Determine if a sound affects a marine animal

How do you determine if a sound affects a marine animal?

There are many factors that influence whether a sound source affects any marine animal. 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 assessed. This process is called ecological risk assessment[1]EPA. (1998). Guidelines for Ecological Risk Assessment (No. EPA/630/R095/002F). Washington, D.C.: U.S. Environmental Protection Agency.[2]EPA. (1992). Framework for Ecological Risk Assessment (No. EPA/630/R-92/001). Washington, D.C.: U.S. Environmental Protection Agency.. 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 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

Schematic of risk assessment procedures[3]EPA. (1998). Guidelines for Ecological Risk Assessment (No. EPA/630/R095/002F). Washington, D.C.: U.S. Environmental Protection Agency.[4]EPA. (1992). Framework for Ecological Risk Assessment (No. EPA/630/R-92/001). Washington, D.C.: U.S. Environmental Protection Agency.

This general model is used in many types of risk assessments in air or underwater.  An example is how a specific underwater sound source could affect a particular marine species. Scientists begin by asking:
  1. What are the characteristics of the sound (e.g. level, frequency, and rise time) at different distances from the source?
  2. Where are marine animals likely to be located relative to the source?
  3. Which species in that area can detect these sounds?

By combining the answers to the questions above, scientists can make an estimate to answer the questions below. The potential effects of sound level on animals will be used as an example.

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

If the sounds are within the animals’ hearing range, the sounds may have an effect. The sounds could interfere with an animal’s ability to hear important sounds, cause the animal to alter its behavior, or physically impair its hearing. The scientists then ask:

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

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

Scientists use data on the way in which the animals respond to similar sounds, sound levels, and the context of exposure to estimate how much the sound might affect their behavior. Exposure to very loud sounds might cause temporary or permanent hearing damage. There has been some research to determine what sound levels and signal durations may affect marine animals (see What are the potential effects of sound on marine mammals? and What are the potential effects of sound on marine fishes?)

It is important to use rigorous scientific methods to answer questions about how sounds might affect marine animals. An example below is from the North Pacific Acoustic Laboratory’s (NPAL) Acoustic Thermometry of Ocean Climate (ATOC) Project on the risk assessment for humpback whales.

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

The first step is to determine the sound field of a source.  The sound field is the level or intensity of sound at different distances and depths as the sound travels away from the source. (For more detailed information on how sound travels, see Why does sound get weaker as it moves?). There are many propagation models that predict how sound travels away from a source. Environmental parameters that are used in these propagation models include the geographic location, the time of year that the source will be used, physical oceanography,  water depth, and properties of the seafloor and sea surface.

The following figure shows the level of sound at different distances and depths as the sound traveled away from the NPAL/ ATOC source towards the island of Kauai. Sound traveled in all directions away from the source, but only the slice related to the humpback whale example is shown. The NPAL/ATOC, moored on the seafloor at a depth of approximately 800 meters (2600 feet), approximately 14.8 kilometers (8 nautical miles) north of Kauai, had a source level of 195 underwater dB at 1 meter and operated at a frequency of 75 hertz. The colors in the picture show the sound level decreasing as sound moves away from the sound source. In addition, 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 indicated with an arrow. The colors show the predicted received levels at various distances and depths. The picture shows the sound level decreases with distance from the sound source. For more information on propagation modeling, please see Jensen, Kuperman, Porter, and Schmidt, 2011.

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

Humpback whale distribution. The blue dots show where humpback whales were sighted near Kauai, Hawaii during January to April 2001. The inner and outer lines around Kauai show where the water is 100 fathoms (600 feet or 183 meters) and 1000 fathoms (6,000 feet or 1,830 meters) deep, respectively. The humpback whales were often sighted in nearshore water, less than 100 fathoms deep. The red dot north of Kauai shows the location of the NPAL/ATOC source.

Scientists must also consider how the animals are distributed throughout the area and how their distribution changes throughout the year. 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, at upper or lower water depths?), 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 must make estimates of the likelihood that animals will be in certain locations.

The distribution of humpback whales sighted near Kauai, Hawaii, during January to April 2001, are shown in the picture to the left. The illustration shows that humpback whales were found in nearshore waters of less than 100 fathoms (183 meters or 600 feet) deep.

3. Which species in the area can detect these sounds?

Species differ in their hearing abilities. To determine whether or not sound of a specific frequency can be detected at a given level, scientists need to know the sensitivity to the frequencies that the animal can hear, which are called the hearing thresholds. The section, What can animals hear?, provides more information about hearing studies in marine animals.

