- LONDON (Reuters) -- Whale-watching
is a big attraction off the northwestern United States but noise from tourist
boats is making life difficult for the giant mammals.
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- Engine rumble is drowning out crucial signals killer
whales send to each other over distances of several kilometres (miles)
and they have to struggle to be heard.
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- "Today's southern resident population of killer
whales is exposed to intense whale-watching activity. This is associated
with considerable boat engine noise," Rus Hoelzel, of the University
of Durham in England, said in the science journal Nature on Wednesday.
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- He and his colleagues recorded and studied the vocal
behaviour of three groups of killer whales in the waters along Washington
state from 1977-1981, 1989-1992 and 2001-2003.
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- They also analysed the length of their calls in the presence
and absence of whale-watching boats.
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- There was no difference during the first two time periods
but during 2001-2003 the whales increased the duration of their calls by
15 percent when the boats were around.
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- Lengthening calls is one strategy used by the whales
to overcome interference from background noise. They can also change frequency
and the amplitude of their signal, according to the scientists.
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- Since the 1990s, the number of whale-watching boats has
increased five-fold and the population of killer whales has declined. During
daylight hours an average of 22 boats each day are tracking the whales.
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