There are ca. 65 species of small toothed whales possessing echolocation (sonar) abilities, of which 14 species, of three different odontocete families produce solely Narrow Band High Frequency (NBHF) echolocation clicks (Kyhn et al. 2010; Kyhn et al. 2013). These species include: Chilean dolphin (Cephalorhynchus eutropia), Commerson’s dolphin (Cephalorhynchus commersonii), Dall’s porpoise (Phocoenoides dalli), Hector’s dolphin (Cephalorhynchus hectori), Hourglass dolphin (Lagenorhynchus cruciger), and, Peale’s dolphin (Lagenorhynchus australis). Studies are limited for many NBHF species, owing in part to elusive behaviour and/or a lack of funding.
All NHBF species produce echolocation clicks at peak frequencies of >120 kHz, 3 dB bandwidth of 6–26 kHz and Q-values between 8 and 20 e.g., Phocoena phocoena, Neophocaena phocaenoides, Kogia breviceps, and Cephalorhynchus hectori. dB bandwidth can be defined as the frequency width from the peak frequency to -3 dB and Q-values are the ‘quality value’, defined as one of the peak or centroid frequency, divided by a given change in bandwidth (Kyhn et al. 2009).
EVOLUTION OF NARROW BAND HIGH FREQUENCY ECHOLOCATION
The precise reason why some species evolved to emit only NBHF echolocation clicks is unknown, but one explanation is that detection of higher frequency sounds is less affected by ambient noise, which is lower in frequency generally (Kyhn et al. 2009); however, with increasing frequency, sound absorption increases, thereby reducing range at which high frequency sonar is effective (Madsen & Wahlberg 2007). NBHF echolocation clicks could also be an adaptation for predator avoidance against killer whales (Orcinus orca) (Morisaka & Connor 2007; Gotz et al. 2010), given that killer whale hearing sensitivity decreases above ca. 60 kHz (Kyhn et al. 2009).
DISTINGUISHING BETWEEN SPECIES
Individual species have their own click characteristics, which correspond to the environment in which they inhabit e.g. deep oceans, shallow inlets, rivers etc. For example, Kyhn et al. (2009) found that hourglass dolphins, which favour generally open, uncluttered waters, have longer Inter-Click-Intervals (ICI) and click durations than those recorded for coastal Hector’s dolphins, inhabiting more cluttered environments. Similarly, when comparing coastal harbour porpoise (Phocoena phocoena) that occupy slightly deeper waters than the coastal finless porpoises (Neophocaena phocaenoides), Akamatsu et al. (2007) established that the 60.4 ms mean ICI of the finless porpoise was considerably shorter than the 80.5 ms mean of the harbour porpoise, the latter of which benefits from exploring further afield in its environment. Evidence for this has also been observed in microchiropteran (sonar-producing) bats, which have been shown to use higher frequencies when echolocating in cluttered habitats such as near vegetation or over water (Jung et al. 2007).
Some sympatric species are distinguished irrefutably through visual observations; however, this is not always the case, and there are significant disadvantages to visual surveys, which may be overcome by the use of Passive Acoustic Monitoring (PAM) surveying (see www.passiveacousticmonitoring.co.uk for further details). PAM systems often employ specialised software allowing the vocal sounds to be visualised, such as MATLAB® (www.mathworks.co.uk), Ishmael (www.bioacoustics.us) and PAMGUARD (www.pamguard.org), which has become the industry standard software for real-time monitoring of marine mammals for mitigation during industrial operations.
Similarities in click characteristics should be evident among sympatric species, i.e. those that share the same habitat. This could lead to difficulties in distinguishing acoustic signals of one species from another; however, for some species, it has been shown that signals produced by closely associated species have subtle differences, for example in centroid frequency or bandwidth. This has indeed been demonstrated by Kyhn et al. (2010) for Peale’s dolphins and Commerson’s dolphins off the Falklands Islands, and by Kyhn et al. (2013) for harbour and Dall’s porpoises in British Columbia, Canada.
THREATS TO NBHF SPECIES
Many NBHF species face threats to their survival, often due to anthropogenic activities such as commercial fishing, which can result in habitat destruction and bycatch. Passive Acoustic Monitoring has proven to be a useful tool in discovering more about the behaviour, abundance, population densities and habitats of these species. Based on the data collected, local governments and conservation organisations are able to implement more effective conservation measures.
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