There are many factors that influence if and how much a sound source affects marine animals. How loud the source is, what frequencies it transmits, where it will be used, and what species might be in the area are all factors that need to be considered. This process is called ecological risk assessment (1, 2). The steps of this scientific process are illustrated in the following diagram. The first step is to define the problem. This involves determining what might cause ecological effects. The next stage involves two steps, estimating the probability of being exposed to the problem and, based on that exposure, determining the types of ecological effects that are expected. Based on these results, the risk can be estimated.

Schematic of Risk Assessment Procedures (3, 4)

1. What is the level of sound at different distances and depths as sound travels away from the source?
2. Where are marine animals likely to be located relative to the source?

3. What are the sound levels to which the animals are likely to be exposed?

4. Can the animal sense these sounds?

5. What effects might these sound levels have on the animals?

As you can see, it is important to use rigorous scientific methods to answer these questions. Each step is discussed below in more detail. As an example, we will walk through an analysis that was done to determine how the sound source for the North Pacific Acoustic Laboratory's Acoustic Thermometry of Ocean Climate (ATOC) Project might affect humpback whales around the island of Kauai, Hawaii. This analysis was part of an environmental impact statement that was approved by the National Oceanic and Atmospheric Administration (NOAA) Fisheries Department that oversees environmental regulations. While this example relates to marine mammals, the exact same process could be used to analyze the effects on fish.

Humpback whales are commonly sighted in nearshore waters near Kauai, Hawaii during the winter months. Photo courtesy of Ann Zoidis.

1. What is the level of sound at different distances and depths as sound travels away from the source?

The first step in determining if a sound source might impact marine animals is to make an estimate of the sound field around the source. A source's sound field is the level of sound at different distances and depths as the sound travels away from the source. (For more detailed information on how sound moves, see Why does sound get weaker as it moves?). There are many models that predict how sound moves away from sources. Several factors are included in these models. The geographic location and time of year that the source will be used affects how sound travels through the water. Water depth also affects how sound travels. Sound energy will be lost through the sea surface and into the sea floor, particularly when sound travels in shallow water. The depth and any movement of the source will also affect the sound field. Finally, the characteristics of the sounds that are transmitted are important. This includes the intensity and duration of the sounds, their frequency, and the number of sounds and the rate that are transmitted.

The following figure shows the level of sound at different distances and depths as the sound travels away from the ATOC/NPAL source towards the island of Kauai. Sound travels in all directions away from the source, but only the slice related to the humpback whale example is shown. The ATOC/NPAL source is moored on the seafloor at a depth of approximately 800 meters (2600 feet), approximately 14.8 kilometers (8 nautical miles) north of Kauai. It has a source level of 195 underwater dB at 1 meter and operates at a frequency of 75 Hertz. In this picture, the source is in the upper left hand corner near the dark red. The gray area represents the sea floor. The colors in the picture show the sound level decreasing as it moves away from the sound source. You can also see that the level of sound varies with both water depth and distance from the source.

In the picture, the sound source (with a source level of 195 underwater dB at 1 meter) is at the upper left hand corner in the dark red color. The colors show the predicted sound intensity at various distances and depths. The picture shows the sound level decreases with distance from the sound source.

This figure shows sound traveling away from the ATOC source towards Kauai (due south). As the sound moves further away from the source, the level of sound decreases.

2. Where are marine animals likely to be located relative to the source?

The second step is very similar to the first step, but instead of thinking about how the sound spreads out in the area around the source, scientists consider how the animals are spread throughout the area. The first question is whether or not a particular species is found in the area at the time of year that the source is being used. For species that might be in the area, the important factors to consider are their distribution patterns (for example, are they found close to shore, far out at sea, near seamounts?), their density or abundance (how many of them might be in the area?), their swim speed (how fast would they be moving compared to the source?), their movement patterns (do they swim in a straight line, or are they milling around?), and their dive patterns (how much time do they spend at the surface, what depths do they dive to and for how long?). The answers to these questions give a picture of how the animals are distributed throughout the area. Scientists cannot predict precisely where individual animals will be located; therefore they make estimates of the likelihood that animals will be in certain locations.