Scientists have not measured hearing directly in many marine mammal species, including the example species used here, humpback whales. Using information from high resolution computed tomography (CT) imaging and direct anatomical measurements, scientists have estimated the hearing of humpback whales to range from approximately 20 Hz to 33 kHz, with best sensitivity near 3 kHz. [5] Tubelli, A. A., Zosuls, A., Ketten, D. R., & Mountain, D. C. (2018). A model and experimental approach to the middle ear transfer function related to hearing in the humpback whale ( Megaptera novaeangliae ). The Journal of the Acoustical Society of America, 144(2), 525–535. https://doi.org/10.1121/1.5048421. These values are consistent with data from vocalization studies of humpback whales. Humpback whales make three types of vocalizations that range from about 50 Hz up to 10 kHz, so it is very likely that they can hear at the frequency of the NPAL/ATOC source transmissions. Scientists were able to conduct hearing threshold tests on false killer whales and Risso’s dolphins to determine their sensitivity to the NPAL/ATOC source [6]Au, W. W. L., Nachtigall, P. E., & Pawloski, J. L. (1997). Acoustic effects of the ATOC signal (75 Hz, 195 dB) on dolphins and whales. The Journal of the Acoustical Society of America101(5), 2973–2977. https://doi.org/10.1121/1.419304.. They found that these toothed whales can just barely hear the NPAL/ATOC source at the frequency and source level of its transmissions.

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

The next step is to combine the sound field prediction with the marine animal data. An example of this modeling can be seen in the figure below.

In the picture, the sound source (with a source level of 195 underwater dB at 1 meter) is indicated with an arrow. The colors show the predicted received levels at various distances and depths. The two black diamonds are marine mammals that are predicted to be in the area while the source is transmitting. For more information on propagation modeling, please see Jensen, Kuperman, Porter, and Schmidt, 2011.

The sound source is indicated with an arrow. 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 model of the sound field with the probable locations of the marine animals, the amount of sound energy to which the animals might be exposed can be estimated.

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

The final step is to estimate the potential impacts of the sound exposure. Exposure to sounds that are very loud compared to the hearing threshold can cause temporary or permanent hearing damage. Sounds added to the animal’s environment may interfere with the animal’s ability to hear important sounds, such as calls from other animals. This is called masking. Some sounds may also disturb the animal and cause it to change its behavior.  Responses can be influenced by a diverse suite of environmental, biological, and operational factors (i.e., context of exposure)[7]Ellison, W. T., Southall, B. L., Clark, C. W., & Frankel, A. S. (2012). A New Context-Based Approach to Assess Marine Mammal Behavioral Responses to Anthropogenic Sounds: Marine Mammal Behavioral Responses to Sound. Conservation Biology, 26(1), 21–28. https://doi.org/10.1111/j.1523-1739.2011.01803.x.. Factors such as group composition (e.g., a mother-calf pair versus a single, large, adult male), spatial orientation (e.g., a source moving towards versus away from the animal), or behavior at time of exposure (e.g., an animal engaged in deep foraging dives versus resting near the surface) may affect the probability and severity of a behavioral response. Responses to sounds may vary dramatically, from no response at all, to minor, momentary reactions, to profound changes in behavior.

Because there were little data available in the mid 1990’s on the potential effects of the NPAL/ATOC source, scientists conducted research during the ATOC Project. Studies were designed to discover if there might be changes in distribution and abundance of humpback whales[8]Mobley, J. R., Grotefendt, R. A., Forestell, P. H., & Frankel, A. (1999). Results of Aerial Surveys of Marine Mammals in the Major Hawaiian Islands (1993-1998): Report to the Acoustic Thermometry of Ocean Climate Marine Mammal Research Program (ATOC MMRP). Ithaca, NY: Cornell University Bioacoustics Research Program.[9]Frankel, A. S., & Clark, C. W. (2002). ATOC and other factors affecting the distribution and abundance of humpback whales (Megaptera novaeangliae) off the north shore of Kauai. Marine Mammal Science, 18(3), 644–662. https://doi.org/10.1111/j.1748-7692.2002.tb01064.x., behavioral reactions of northern elephant seals[10]Costa, D. P., Crocker, D. E., Gedamke, J., Webb, P. M., Houser, D. S., Blackwell, S. B., … Le Boeuf, B. J. (2003). The effect of a low-frequency sound source (acoustic thermometry of the ocean climate) on the diving behavior of juvenile northern elephant seals, Mirounga angustirostris. The Journal of the Acoustical Society of America, 113(2), 1155–1165. https://doi.org/10.1121/1.1538248., behavioral responses of humpback whales[11]Frankel, A. S., & Clark, C. W. (2000). Behavioral responses of humpback whales ( Megaptera novaeangliae ) to full-scale ATOC signals. The Journal of the Acoustical Society of America, 108(4), 1930–1937. https://doi.org/10.1121/1.1289668.[12]Frankel, A. S., & Clark, C. W. (1998). Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae , in Hawai′i. Canadian Journal of Zoology, 76(3), 521–535. https://doi.org/10.1139/z97-223., behavioral responses of fishes[13]Klimley, A. P., & Beavers, S. C. (1998). Playback of acoustic thermometry of ocean climate (ATOC) -like signal to bony fishes to evaluate phonotaxis. The Journal of the Acoustical Society of America, 104(4), 2506–2510. https://doi.org/10.1121/1.423756., changes in vocalizations of humpback whales, and hearing sensitivities of two dolphin species[14]Au, W. W. L., Nachtigall, P. E., & Pawloski, J. L. (1997). Acoustic effects of the ATOC signal (75 Hz, 195 dB) on dolphins and whales. The Journal of the Acoustical Society of America, 101(5), 2973–2977. https://doi.org/10.1121/1.419304..

