Effects of anthropogenic noise on cetaceans: a review.

Abstract

For animals in the marine environment, there have been increasing levels of stress due to the rise in ambient noise caused by humans. Shipping, seismic surveying and industrial activity are just some of the man-made activities contributing to the noise pollution. This noise interferes with marine mammals, especially cetaceans, which use sound as their prime mechanism to learn about their environment, as well as communicate, navigate, and forage. Here I review the various effects of anthropogenic noise on cetaceans. Conservation biologists need to explore how cetaceans are responding to this increase in noise pollution and determine what further research needs to be done in order to protect these animals and their habitat.

Introduction

Cetaceans are divided into two suborders: Mysticeti (baleen whales) and Odontoceti (toothed whales, which include dolphins and porpoises). The toothed whales use their teeth for catching fish, squid or other marine life that they then swallow whole (Rice 1998). Baleen whales do not have teeth, but have plates made of keratin that act as a giant filter, straining small animals from the seawater. Not all Mysticeti feed on plankton: the larger whales tend to eat small shoaling fish, such as herrings and sardine. One species of Mysticeti, the Gray Whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea floor crustaceans (Rice 1998).

Uses of Sounds

Cetaceans may be especially sensitive to anthropogenic noise because some species of toothed whales have developed a special sense called echolocation that allows them to hunt and gather information about their environment in turbid and deep water where no light penetrates (Richardson et al. 1995). They send out a sound into the water and then use the returning echo to identify the objects that have reflected the sound. The differences in the returning echo provide the animal with information about the size, shape, orientation, direction, speed, and composition of the object or landscape.

Baleen whales live throughout the world's oceans and produce acoustic signals at low to moderate frequencies from 12 Hz-8 kHz (Ridgway and Harrison 1985; Richardson et al. 1995). These cetaceans lack an echolocation system. Toothed whales live in oceans around the world and in some rivers (Jefferson et al. 1993). They signal at moderate to high frequencies from 1-20 kHz and use echolocation at high frequencies from 20-150 kHz (Richardson et al. 1995). Cetaceans have inner ears that are so well developed that they can not only hear sounds tens of miles away, but are also able to discern the direction from which the sound comes.

Cetaceans not only use sound for navigation and feeding, but also to communicate, through methods such as groans, moans, whistles, clicks, or complex singing (Caldwell 1972). Communication is a behavior by one individual which causes a change in behavior or state of another individual, the receiver, through conveying information (Alcock 2005). Cetaceans use sound to communicate information about the location and identity of individuals or groups, reproductive status, food sources, predation risk, and dominance (Tyack 1999). For example, humpback whale males compete with other males through complex songs that can last for several hours, and females choose between males based on these vocalizations (Gaskin 1982).

Cetaceans communicate with each other using individualized vocalizations. Bottlenose dolphins, for example, have signature whistles used in communication (Tyack 1999). This whistle not only sends information on location, but also gives the identity of the individual vocalizing.

Sound is also important for marine mammals because it allows them to locate food sources. Echolocation is typically used by toothed whales to capture single prey items such as fishes or squid. With this ability cetaceans can search for, chase and catch fast-swimming prey in total darkness (Richardson et al. 1995).

Cetaceans can also use echolocation in the navigation of their underwater environment. They are able to orient themselves by listening to sounds, such as waves crashing on the beach. By detecting and localizing sounds, individuals are able to avoid certain objects (Richardson et al. 1995).

Noise

The ability of cetaceans to communicate, navigate, and echolocate can be compromised by human noise. Noise is a disturbance, usually random and persistent, that obscures or reduces the clarity of a signal (Richardson et al. 1995). The ocean is naturally noisy, which would seem to present problems for marine mammals using sound, however, they are well-adapted to natural levels of ambient noise (NRC 2003). However, masking can occur, which is the blocking of the perception stimulus, resulting from the presence of another stimulus in the same range (Richardson et al. 1995).

Naturally Occurring

Ambient noise in the marine environment is caused by biological and non-biological sources (NRC 2003). Non-biological ambient noise consists of wind, precipitation, tectonic processes, seismic energy, and even movement of sediment over the ocean floor. These processes all contribute to the ambient noise level from frequencies below 1 Hz to 50 kHz (NRC 2003). Ice cover also affects ambient noise levels by isolating the environment from surface level sounds.

Calls by groups of cetaceans, such as dolphins, contribute some of the biological ambient noise found in the marine environment (NRC 2003). An unlikely source of ambient noise comes from small snapping shrimp, which produce high levels of sound during defense of their territories and can cause such a significant amount of noise that bottlenose dolphins have to change the frequency of their echolocation clicks in order to prevent masking (Au 1993).

Man-made

Anthropogenic noise contributes greatly to overall ocean noise. Sound is frequently created by humans in the marine environment. Man-made noise comes from a variety of activities: shipping, seismic surveying, sonars, explosions, industrial activity, and more (Richardson et al. 1995). Anthropogenic noises consist of frequencies from 1 Hz to 200 kHz and even higher; therefore cetaceans can hear and possibly be affected by some type of man-made noise (NRC 2003).

