Environmental Engineering Reference
In-Depth Information
at these frequencies and exceeds the sound pressure level (i.e., above the line) on
a given species' audiogram, the organism will be able to detect the sound. There
is a wide range of sensitivity to sound among marine fish. Herrings (Clupeoidea)
are highly sensitive to sound due to the structure of their swim bladder and audi-
tory apparatus, whereas flatfish such as plaice and dab (Pleuronectidae) that have
no swim bladder are relatively insensitive to sound (Nedwell et al., 2004). Possible
responses to the received sound may include altered behavior (e.g., attraction, avoid-
ance, interference with normal activities) (Nelson, 2008) or, if the intensity is great
enough, hearing damage or mortality. For example, fish kills have been reported in
the vicinity of pile-driving activities (Caltrans, 2001; Longmuir and Lively, 2001).
The National Research Council (2000) reviewed studies that demonstrated a
wide range of susceptibilities to exposure-induced hearing damage among different
marine species. The implications are that critical sound levels will not be able to be
extrapolated from studies of a few species (although a set of representative species
might be identified), and it will not be possible to identify a single sound level value
at which damage to the auditory system will begin in all, or even most, marine mam-
mals. Participants in a recent National Oceanic and Atmospheric Administration
(NOAA) workshop (Boehlert et al., 2008) suggested that sounds that are within
the range of hearing and “sweep” in frequency are more likely to disturb marine
mammals than constant-frequency sounds. Thus, devices that emit a constant fre-
quency may be preferable to ones that vary. They believed that the same may be true,
although perhaps to a lesser extent, for sounds that change in amplitude.
Moore and Clarke (2002) compiled information on the reactions of gray whales
(
Eschrichtius robustus
) to noise associated with offshore oil and gas development
and vessel traffic. Gray whale responses included changes in swim speed and direc-
tion to avoid the sound sources, abrupt but temporary cessation of feeding, changes
in calling rates and call structure, and changes in surface behavior. They reported
a 0.5 probability of avoidance when continuous noise levels exceeded about 120 dB
re 1 µPa and when intermittent noise levels exceeded about 170 dB re 1 µPa. They
found little evidence that gray whales travel far or remain disturbed for long as a
result of noises of this nature
Weilgart (2007) reviewed the literature on the effects of ocean noise on cetaceans
(whales, dolphins, porpoises), focusing on underwater explosions, shipping, seismic
exploration by the oil and gas industries, and naval sonar operations. She noted that
strandings and mortalities of cetaceans have been observed even when estimated
received sound levels were not high enough to cause hearing damage. This suggests
that a change in diving patterns may have resulted in injuries due to gas and fat emboli
(a fat droplet that enters the blood stream). That is, aversive noise may prompt ceta-
ceans to rise to the surface too rapidly, and the rapid decompression causes nitrogen
gas supersaturation and the subsequent formation of bubbles (emboli) in their tissues
(Fernandez et al., 2005). Other adverse (but not directly lethal) impacts could include
increase stress levels, abandonment of important habitats, masking of important
sounds, and changes in vocal behavior that may lead to reduced foraging efficiency
or mating opportunities. Weilgart (2007) pointed out that responses of cetaceans to
ocean noise are highly variable among species, age classes, and behavioral states, and
many examples of apparent tolerance of noise have been documented.
