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Oceans of noise - Whale and Dolphin Conservation Society

Oceans of noise - Whale and Dolphin Conservation Society

4. The Use

4. The Use of Sound by CetaceansChris Parsons and Sarah DolmanAn understanding of cetacean hearing and aural mechanisms is essential in order to assess thepotential effects of anthropogenic noise on them.Cetaceans live in an environment in which vision is not the primary sense; this is because lightdoes not penetrate far beneath the surface of the ocean. As you dive down from the sea’s surface,longer wavelength light is the first to vanish, with red light no longer being visible at depths of10m, followed by yellow, then green and finally blue light. Hardly any light penetrates depthsgreater than 200m. Therefore, cetaceans have become reliant upon sound, instead of light, as theirprimary sense for communication and being aware of their surrounding environment.Moreover, many cetaceans inhabit underwater environments that restrict visibility further, such asturbid rivers and estuaries, or plankton rich oceanic waters. These species rely on sound evenmore heavily as their primary sense; with the extreme being the river dolphins, whose visualsenses are greatly degenerated to the extent of being limited to the simple detection of dark andlight areas.4.1. EcholocationEcholocation is the ability by which animals can produce mid- or high-frequency sounds anddetect the echoes of these sounds that bounce off of distant objects, to determine the physicalfeatures of their surroundings. To date, toothed whales (odontocetes) are the only marinemammals known to produce echolocation sounds.Echolocation provides accurate and detailed information about the cetacean’s surroundings andallows odontocetes to detect objects only a few centimetres across at distances of tens of metres.They can even distinguish differences in the composition of objects that are externally identical(Kamminga and Van der Ree 1976). Echolocation sounds tend to be produced at highfrequencies. Bottlenose dolphins produce echolocation clicks in frequencies of 50 kHz up to 130kHz (Au 1993), whereas porpoises produce echolocation clicks of 110-150 kHz. (Kamminga andWiersma, 1981). The higher the frequency used by the cetacean, the greater resolution the clickshave (and the smaller an object that can be detected), however, higher frequency sounds have amore limited range underwater.Echolocation is vital to odontocetes. They not only rely on the process for detecting and catchingprey species, but also use it for ‘seeing’ the environment around them. An odontocete unable toproduce or hear echolocation clicks would effectively become ‘blind’ and presumably quicklydie. Even a slightly reduced ability to echolocate would severely impact the health of cetaceans,particularly those inhabiting low visibility habitats such as rivers and estuaries.4.2. NavigationMysticete cetaceans are known to produce low frequency calls of high source level. As lowfrequency sounds attenuate less quickly in a marine environment, these sounds could theoreticallytravel great distances. In addition, the hydrography of oceanic water causes the production ofsubmarine sound channels, where water layers of differing temperatures and densities causesound waves to be concentrated and channelled for great distances (particularly sounds of low45

frequencies). These sound channels would allow low frequency sounds produced by mysticetes totravel even further.It has been estimated that a 20 Hz call from a fin whale could be detected at a distance of severalhundred kilometres from the calling whale (Spiesberger and Fristrup 1990). It has been suggestedthat mysticete whales use low frequency calls to orientate and navigate in a way similar toecholocation (Norris 1969; Payne and Webb 1971). Low frequency calls would echo back fromthe seabed in a fashion similar to a depth sounder (Thompson et al. 1979), or distantoceanographic features, such as a continental shelf edge, submarine mountain range or islandchain (e.g. Hawaii, the Azores or Bermuda) (Tyack 1997). Such a form of navigation would seemessential to assist whales navigating on their long migrations. Polar-dwelling cetaceans such asthe bowhead whale could also use these calls to monitor the location of the ice edge (Ellison et al.1987) which could be vital, not only in finding prey species which concentrate near the ice edge,but also in ensuring that the whale does not travel too far into pack and become trapped.Unfortunately, many forms of anthropogenic sound are also produced at these low frequencies. Awhale unable to migrate or manoeuvre around the ice edge safely due to high levels ofanthropogenic noise would presumably have seriously reduced chances of survival.4.3. CommunicationCommunication is the production of a stimulus or signal that is received by another organismeliciting a response. Cetaceans communicate within and between species in a variety of ways,although due to the environment in which they the live, as explained above, the majority of thiscommunication is in the form of acoustic signals.Cetacean communication has a variety of functions such as:______intrasexual selection;intersexual selection;mother/calf cohesion;group cohesion;individual recognition; anddanger avoidance.4.3.1. Intrasexual selectionIntrasexual selection, incorporates a variety of behaviours that maintains social orders within thesexes, such as hierarchies of dominance, or maintenance of territories. This type ofcommunication is clearly seen in the humpback whale. It is believed that only the male humpbackwhales sing and their song has been demonstrated to maintain distances between whales (Tyack1981; Helweg et al. 1992). Aggressive interactions occur between singing humpack whales andother males (Tyack 1982) and similar results have also been recorded for singing minke whales(Gedamke et al. 2001).4.3.2. Intersexual selectionVertebrate species use vocal calls as an “honest” means of demonstrating their fitness (Daviesand Haliday 1978; Clutton-Brock and Albon 1979), the call being a costly and, therefore, honestsignal (Zahavi 1987; Grafen 1990a, 1990b). It allows a female to choose the best male possible tofather her offspring (ensuring that the offspring would be as healthy and as viable as possible). It46

Oceans of noise - Whale and Dolphin Conservation Society
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