Ocean Noise: Turn it down - Ocean Conservation Research

Page 10
Man-made ocean noise:
the invisible pollution
Commercial shipping
Since their introduction in the 19th Century propeller-driven
ships have become the single most dominant source of
man-made low frequency noise throughout much of the
world’s oceans. The noise is caused primarily by ‘propeller
cavitation’, which occurs when the low pressure generated
by the propeller causes thousands of tiny bubbles to form
in the water. The sound these bubbles make when they
burst is the major source of noise from boats under power.
Shipping noise interacts with sounds made by many
species of whale across a broad range of frequencies
and particularly with the 20-500 Hz vocalisation range of
large baleen whales (see Figure 2, page 19). The noise
created by shipping is immense. Commercial shipping
generates sound pressure levels that range between 150
and 195 dB at source. 5 Moreover, these sounds are
continuous.
With more than 90 per cent of the world’s commerce
reportedly dependent on ocean transport it is no surprise
that the commercial shipping fleet has been growing fast.
The gross tonnage of commercial vessels nearly quadrupled
from 160 million GT in 1965 to 605 million GT in 2003, with a
similar increase in propulsion power. 6 Today the world’s
merchant fleet of vessels of 100 GT and above numbers
97,000 7 , including some 11,000 tankers. The largest of these
tankers displaces over half a million tonnes. By 2025 the
amount of cargo shipped around the world is forecast by
shipping analysts to double or even triple.
With a trend towards ever larger, faster and more powerful
ships the commercial fleet of tomorrow is likely to generate
even more noise. The effects of global warming are also
opening up new areas of the world’s oceans to commercial
shipping: Arctic ice shrinkage may soon enable ships to
sail regularly over the top of the world for the first time.
Man-made ocean noise: the invisible pollution
160 million 605 million
1965
2003
Gross tonnage (GT) of commercial vessels
Number of commercial vessels
5 Hildebrand, 2004 reports that peak spectral densities for
individual ships range from 195 dB re µPa2 /Hz @ 1 m for
fast moving supertankers, to 140 dB re µPa2 /Hz @ 1 m for
small fishing vessels.
89,899
41,865
Figure 1: Increase in commercial shipping
By 2025 the amount of cargo
shipped around the world is
forecast by shipping analysts
to double or even triple.
6 Ross, D.G., 1993
7 Lloyds Register, Merchant Fleets of the World, 2007. 50,000
cargo carrying vessels and 47,000 miscellaneous vessels
including tugs, dredgers and fishing boats.