Distribution of humpback whales sighted near Kauai, Hawaii during January - April 2001 are shown in the full version of the following picture. The illustration shows that humpback whales largely prefer to be in nearshore water that is less than 100 fathoms deep.

Humpback whale distribution. The blue dots show where humpback whales were sighted near Kauai, Hawaii during January - April 2001. The inner and outer lines around Kauai show where the water is 100 fathoms (600 feet or 183 meters) and 1000 fathoms (6000 feet or 1830 meters) deep, respectively. The humpback whales were often sighted in nearshore water, less than 100 fathoms deep. The red dot north of Kauai shows where the ATOC/NPAL source is located.

3. What are the sound levels to which the animals are likely to be exposed?

The third step is to combine the sound field created in the first step with the marine animal field created in the second step. One example of what a slice of this picture could look like is found below.

In the picture, the sound source (with a source level of 195 underwater dB at 1 meter) is at the upper left hand corner in the dark red color. The colors show the predicted sound intensity at various distances and depths. The picture shows the sound level decreases with distance from the sound source. The two black diamonds are marine mammals that are predicted to be in the area while the source is transmitting. By combining the picture of the sound field with the probable locations of the marine animals, the amount of sound energy the animals might be exposed to can be estimated.

This is the same sound field shown in the picture above. As before, the sound source is at the upper left hand corner near the dark red. The gray area represents the sea floor. The two black diamonds are humpback whales that are predicted to be in the area while the source is transmitting. By combining the picture of the sound field with the probable locations of the marine animals, the amount of sound energy the animals might be exposed to can be estimated.

4. Can the animal sense these sounds?

Different animals are sensitive to different frequencies. Ultrasonic whistles can be heard by dogs, for example, but not by people. Baleen whales such as humpback and blue whales are sensitive to very low frequencies, while dolphins are sensitive to much higher frequencies. To determine whether or not sound of a specific frequency can be detected at a given level, scientists need to know the quietest sounds that the animals can sense at different frequencies, which is called the hearing threshold. The section What sounds can we hear? provides more information about what frequencies marine animals can hear. The sounds do not affect the animals if they cannot sense them, unless the sound levels are so high that they cause direct physical injury to the animal.

In the example of the humpback whales, scientists have not been able to conduct hearing sensitivity tests on such big animals. Based on data from other species, though, scientists estimate that humpback whales have their most sensitive hearing at the frequencies where they vocalize. Humpback whales make three types of vocalizations that range from about 50 Hertz up to 10 kHz, so it is very likely that they can hear at the frequency of the ATOC/NPAL source transmissions. Scientists were able to conduct hearing threshold tests on false killer whales and Risso's dolphins to determine their sensitivity to the ATOC/NPAL source (5). They found that these dolphins can just barely hear the ATOC/NPAL source at the frequency and source level of its transmissions.

5. What effects might these sound levels have on the animals?

The final step is to estimate the potential impacts of the sound. If the animal can hear the sound, it may prevent it from hearing other important sounds, such as calls from other animals. This is called masking. The sound might also cause the animal to change its behavior. Scientists use data on the way in which the animals react to similar sounds and sound levels to estimate how much the sound might affect their behavior. Like people, responses to different sounds vary dramatically, from no response at all, to minor, momentary reactions, to profound changes in behavior.

Exposure to very loud sounds can cause temporary or permanent hearing damage, just as is the case for teenagers who go to too many rock concerts without wearing earplugs. Finally, it is possible for sound levels to be so high that they cause direct physical injury. Explosions can cause injury to the inner ear, for example.

Because there were little data on the potential effects of a source such as the ATOC/NPAL source, scientists conducted associated Marine Mammal Research Projects (MMRPs) during the ATOC Project. Studies were designed to discover if there might be changes in distribution and abundance of humpback whales (6, 7), behavioral reactions of northern elephant seals (8), behavioral responses of humpback whales (9, 10), behavioral responses of fishes (11), changes in vocalizations of humpback whales, and hearing sensitivities of two species of dolphins (12). There were no obvious effects due to the sound source. After intensive statistical analyses, subtle effects were detected, such as the distance and time between successive humpback whale surfacings increased slightly with increasing sound levels.

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