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

Report No. 6945
BBN Systems and Technologies Corporation
An analysis was not performed for gray whales and fin whales which may be
in this area during part of the winter. Because of their assumed good hearing
sensitivity at low frequencies, the SER values which would be obtained for
these species would be high and similar to those obtained for the spring -
summer period, with some changes due to different population dens it ies .
5.3 Zone of Influence Estimates for Major Noise Sources
Observations on whale behavior as related to quantified acoustic exposure
levels have been obtained for a few species such as the gray (Malme et al.
1983, 1984) ; bowhead (Richardson et al. 1985; Ljungblad et al. 1989) ; humpback
(Malme et al. 1985) ; and white whales (Aubrey et al. 1986). These studies
have used movement away from a sound source (avoidance) as one of the main
indicators of a desire to reduce sound exposure. Analyses of response data
obtained from playback experiments and tests using air gun sources have
provided estimates of the probability of avoidance for a limited set of
industrial noise sources. The disturbance criteria are usually given in terms
of the sound exposure level which will produce avoidance behavior in 50% of
the animals exposed. For playback experiments with gray whales the sound
level which produced a 50% probability of avoidance was found to vary within a
range of 117 to 125 dB depending on the playback stimulus. The stimuli used
were recordings of a drillship, drilling platform, semi-submersible platform,
production platform and helicopter. The effective level of the playback
signal was determined using the dominant bandwidth of the signal as defined
previously in Section 3.
Studies of the behavioral responses of bowhesd whales to playback of
drillship and dredge noise and to noise from nearby boats have developed a
somewhat different criterion (Richardson et al. 1985; Miles et al. 1987). In
these studies the ratio of the received level in the strongest 1/3 octave band
of the stimulus signal to the ambient noise in the same band was used as the
primary measurement parameter. It was found that, while individual responses
were highly variable, a S/N ratio of 30 dB caused about 50% of bowhead whales
to exhibit avoidance behavior during drillship and dredge noise playbacks.
Reactions to boats seemed to occur at lower S/N ratios.
In developing the estimates of the range at which a 50% probability of
avoidance would occur for the major noise sources determined in this study, we
have considered both the constant effective level criterion, using 120 dB as
representing an average avoidance level for the various sources, and the 30 dB
S/N criterion using the highest 1/3 octave band in the signal. As shown by
the SER ratings determined in the previous section, the gray whale is the
species that is potentially the most impacted by the major underwater sound
sources considered. This is a result of the assumed high sensitivity of this
species to low frequency sound and its high abundance relative to most other
baleen whales in Alaskan waters. Presumably the hearing characteristics of
the bowhead, humpback, and fin whales are comparable to the assumed characteristic
of the gray whale, and as a result they also are potentially more
influenced by the low frequency noise sources considered here than are the
pinnipeds and odontocetes.