Potential Consequences of Noise

The presence of anthropogenic noise has several potential impacts on cetaceans. Noise can cause cetaceans to alter their behavior, can prevent marine animals from hearing important sounds through masking, and can cause hearing loss (temporary or permanent) or tissue damage in marine animals (Richardson et al., 1995). The impacts on cetaceans are affected by many factors: sound level, and other characteristics of the sounds, such as frequency (harmonics vs. pure tones); the hearing sensitivity, age, sex, and behavior of the animals; and the environmental conditions under which the animals experience the sound (Kastelein et al. 2001).

Behavioral Responses

The effects of noise on cetaceans have been found to vary from the short- to the long-term. Some studies have shown there to be little or no effect from noise on cetaceans.

Short-term behaviors consist of termination of feeding, resting, and social interactions (Watkins and Schevill et al. 1975, Watkins et al. 1985, Miller et al. 2000). Cetaceans have also shown changes in surfacing, respiration, and diving cycles (Kastelein et al. 2001). Avoidance has been observed in some cases as well, which is the displacement of an animal from their habitat for a certain period of time (Gordon et al. 2003).

Marine seismic surveys are some of the harshest anthropogenic noises cetaceans are subjected to (Gordon et al. 2003). Seismic surveys use reflected seismic waves to produce images of the Earth's subsurface (Gordon et al. 2003). The method requires a controlled seismic source of energy, such as dynamite or a specialized air gun. By noting the time it takes for a reflection to arrive at a receiver, it is possible to estimate the depth of the feature that generated the reflection. In this way, reflection seismology is similar to echolocation (Richardson et al. 1995). The reactions of cetaceans to seismic surveys have included surprise, fright, stress, and avoidance. Mysticeti and odontocetes have also shown changes in behavior and vocalization patters such as disruption of foraging, avoidance of particular areas, altered dive and respiratory patterns, and disruption of mating systems (Gordon et al. 2003).

There have been few studies detailing the long-term effects of disturbance on cetaceans. A study by Morton and Symonds found that killer whale displacement occurred for several years from an introduction of noise into their environment (Morton and Symonds 2002). Beluga whales will flee from approaching ice-breaking vessels, traveling up to 80 km away from productive feeding areas, and remain displaced for several days (Erbe and Farmer 2000). Gray whales also show avoidance behavior through leaving the breeding grounds, such as the breeding lagoon in Baja California, during extensive industrial activity and not returning to the habitat until the shipping decreases (Bryant et al. 1984). A study conducted in Australia found that long-term disturbance negatively affected populations of bottlenose dolphins (Bejder et al. 2006). Cetacean-watching tourism has increased over the years and this has lead to a decline in dolphin abundance in this area. These cases show examples of animals not being able to cope in a changing environment and suggest that cetaceans may not be adjusting to long-term anthropogenic noises.

Determining average behavioral reactions of cetaceans to anthropogenic noise is problematic because there is such high variability in terms of behavioral response between species and even between individuals within a species (NRC 2003). Additionally, these responses have been highly variable, ranging from attraction of dolphins to the bow of a ship to long-term displacement (Richardson et al. 1995). Harbour porpoises have also responded with great variability, with habitat avoidance in some studies (Culik et al. 2001) and exploratory behavior in others (Koschinski et al. 2003). Some sounds may not cause any observable responses, and while other sounds may cause subtle changes in diving, surfacing, or vocalization patterns, these changes in behavior are not necessarily a cause for concern if they are temporary or do not cause the animal to leave an area important to their survival (such as breeding or feeding areas). For example, Pilot whales respond vocally to military sonar pulses from a considerable range over long periods of time (Rendell and Gordon 1999). These long-term induced responses are not necessarily deleterious, but they are unlikely to be adaptive.

Masking

Once ambient noise levels reach certain amplitude they can possibly cause the hearing threshold for one sound to be raised by the presence of another--the process known as masking (Richardson et al. 1995). Masking depends on the relative intensities and the frequency compositions of both signal and noise. As amplitude of a noise increases, it becomes harder for an animal to detect a signal. In addition, as the bandwidth (the measure of the capacity of a communications channel) of a noise source gets closer to the bandwidth of the signal interference with that signal can occur and make it harder for an animal to detect the signal. The higher a channel's bandwidth, the more information it can carry. Cetaceans can decrease these effects by signaling at different frequencies from the noise, and at higher amplitudes (Snowdon & de la Torre 2002). Researchers found that icebreaker noise, as well as naturally occurring sounds from cracking ice, masked the calls of a Beluga whale (Erbe and Farmer 2000). However, by changes in behavior through the production of more calls, longer calls, or shifting of the frequency of the calls animals are able to minimize the effects of masking. Studies show that these Belugas can change their signals in environments with high levels of ambient noise by raising the frequency of their own call in order to make their signals heard (Thomas et al. 1990). A study that experimentally moved a Beluga whale into a noisier environment showed similar results (Au et al. 1985). The Beluga whale was able to reduce the effects of masking by changing the frequency of its own calls. Studies have shown that dolphins will vary in their signal frequencies depending on the ambient noise level in their habitat as well. Habitats with less noise were home to dolphins that whistled at varying frequencies and high modulation, while habitats with more noise were home to dolphins that whistled at lower frequencies and less modulation (Morisaka et al. 2005). These studies show examples of how cetaceans are flexible in their behaviors and are able to adjust to changes in their environments.