There were no obvious or measurable effects as a result of exposure to the NPAL/ ATOC sound source. After extensive statistical analyses, subtle effects were found to be correlated with increasing received sound levels, such as a slight increase in the distance and time between successive humpback whale surfacings [15]Frankel, A. S., & Clark, C. W. (2000). Behavioral responses of humpback whales ( Megaptera novaeangliae ) to full-scale ATOC signals. The Journal of the Acoustical Society of America, 108(4), 1930–1937. https://doi.org/10.1121/1.1289668.[16]Frankel, A. S., & Clark, C. W. (1998). Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae , in Hawai′i. Canadian Journal of Zoology, 76(3), 521–535. https://doi.org/10.1139/z97-223..

The content on DOSITS is based on well understood scientific principles, peer-reviewed literature, and high-quality sources of scientific data. Independent experts who specialize in underwater acoustics have reviewed the material in this section.

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References

Cited References   [ + ]

1, 3. EPA. (1998). Guidelines for Ecological Risk Assessment (No. EPA/630/R095/002F). Washington, D.C.: U.S. Environmental Protection Agency.
2, 4. EPA. (1992). Framework for Ecological Risk Assessment (No. EPA/630/R-92/001). Washington, D.C.: U.S. Environmental Protection Agency.
5. Tubelli, A. A., Zosuls, A., Ketten, D. R., & Mountain, D. C. (2018). A model and experimental approach to the middle ear transfer function related to hearing in the humpback whale ( Megaptera novaeangliae ). The Journal of the Acoustical Society of America, 144(2), 525–535. https://doi.org/10.1121/1.5048421.
6. Au, W. W. L., Nachtigall, P. E., & Pawloski, J. L. (1997). Acoustic effects of the ATOC signal (75 Hz, 195 dB) on dolphins and whales. The Journal of the Acoustical Society of America101(5), 2973–2977. https://doi.org/10.1121/1.419304.
7. Ellison, W. T., Southall, B. L., Clark, C. W., & Frankel, A. S. (2012). A New Context-Based Approach to Assess Marine Mammal Behavioral Responses to Anthropogenic Sounds: Marine Mammal Behavioral Responses to Sound. Conservation Biology, 26(1), 21–28. https://doi.org/10.1111/j.1523-1739.2011.01803.x.
8. Mobley, J. R., Grotefendt, R. A., Forestell, P. H., & Frankel, A. (1999). Results of Aerial Surveys of Marine Mammals in the Major Hawaiian Islands (1993-1998): Report to the Acoustic Thermometry of Ocean Climate Marine Mammal Research Program (ATOC MMRP). Ithaca, NY: Cornell University Bioacoustics Research Program.
9. Frankel, A. S., & Clark, C. W. (2002). ATOC and other factors affecting the distribution and abundance of humpback whales (Megaptera novaeangliae) off the north shore of Kauai. Marine Mammal Science, 18(3), 644–662. https://doi.org/10.1111/j.1748-7692.2002.tb01064.x.
10. Costa, D. P., Crocker, D. E., Gedamke, J., Webb, P. M., Houser, D. S., Blackwell, S. B., … Le Boeuf, B. J. (2003). The effect of a low-frequency sound source (acoustic thermometry of the ocean climate) on the diving behavior of juvenile northern elephant seals, Mirounga angustirostris. The Journal of the Acoustical Society of America, 113(2), 1155–1165. https://doi.org/10.1121/1.1538248.
11, 15. Frankel, A. S., & Clark, C. W. (2000). Behavioral responses of humpback whales ( Megaptera novaeangliae ) to full-scale ATOC signals. The Journal of the Acoustical Society of America, 108(4), 1930–1937. https://doi.org/10.1121/1.1289668.
12. Frankel, A. S., & Clark, C. W. (1998). Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae , in Hawai′i. Canadian Journal of Zoology, 76(3), 521–535. https://doi.org/10.1139/z97-223.
13. Klimley, A. P., & Beavers, S. C. (1998). Playback of acoustic thermometry of ocean climate (ATOC) -like signal to bony fishes to evaluate phonotaxis. The Journal of the Acoustical Society of America, 104(4), 2506–2510. https://doi.org/10.1121/1.423756.
14. Au, W. W. L., Nachtigall, P. E., & Pawloski, J. L. (1997). Acoustic effects of the ATOC signal (75 Hz, 195 dB) on dolphins and whales. The Journal of the Acoustical Society of America, 101(5), 2973–2977. https://doi.org/10.1121/1.419304.
16. Frankel, A. S., & Clark, C. W. (1998). Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae , in Hawai′i. Canadian Journal of Zoology, 76(3), 521–535. https://doi.org/10.1139/z97-223.