Dolphin-friendly tuna may still kill
Dolphin numbers have not recovered from the effects of tuna fishing as fishing boats can separate mothers from their calves, according to a new study on dolphin swimming behaviour.
The research also helps 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.
Aerospace engineer Professor Daniel Weihs, from the Israel Institute of Technology, published research into how dolphin calves swim with their mothers in the current issue of the open access Journal of Biology.
The shape and motion of a dolphin is like an ellipse, so Weihs calculated the most energy-efficient way two ellipses could move forward in a body of water.
He found that if a calf's centre of mass was about two-thirds of the mother's length, the mother could provide up to 90% of the thrust needed for the calf to swim at about 8.5 kilometres per hour.
Analysis of aerial photographs of dolphins swimming revealed that dolphin calves swam the way Weihs predicted.
"As the mother moves through the water, she pushes the water in front of her forwards and to the sides, to make space for her body," Weihs said.
"As she moves, the space behind her is filled with water moving forward and inward. If the baby is [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 mum.
Other creatures hitch a ride this way but not human swimmers, who the researchers said thrashed around too much.
For example, schooling fish and baby birds that swim in formation behind their mothers hitch an almost free ride from the lead swimmers.
Likewise, birds that fly in a v-shaped formation ride the first bird's airwave, instead of water.
"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," Weihs said.
Without human or predator intrusion, this energy-saving technique helps baby dolphins keep up with their mothers.
But Weihs said tuna fishing boats chased dolphins at speeds too high for water riding, otherwise known as drafting, to work.
Fishing boats follow schools of dolphins because tuna tend to gather beneath them.
In a type of fishery known as a 'purse seine' fishery, for example, a helicopter and a fast, main boat are used to track dolphins.
Once dolphins are spotted, speedboats are launched to herd dolphins away, while the main boat drops a kilometre-long curtain of net.
Weihs said that after fishing boats leave a site, lactating mother dolphins are often found without calves, and calves without mothers.
"The really young ones die of simple lack of nourishment, while for older calves, some may survive," he said.
U.K. biologist Professor R McNeill Alexander, at the University of Leeds, agreed with the study in a commentary in the same issue of the journal.
"So far, no metabolic measurements are available to confirm [Weihs'] conclusions. But it seems clear that mother dolphins give their calves substantial help.
"Just as baby monkeys could not keep up with their troop if their mothers did not carry them, dolphin calves might be unable to keep up with the adults if they did not keep close to their mothers."
U.S. marine biologist Dr Elizabeth Edwards, from the National Marine Fisheries Service in San Diego, hoped future research on the limits of drafting would help determine if the separation of mothers and calves was to blame for dolphin deaths in the eastern tropical Pacific Ocean, where tuna was fished.