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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).