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drops of up to 58 per cent in cetacean communication and sounds when exposed to vessel traffic (Jensen et al., 2009). Noise from merchant ships elevates the natural ambient noise level by 20–30 dB in many areas, with especially high frequency sounds, to which some cetaceans are very sensitive (Frankel et al., 1995; Arveson and Vendittis, 2000). Boat noise is easily audible to killer whales (Or- cinus orca) as far as 15 km away (Erbe, 2002), minke whale (Balae-

noptera acutorostrata) “boings” have been picked up at over 100 km distance (Oswald et al., 2011). Distances between communicating humpback whales in one study were over 5 km. Recent research using the underwater microphones of the Sound Surveillance Sys- tem (SOSUS) can track singing blue, fin, humpback and minke whales and has revealed that whale song can probably be heard across several thousand kilometres (Croll et al., 2002).

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