. However, recent improvements in research methods are providing unprecedented views of marine mammal behavior, and the resulting studies are beginning to establish a rational basis for assessing risk, and for identifying the most critical unresolved issues.