Habituation 

Habituation, a decreased response by an animal to man-made noises due to a lowered perception of risk, is known to occur with porpoises when, during experiments, they no longer respond to sounds (Koschinski and Culik 1997). Habituation shows that cetaceans, while temporarily disturbed by anthropogenic noise, can possibly return to normal behaviors by adjusting to man-made noises. However, habituation can also lead to behaviors that are detrimental for cetaceans. Ship strikes are the largest contributor to anthropogenic deaths of the North Atlantic right whale (Nowacek et al, 2003). The right whales ignore the ships possibly because of habituation to the sounds of approaching vessels (Richardson et al. 1995, Laist 2001).

Future Research

Current studies show little response or a return of normal behaviors of many cetaceans to anthropogenic noise. However, more data needs to be collected before conclusions are drawn. There are many past studies that have suggested negative effects of anthropogenic noise and current population declines of certain cetaceans make this subject a needed focus for future research. I will discuss possible topics for future research, some of the current situations of cetaceans and the current methods being used in order to improve these conditions.

The ability of cetaceans to detect man-made noise does not mean that they are negatively affected by that noise. There are many examples of cetaceans that often occur close to noise sources and the activities of these animals seem unaffected by the noise (Richardson et al. 1995). However, we do not have enough research to determine if the numbers of animals present in noise-polluted areas are similar to the numbers that would exist if the manmade noise was absent. Also, the lack of data on the importance of natural sounds to cetaceans makes it difficult to determine the effects of anthropogenic noise on the animals.

There are 119 marine mammal species, and only 20 percent have direct behavioral or physiological hearing data available on them (NRC 2003). More studies need to focus on determining the hearing abilities of all the species of marine mammals. Without this basic knowledge we will not be able to determine the effects of anthropogenic noise on cetaceans.

There is a wide variation in response to disturbance between different cetacean species and even within individuals of the same species (Richardson et al. 1995, Wartzok et al. 2003). These responses indicate the real differences in sensitivity between individuals at different times. Studies of such behavior has led to conflicting data on the responsiveness of animals to noise and this makes it difficult to determine a set of guidelines in predicting the behavioral reactions of cetaceans. Larger sample sizes are needed as well as more behavioral studies of wild animals, in addition to captive individuals in determining the average behavioral reactions to anthropogenic noise.

Since terrestrial mammals are easier to study than marine ones, it may be useful for future studies to look at closely related terrestrial mammals as a model for determining effects on cetaceans and marine mammals in general. Much more is known about how environmental factors affect communication in terrestrial mammals (Au 1990). Observational studies are not sufficient in determining the effects of anthropogenic noise, which is where most data on cetaceans is collected. The results from captive studies, where manipulation of the environment occurs, consist of very small sample sizes and these results cannot necessarily be applied to the larger wild populations.

Today, twenty-five marine mammal populations are currently classified as either endangered or threatened under the U.S. Endangered Species Act (ESA) (NRC 2003). Other anthropogenic effects such as commercial whaling, ship strikes, fishing, chemical pollution and the destruction of habitat are contributing to the decline of marine mammal populations. The population of resident Orcas in Puget Sound has been declining for several years and has been found to have levels of contamination twenty times higher than the minimum threshold for classifying a substance as toxic waste (Wursig 2006).

Currently, there are some methods that are being used in order to moderate the effects of human activities on marine mammals (NRC 2003). The avoidance of marine mammal habitats is an obvious way to protect the animals. By knowing the times and locations for breeding seasons or knowing areas that are important feeding grounds for certain species allows humans to avoid certain habitats at certain times of the year. The removal of a sound source or modification in some way may also moderate the effects on marine mammals. Some sound signals that cannot be removed have developed a ramp-up method in which the signal progressively gets louder and louder in order to allow individuals time to move away from the signal (Richardson et. al 1995). Sound screening can be used as well. It involves the muffling of sound through something such as a bubble wall that prevents the sound from traveling as far in the marine environment. Marine mammal reserves have been created in order to protect animals from human interference. The creation of more reserves and the expanding of current ones can help to further protect marine animals from anthropogenic effects.

Conclusion

Research shows that cetaceans are being affected by anthropogenic noise. In some cases these animals are able to adjust through behavior modification and continue to survive. Though most cetaceans appear to habituate to noises or have short-term behavioral changes that do not appear to greatly negatively affect them, anthropogenic noise may be having a greater effect on all marine mammals than currently speculated. Cetaceans have shown a high level of phenotypic plasticity, but the stresses being placed on them long term might lead to depleted populations and eventual extinction.

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Lauren Riegler (1)

(1) 336 Spanish Canyon Trail, West Point, Texas 78963 [e-mail]

Lauren Riegler will graduate in May of 2007 with a BS in Biology at Trinity University, working in areas of animal behavior and invasive species. She plans to continue studies in conservation biology and animal behavior and pursue a master's degree.