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Richard Black By Richard Black Environment correspondent, BBC News
Noise from ships stresses whales nearby, researchers have shown.
Ships' propellers emit sound in the same frequency range that some whales use
for communicating, and previous studies have shown the whales change their
calling patterns in noisy places.
Now, researchers have measured stress hormones in whale faeces, and found they
rose with the density of shipping.
The species studied in the Bay of Fundy in Canada, the North Atlantic right
whale, is listed as endangered.
It had been thought that hunting by the Basque people a few hundred years ago
brought a robust population down to barely sustainable levels.
But recent research suggests the big population decline happened much earlier,
for reasons that are unclear.
Dr Rosalind Rolland of the New England Aquarium in Boston, US, who led the new
study, said the population was now up to an estimated 490 individuals from
about 350 a decade ago.
North Atlantic right whales (Eubalaena glacialis) roam up and down the east
coast of North America, coming to the Bay of Fundy typically in late summer to
feed.
Aquarium scientists have been studying them in the bay since 1980.
But the new study, reported in the Royal Society journal Proceedings B, came
about through chance.
Quiet period
Following the al-Qaeda attacks on New York and Washington DC on 11 September
2001, ship traffic in the bay dropped off.
Right whale mother and calf Mothers and calves communicate with each other and
other whales using low-frequency sound
Whale researchers registered a 6 decibel (dB) fall in the intensity of
underwater noise, with the change particularly pronounced at frequencies below
150Hz.
Fortuitously, another team had just begun a five-year project to gather and
examine faeces from the right whales.
Trained dogs are taken on boats, their noses guiding researchers to the bobbing
faecal matter, which is then pulled inboard in nets.
"We were working on different boats, we knew the different studies were going
on without any real interaction," Dr Rolland told BBC News.
"And it was only when I was preparing for a workshop on ocean noise and stress
in 2009 that I realised we had this data and analysed it this way - it was just
one of those opportunistic things."
Faeces gathered during the 2001 period of light shipping showed a significantly
lower level of metabolites of glucocorticoid hormones, which are associated
with stress, than in subsequent summers when marine traffic returned to normal
levels.
"This is the first time that anyone's documented any physiological effect -
these are after all 50 tonne animals so they don't make terribly easy things to
study," said Dr Rolland.
Continue reading the main story
Guide to whales (BBC)
Guide to the oceans' great whales
"Past studies have shown they alter their vocalisation pattern in a noisy
environment just like we would in a cocktail party, but this is the first time
the stress has been documented physiologically."
Precisely how much it matters to the animals is unclear.
But ocean noise has risen substantially in recent decades along with the growth
in global shipping; one analysis showed that the north-eastern Pacific is
10-12dB louder now than in the 1960s.
Past years have seen significant numbers of right whales hit by ships and
entangled in fishing nets.
In the Bay of Fundy, relocating shipping lanes away from the feeding grounds in
2003 has reduced ship strikes by 80-90%, while similar measures have been taken
elsewhere along the coast.
The research team would now like to establish a study that could relate stress
hormones to ocean noise in a range of locations.
This could include studying the differences between the North Atlantic right
whales and their close relatives in the southern hemisphere, the Southern right
whales, whose numbers are increasing much more vigorously since the era of
industrial hunting.