10 MARINE MAMMALS AND SEA TURTLES - Hebron Project

Hebron Project Comprehensive Study Report
Marine Mammals and Sea Turtles
(Kastak et al. 1999, 2005; Ketten et al. 2001; cf. Au et al. 2000). However,
TTS onset in the California sea lion and northern elephant seal may occur at
a similar sound exposure level as in odontocetes (Kastak et al. 2005).
There have been few studies that have directly investigated hearing or
noise-induced hearing loss in sea turtles. The apparent occurrence of TTS in
loggerhead turtles exposed to many pulses from a single airgun less than or
equal to 65 m away (Moein et al. 1994) suggests that sounds from an airgun
array could cause at least temporary hearing impairment in sea turtles if they
do not avoid the (unknown) radius where TTS occurs. There is also the
possibility of permanent hearing damage to turtles close to the airguns.
However, there are few data on temporary hearing loss, and no data on
permanent hearing loss in sea turtles exposed to airgun pulses.
PTS: When PTS occurs, there is physical damage to the sound receptors in
the ear. In some cases, there can be total or partial deafness, while in other
cases, the animal has an impaired ability to hear sounds in specific frequency
ranges. There is no specific evidence that exposure to pulses of airgun sound
can cause PTS in any marine mammal, even with large arrays of airguns.
However, given the likelihood that some mammals close to an airgun array
might incur at least mild TTS (see Finneran et al. 2002), there has been
speculation about the possibility that some individuals occurring very close to
airguns might incur PTS (Richardson et al. 1995, p. 372). The specific
difference between the PTS and TTS thresholds has not been measured for
marine mammals exposed to any sound type. When exposure is measured in
SEL units, Southall et al. (2007) concludes the PTS-onset to TTS-onset for
marine mammal exposure to impulse sound is at least 15 dB. Based on data
from terrestrial mammals, a precautionary assumption is that the PTS
threshold for impulse sounds (such as airgun pulses as received close to the
source) is at least 6 dB higher than the TTS threshold on a peak-pressure
basis, and probably more than 6 dB.
Although it is unlikely that airgun operations during most seismic surveys
would cause PTS in marine mammals, caution is warranted given the limited
knowledge about noise-induced hearing damage in marine mammals,
particularly baleen whales. Commonly applied monitoring and mitigation
measures, including visual monitoring, ramp-ups, and power-downs of the
airguns when mammals are seen within the ―safety radii‖, are expected to
minimize the already-low probability of exposure of marine mammals to
sounds strong enough to potentially induce PTS.
The study by Moein et al. (1994) indicates that sea turtles can experience
TTS when exposed to moderately strong airgun sounds. However, there are
no data to indicate whether or not there are any plausible situations in which
exposure to repeated airgun pulses at close range could cause permanent
hearing impairment in sea turtles.
Non-auditory Physiological Effects: Possible types of non-auditory
physiological effects or injuries that could theoretically occur in marine
mammals exposed to strong underwater sound might include stress,
neurological effects, bubble formation, and other types of organ or tissue
10-46 June 2010