Friendly fishing still kills dolphins
David Adam, science correspondent
4th May 2004
Danger to mammals remains as calves swimming in slipstream left vulnerable when mothers flee pursuing vessels
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.
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.
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.
"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.
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.
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.
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.
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.
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.
"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.
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 swimming 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.
"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.
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.
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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