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6.4 Assessing Impact
Temporary or chronic exposure to noise can have an impact in cetaceans. That
impact can range from changes in the behavior displayed to permanent hearing loss
and potentially even death (e.g. Cox et al. 2006; Clark et al. 2009; Mooney et al.
2009; Mann et al. 2010). Passive acoustics can also be used to assess impact of noise
on cetaceans. For this, two main approaches can be taken: experimental or opportunistic. The experimental consists of presenting a stimulus and measuring the
response of cetaceans, such as has been done within the multidisciplinary project
called SOCAL-BRS, in which researchers from several institutions try to assess
impact of military sonar on different cetacean species in the South of California.
Alternatively, one can choose an opportunistic approach, in which we measure the
response of individuals using already existing alterations in the ambient noise levels. This last method is less invasive but also requires more time of acoustic data to
make reliable comparisons.
One way of measuring impact consists of using a Bayesian approach (Melcón
et al. 2012b). In this way, we can calculate the probability of a particular odontocete
vocalizing given a particular anthropogenic noise and compare the probability of
the same species vocalizing given no anthropogenic noise. In that particular paper,
the authors studied blue whale foraging calls given sonar or no sonar. They found
that with sonar animals are more silent than without sonar, probably implying that
when blue whales hear sonar they stop foraging.
A similar approach can be taken to determine at which received level of sonar,
for example, the probability of blue whales calling decreases to half. For this, we
plot the probability of an animal vocalizing as a function of the received level of the
noise, and we fit the data to a logit function (see Fig. 6.3).
Besides the change in behavior, ambient and anthropogenic noise can cause
masking. This phenomenon can occur when there is an overlap of frequencies
between the animal’s acoustic signal and the noise, such that the noise “buries”
partially the call of interest. To measure the effect of masking, one type of analysis
that can be conducted consists of calculating the acoustic space reduction
(Hermannsen et al. 2014; Clark et al. 2009). In other words, and based on the minimum signal-to-noise ratio required by the auditory system of the animal to evaluate
the signal of interest, how much does the effective area get reduced with a certain
level of background noise. This means that two individuals will have to be closer for
communication to be effective with considerable background noise, or echolocation
will be less effective at long distances.
While acoustics cannot address the whole spectrum of potential impacts that
noise can have on marine mammals, there are certain measures that can be assessed
using passive acoustics and can already tell us something about the effects of human
activity in the water on marine life. Thus, while not overarching, we recommend the
use of this method to learn more about the impact of noise pollution on cetaceans in
Latin America, and in this way make the corresponding recommendations to advocate in an informed manner for the conservation of odontocetes in our regions.
M.L. Melcón et al.
6.4 Assessing Impact
Temporary or chronic exposure to noise can have an impact in cetaceans. That
impact can range from changes in the behavior displayed to permanent hearing loss
and potentially even death (e.g. Cox et al. 2006; Clark et al. 2009; Mooney et al.
2009; Mann et al. 2010). Passive acoustics can also be used to assess impact of noise
on cetaceans. For this, two main approaches can be taken: experimental or opportunistic. The experimental consists of presenting a stimulus and measuring the
response of cetaceans, such as has been done within the multidisciplinary project
called SOCAL-BRS, in which researchers from several institutions try to assess
impact of military sonar on different cetacean species in the South of California.
Alternatively, one can choose an opportunistic approach, in which we measure the
response of individuals using already existing alterations in the ambient noise levels. This last method is less invasive but also requires more time of acoustic data to
make reliable comparisons.
One way of measuring impact consists of using a Bayesian approach (Melcón
et al. 2012b). In this way, we can calculate the probability of a particular odontocete
vocalizing given a particular anthropogenic noise and compare the probability of
the same species vocalizing given no anthropogenic noise. In that particular paper,
the authors studied blue whale foraging calls given sonar or no sonar. They found
that with sonar animals are more silent than without sonar, probably implying that
when blue whales hear sonar they stop foraging.
A similar approach can be taken to determine at which received level of sonar,
for example, the probability of blue whales calling decreases to half. For this, we
plot the probability of an animal vocalizing as a function of the received level of the
noise, and we fit the data to a logit function (see Fig. 6.3).
Besides the change in behavior, ambient and anthropogenic noise can cause
masking. This phenomenon can occur when there is an overlap of frequencies
between the animal’s acoustic signal and the noise, such that the noise “buries”
partially the call of interest. To measure the effect of masking, one type of analysis
that can be conducted consists of calculating the acoustic space reduction
(Hermannsen et al. 2014; Clark et al. 2009). In other words, and based on the minimum signal-to-noise ratio required by the auditory system of the animal to evaluate
the signal of interest, how much does the effective area get reduced with a certain
level of background noise. This means that two individuals will have to be closer for
communication to be effective with considerable background noise, or echolocation
will be less effective at long distances.
While acoustics cannot address the whole spectrum of potential impacts that
noise can have on marine mammals, there are certain measures that can be assessed
using passive acoustics and can already tell us something about the effects of human
activity in the water on marine life. Thus, while not overarching, we recommend the
use of this method to learn more about the impact of noise pollution on cetaceans in
Latin America, and in this way make the corresponding recommendations to advocate in an informed manner for the conservation of odontocetes in our regions.
M.L. Melcón et al.