- Dolphin-friendly tuna fishing could still be killing
thousands of the marine mammals each year by separating vulnerable calves
from their mothers, a scientist has found.
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- Hydrodynamics expert Daniel Weihs at the Israel Institute
of Technology in Haifa has discovered a powerful underwater attraction
between the animals that literally sucks baby dolphins along with their
mothers. The attraction only works if the dolphins are swimming very close
together. Experts say fleeing from fishing boats could disrupt the positioning
of the pairs, causing the young to get left behind and permanently separated.
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- This could explain why the introduction of dolphin-friendly
techniques has not led to a recovery in populations in the eastern Pacific,
despite fewer being killed by fishing boats that target them because they
are found alongside tuna.
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- "Chases by fishing vessels can easily cause the
loss of the mother-calf connection," Dr Weihs said. Fishing prior
to the restrictions could have disrupted dolphin schools, causing many
dolphins to die before they were able to breed.
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- The new study shows that a mother's slipstream can provide
up to 90% of the thrust needed to propel the younger animal through the
water. "But if the mother really gets scared and starts moving really
fast the calf just can't keep up," said Dr Weihs. Stranded dolphin
calves almost always die because they rely on their mothers for milk until
they are about 18 months old.
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- Dr Weihs usually studies aircraft but was asked to look
into the dolphin problem by the US National Marine Fisheries Service in
California, which was concerned by regular finds of orphaned calves and
lactating mothers without their young. He previously helped the service
develop fishing techniques in which the giant tuna nets are lowered in
the water slightly to encourage dolphins to leap out.
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- The service asked him to investigate a phenomenon known
as drafting, where dolphin calves position themselves close to their mother's
side in an apparent attempt to reduce the effort needed to swim.
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- Using a computer to look at the interactions between
two dolphin shaped objects in water he found two forces at play, both of
which help younger animals stay with their mothers.
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- The first is very similar to the slipstream effect seen
with racing cyclists: water behind the mother rushes forward to fill the
hole left as she moves, effectively giving the calf a free ride. The second
force - called the Bernoulli effect - tends to pull the calf sideways towards
the mother's flank, where the slipstream is strongest.
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- "Hold a piece of paper, blow over it and you'll
see it lift up. That's the Bernoulli effect," Dr Weihs said. It works
because air or liquid moving across a surface generates drag, reducing
the pressure and producing suction - or in the case of an aircraft wing,
lift.
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- His dolphin model predicts the maximum thrust when the
calf is about two-thirds of the way down the mother's length; aerial photographs
of swim ming Eastern Spinner dolphins confirm they tend to adopt this position.
The study also shows the attraction can survive dolphins' leaps from the
water, as long as both animals leave and enter at an angle of about 45
degrees. The results are published today in the Journal of Biology.
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- "The surprising thing is how strong the effect is,"
Dr Weihs said. "The calf doesn't move its tail, the mother does and
both of them move. It's amazing." The attraction is even strong enough
for childless female dolphins to deliberately baby-snatch, by swimming
past the mother-calf pair at high speed. Biologists studying Bottlenose
dolphins regularly report such "bolting with infant" events.
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- The dolphin pair have to be close (typically less than
20cm) for the attraction to pull along the smaller animal. The animals
must move much further apart to accelerate quickly, suggesting that dolphin
numbers will only recover if the boats pursuing them are slowed. Dolphin
schools are usually first spotted by helicopter and then speedboats chase
them, separate some and herd the animals towards the net.
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- The results will also affect a proposed relaxation of
the dolphin-safe definition, which would allow tuna fishing as long as
no dolphins are killed or injured in the nets. The new study suggests relying
on observations of injuries to dolphins underestimates damage inflicted
on their populations.
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- Guardian Unlimited © Guardian Newspapers Limited
2004 http://www.guardian.co.uk/conservation/story/0,13369,1208992,00.html
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