1. Overview
9
response. Most of the responses were to the feeding call. Feeding call and
social sounds produced changes in both separation and whale speed, indicating that these sounds can alter a whale's behavior. The M. novaeangliae
responded to sounds as low as 102 to 105dB but the strongest responses
occurred when the sounds were 111 to 114 dB re 1 ~Pa.
All of the playback experiments suggest that sounds must be between 85
and 120dB before whales will react to them. These levels are very high compared to what dolphins can hear and may suggest that it is very difficult to
relate reaction to hearing sensitivity. Whales may not be reacting strongly
unless the sounds are much higher than their hearing threshold.
Determining the hearing sensitivity or audiogram of baleen whales
represents an extremely difficult challenge and will probably require
using some sort of electrophysiological technique. Dolphin (Chapter 7)
discusses auditory evoked potential measurements and techniques with
odontocetes. Perhaps a technique measuring auditory evoked potentials
with beached whales may provide a way to estimate hearing sensitivity.
Even with evoked potential measurements, there are many issues that have
to be considered. For example, if an airborne source is used, the results
cannot be translated directly to the underwater situation. If a sound source
is placed on a whale's head, relating that to' a whale receiving a plane
wave will also not be simple. Measurement of evoked potential may not
be simply because of the amount of flesh, muscle, and blubber that the
brain waves would have to travel through in order to reach the measurement electrodes.
3.1 Effects of Low-Frequency Anthropogenic
Sounds on Cetaceans
The introduction of the Acoustic Thermometry of Ocean Climate (ATOC)
program has caused many marine mammologists and environmentalists to
voice concern over the effects of low-frequency anthropogenic noise on
marine mammals. Au et al. (1997) tested the low-frequency hearing of P
crassidens and G. griseus and found that at 75 Hz, (the center frequency of
the ATOC signal) the animals' pure tone hearing thresholds were 140.7 and
142.2 dB re 1 ~Pa, respectively. When the actual ATOC signal was used,
the thresholds shifted to 139.0 and 140.8 dB re 1~Pa for P crassidens and
G. griseus, respectively. These results indicate that for the ATOC source
located at a depth of 850 m, dolphins swimming directly over the source
would not hear the signal until they dove to a depth of about 400m. Therefore, the ATOC signal will most likely not affect odontocetes, with the possible exception of sperm whales. Au et al. (1997) also estimated that for
ranges greater than about 0.5 km, the maximum sound pressure level above
a depth of 560 m will be approximately 130 dB. This is not a very loud sound,
even for baleen whales since they typically produce much louder sounds
that their conspecifics must listen to.
9
response. Most of the responses were to the feeding call. Feeding call and
social sounds produced changes in both separation and whale speed, indicating that these sounds can alter a whale's behavior. The M. novaeangliae
responded to sounds as low as 102 to 105dB but the strongest responses
occurred when the sounds were 111 to 114 dB re 1 ~Pa.
All of the playback experiments suggest that sounds must be between 85
and 120dB before whales will react to them. These levels are very high compared to what dolphins can hear and may suggest that it is very difficult to
relate reaction to hearing sensitivity. Whales may not be reacting strongly
unless the sounds are much higher than their hearing threshold.
Determining the hearing sensitivity or audiogram of baleen whales
represents an extremely difficult challenge and will probably require
using some sort of electrophysiological technique. Dolphin (Chapter 7)
discusses auditory evoked potential measurements and techniques with
odontocetes. Perhaps a technique measuring auditory evoked potentials
with beached whales may provide a way to estimate hearing sensitivity.
Even with evoked potential measurements, there are many issues that have
to be considered. For example, if an airborne source is used, the results
cannot be translated directly to the underwater situation. If a sound source
is placed on a whale's head, relating that to' a whale receiving a plane
wave will also not be simple. Measurement of evoked potential may not
be simply because of the amount of flesh, muscle, and blubber that the
brain waves would have to travel through in order to reach the measurement electrodes.
3.1 Effects of Low-Frequency Anthropogenic
Sounds on Cetaceans
The introduction of the Acoustic Thermometry of Ocean Climate (ATOC)
program has caused many marine mammologists and environmentalists to
voice concern over the effects of low-frequency anthropogenic noise on
marine mammals. Au et al. (1997) tested the low-frequency hearing of P
crassidens and G. griseus and found that at 75 Hz, (the center frequency of
the ATOC signal) the animals' pure tone hearing thresholds were 140.7 and
142.2 dB re 1 ~Pa, respectively. When the actual ATOC signal was used,
the thresholds shifted to 139.0 and 140.8 dB re 1~Pa for P crassidens and
G. griseus, respectively. These results indicate that for the ATOC source
located at a depth of 850 m, dolphins swimming directly over the source
would not hear the signal until they dove to a depth of about 400m. Therefore, the ATOC signal will most likely not affect odontocetes, with the possible exception of sperm whales. Au et al. (1997) also estimated that for
ranges greater than about 0.5 km, the maximum sound pressure level above
a depth of 560 m will be approximately 130 dB. This is not a very loud sound,
even for baleen whales since they typically produce much louder sounds
that their conspecifics must listen to.