Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
There is behavioral evidence that at least some baleen whales detect
faint calls from conspecifics many kilometers away, and head toward the
calling animals (Watkins 1981b; Tyack and Whitehead 1983). Cwnmings and
Thompson (1971) showed that gray whales swim rapidly away when killer whale
sounds are projected into the water. Subsequently, Malme et al. (1983) found
that gray whales detected killer whale sounds when their signal to noise ratio
was about 0 dB. Various species of baleen whales have been found to move away
from sources of industrial sounds. The directional responses to calling
conspecifics, killer whale sounds, and industrial noise demonstrate that
baleen whales have directional hearing capabilities.
The thresholds of other marine mammals range between 30 and 80 dB re
1 pPa at the frequencies to which they are most sensitive (see Figures 2.25
and 2.26). If baleen whales have similar sensitivities, but shifted to
frequencies below 1 kHz, oceanic ambient noise--even in the absence of
industrial activity--rather than absolute detection threshold would be the
factor limiting hearing. Even In quiet conditions (sea state I ), average,
ambient noise levels in the ocean are above 75 dB re 1 pPa in all 1/3 octave
bands below 1000 Hz (Greene 1987, based on Knudsen et al. 1948). As noted
earlier, masking bandwidths may exceed 1/3 octave at low frequencies, in which
case ocean noise levels in masking bands would be even higher.
Though ambient noise probably limits low frequency hearing in baleen
whales, the possible situation above 1 kHz is less clear.' Ambient noise
levels fall as frequency rises, and are, therefore, less likely to limit
hearing. Cochlear structure suggests that the high frequency cutoff of baleen
whales is about 20 kHz (G. Fleischer, pers. comm.).
Although audition data are totally lacking for baleen whales, auditory
attributes such as critical ratio and sound localization ability may not be
radically different than those of other mammals. This may be true even though
low frequency sounds are probably the most important sounds for baleen whales.
All vertebrates studied to date can localize sound, with humans and bottlenose
dolphins having the most precise abilities of any species studied. Between
250 and 1000 Hz, humans have minimum audible angles below 2O (Gourevitch
1980). The baleen whale's ear is well isolated acoustically from the skull, a
prerequisite for extremely accurate sound localization underwater (Fleischer
1978). The ears of pinnipeds are not perfectly isolated from the skull
(Repenning 1972) ; thus the localization abilities of baleen whales may be
superior to those of pinnipeds. The relatively great distance between the
ears of large whales may greatly enhance their ability to localize sound cues
(see Gourevitch 1980 for a detailed discussion of localization). Norris
(1981) suggested that baleen whales may be able to find prey concentrations by
localizing the sounds produced by swimming fish (e.g., Moulton 1960).
Critical ratio functions are similar among many vertebrates, and those of
the baleen whales may be comparable. Baleen whales may also have lower
critical ratios when signal and noise are angularly separated. Given the
large size of baleen whales' heads, this improvement' in critical ratio as a
result of directional phenomena may extend to lower frequencies than in other
mammals.