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Investigations into the auditory physiology of marine animals with special reference to the effect of anthropogenic noise pollution on cetaceans
In the frequently murky waters of the seas, an acute sense of hearing is of central importance. It is used to retain cohesion in social groups, for echolocation and the detection of approaching predators.
A NATO
investigation conceded that a mass stranding of Beaked Whales in Greece in 1996
could not have been caused naturally. More recently, 15 beaked whales died after
being washed up on the Canary Islands following military manoeuvres in the
region of Nato's Mediterranean fleet.
A thorough search of the literature has been unable to find any references on Scanning Electron Microscope (SEM) examinations of the inner ear ultrastructure from any of the cetacean species. Damage to the ear may contribute significantly to the reported disorientation experienced by cetaceans which have become stranded live, yet do not present with any obvious signs of injury.
With rising levels of man-made noise in rivers and oceans, it is becoming increasingly important to be able to form objective estimates of the effect of noise within the hearing range of a particular organism. Thus, there is a need to generate a concise audiogram for D. delphis in response to the sound pressure and particle motion components of an acoustic signal, with special emphasis on the reception of low frequencies (50 Hz to 5 kHz). The hearing frequencies or audiograms for a number of odontocetiformes are well characterised, and have been produced using both physiological and behavioural approaches (see Nachtigall et al., 1995; Kastelein, et al., 2003; Sauerland and Dehnhardt, 1998; Gerstein et al., 1999; Kastelein et al., 2002), though the audiogram for D. delphis has as yet to be produced. The bottlenose dolphin (T. truncates) hears frequencies from 100 Hz to 150 kHz (Johnson, 1966; 1967), and the striped dolphin (Stenella coeruleoalba) hears frequencies ranging from around 500 Hz to 150 kHz (Kastelein, 2003; Brill et al., 2001), with both producing broadband clicks for echolocation that range in frequency from 20 Hz (well below the published audible frequencies for this animal) to around 200 kHz. P. phocoena hears frequencies between 300 Hz (Kastelein et al., 2002), up to as high as 190 kHz (Bibikov, 1992; Popov, 1986; Kastelein et al., 2002), and utilises a narrow band high frequency sonar of around 120 to 140 kHz (Busnel and Dziedzic, 1966a). It is clear from studying the hearing of small cetaceans, that low frequency sound reception appears to be poor in these animals compared to generalist fish species (Lovell et al., 2005 D); as previously mentioned, these findings could be indicative of the inefficiency of ceramic transducers (used in many of the experiments) at frequencies below 2 or 3 kHz. Audiograms produced for the majority of non-mammalian marine vertebrates show that lower hearing thresholds can be obtained in a particle motion dominated sound field. However, the sensitivity of the cetacean ear to particle motion is not known, even though this information may have considerable or even critical implications in future mitigation strategies. Link to Underwater Noise Pollution
In addition to the proposed ultrastructural examination of cetacean hearing, ARIA Marine intends to conduct hearing tests of animals that strand live, using the Auditory Brainstem Response (ABR) technique. An ABR system that functions underwater using submerged transducers has been developed in Plymouth, which is both portable, and can be used in a variety of situations (such as on a beach or from a boat). •
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Post-mortem Ultrastructural Examination
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Post mortem examinations showed
they had suffered brain lesions and inner ear damage, raising concerns about the
impact of the sonar. Exposure to loud noise results in an increases in
thresholds or hearing loss, and can be either temporary (TTS) or permanent
(PTS). Symptoms of TTS include the temporary loss of hearing ability,
pain, vertigo and tinnitus, though hearing thresholds return to pre-exposure
values (no permanent injury to the ear). Permanent Threshold Shift (PTS)
symptoms include ossicular fracture or dislocation, round and oval window
rupture with cerebrospinal fluid leakage into middle ear along with cochlear and
saccular damage. Thresholds do not return to pre-exposure value.
