Oceans of noise - Whale and Dolphin Conservation Society

However, despite the reported low auditory sensitivity of beluga whales to low frequencies
(Awbrey et al. 1988; Johnson et al. 1989) they were able to detect, and reacted to, low frequency
noises at distances of up to 85km. The beluga whale reactions occurred at a distance much greater
than would have been predicted by mathematical sound propagation models, using data on
auditory thresholds (the minimum sound level required before an animal can hear a sound of a
certain frequency). Acoustic sensitivity data gathered from captive animals predicted that beluga
whales would not have been able to hear the approach of shipping vessels any greater than 20km
away (assuming a threshold of 104 dB re 1µPa at 1 kHz; Johnson et al. 1989). It can not be
completely ruled out, however, that the belugas were reacting to higher harmonics of the
generally low frequency shipping noise or that modelled propagation conditions did not represent
actual conditions.
Most auditory sensitivity experiments are conducted on cetaceans in captivity. The tanks in which
captive cetaceans are kept frequently have a high level of ambient noise from human activities
nearby, machinery associated with the tanks (filters, pumps and chillers) and the nature of the
tanks often causes cetacean vocalisations and echolocation clicks to reflect multiple times, further
ensonifying the tanks. It is highly likely that animals kept in such environments may suffer some
hearing impairment through living in a high noise environment. Doubt, therefore, should be
placed on studies of cetacean hearing capabilities produced in captive environments, and freeranging
cetaceans are likely to be more sensitive to sounds, at lower levels, than portrayed in the
cited studies above.
In addition, low frequency sounds may be detected by some mechanism other than conventional
hearing. This has been suggested to be the case for bottlenose dolphins (Turl 1993) when research
showed that bottlenose dolphins could detect frequencies of only 50-150Hz. The skin of toothed
whales is extremely sensitive (Palmer & Weddell 1964; Yablokov et al. 1974) and sensitive to
vibrations (Ridgeway 1986) or small pressure changes in the area surrounding the eyes, blowhole
and head region (Kolchin and Bel’Kovich 1973; Bryden and Molyneux 1986). These authors
suggest that dolphin skin receptors can detect changes in hydrodynamic and hydrostatic pressure,
including low frequency sound.
Most studies investigating the acoustic sensitivities of cetaceans are, as mentioned above,
conducted on captive animals. As mysticetes are too large to keep in captivity, experiments have
largely not been conducted to determine to auditory sensitivity of baleen whales. Ridgeway and
Carder (2001), however, attempted to determine the auditory sensitivity of a stranded and
rehabilitated gray whale calf. This study suggested that the calf was most sensitive to frequencies
of 3 kHz, 6 kHz and 9kHz although the authors had difficulties in obtaining definitive results
from the experiment (Ridgeway and Carder 2001). However, behavioural reactions from gray
whales in the wild to differing frequencies, suggest that they are most sensitive to frequencies of
between 0.8 and 1.5 kHz (Dalheim and Ljungblad 1990).
4.6. Cetacean vocalisations
The vocalisations of cetaceans also give us an idea of their hearing sensitivities, i.e. we expect
that they will have very sensitive hearing for sound frequencies that are the same as their social
calls and echolocation clicks. Cetacean vocalisations extend over a wide range of frequencies
from the ultrasonic pulses of porpoises (130-150 kHz) (see Table 4.1) to the low frequency moans
of blue whales (10-15 Hz) (Table 4.2).
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