Young dolphins keep up with their mothers by adopting a position to get a “free ride” in the mother’s slipstream. Sudden fleeing from fishing boats is likely to disrupt the positioning of mother-calf dolphin pairs.Dolphin-friendly tuna fishing could still be killing thousands of the marine mammals each year by separating baby dolphins from their mothers, an Israeli scientist has reported.
This could help explain why despite efforts by the tuna fishing industry over the past 20 years to protect dolphins, their numbers have failed to recover, according to the study on the swimming behavior of dolphins by Hydrodynamics expert Daniel Weihs at the Technion Israel Institute of Technology.
The research on dolphin swimming conducted by Weihs also helps to explain why bike and car racers travel in packs, why baby birds swim behind their mother, and why many birds fly in a v-shaped formation.
Weihs of the Faculty of Aerospace Engineering conducted his research at the invitation of National Oceanic and Atmospheric Administration. Exploration of the subject first began in the 1970s when it became apparent that tuna fishermen were catching dolphins in their nets. The dolphins would die when they became trapped in the nets. In light of this, the U.S. government passed a law forbidding the sale of tuna if it was found that during its fishing dolphins also died.
At that time, Weihs was also asked to express his opinion about the problem. He researched it and found that above a certain speed, dolphins jump higher. Therefore, special boats were brought in order to frighten the dolphins and since then, they jump over the nets and their lives are saved.
The service then 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.
In his study which was published in the Journal of Biology, Weihs modeled the complex hydrodynamic interactions between two dolphin-shaped objects traveling through 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,” 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.
Weihs’s 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
Young dolphins keep up with their mothers by adopting a position to get a “free ride” in the mother’s slipstream. Sudden fleeing from fishing boats is likely to disrupt the positioning of mother-calf dolphin pairs, causing the younger dolphins to get permanently separated from their mothers.
“As the mother (dolphin) moves through the water, she pushes the water in front of her forwards and to the sides, to make space for her body,” Weihs told Discovery News. “As she moves, the space behind her is filled with water moving forward and inward. If the baby is (positioned to the right) obliquely behind, it gets dragged along by the forward-moving water.”
The fast-moving water also reduces nearby pressure, which helps to pull the calf inward and close to mom.
Weihs uses other examples to explain the phenomena. “In bicycle and car racing, the lead cyclist produces a flow field similar to the dolphin mum, except there is no tail flapping, so the second cyclist (or car) riding directly behind the first, is ‘sucked’ forwards and uses less energy to move at the same speed,” he explained.
In the ideal position, the mother can provide close to 90 percent of the thrust needed for the young cetacean to move at around 5 mph. “But if the mother really gets scared and starts moving really fast the calf just can’t keep up,” said Weihs.
Without human or predator intrusion, the energy-saving technique helps baby dolphins to keep up with their mothers. Tuna fishing boats, however, chase dolphins at speeds above which the drafting, or water riding, works, according to Weihs.
Fishermen follow dolphins because tuna tend to gather beneath schools of dolphins. Purse-seine fisheries, for example, use a helicopter and a fast, main boat to track dolphins. Once dolphins are spotted, speedboats are launched to herd dolphins away, while the main boat drops a mile-long curtain of net.
Weihs said that after fisherman leave a site, lactating mother dolphins often are found without calves, and calves are found without mothers.
“The really young ones die of simple lack of nourishment, while for older calves, some may survive,” he said.
Stranded dolphin calves almost always die because they rely on their mothers for milk until they are about 18 months old.
“Chases by fishing vessels can easily cause the loss of the mother-calf connection,” Weihs said.
Elizabeth Edwards, a marine biologist at the National Marine Fisheries Service in San Diego told Discovery News that she hopes future research on the limits of drafting will help to determine if the separation of mothers and babies is to blame for dolphin deaths observed in the eastern tropical Pacific Ocean, where tuna fishing takes place.