Staying close to mother helps baby dolphins swim

Why has the introduction of dolphin-friendly tuna-fishing methods not led to a recovery of dolphin populations? A new study shows how chase by tuna fishermen may sever the link between mother and baby dolphins, to disastrous effect.

Young dolphins keep up with their mothers, who are more powerful swimmers, by adopting the ideal position to get a 'free ride' in the mother's slipstream, according to an article published this week in the top-tier Open Access journal, Journal of Biology. 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.

Daniel Weihs, an aerospace engineer at Technion, the Israel Institute of Technology, has modeled the complex hydrodynamic interactions between two dolphin-shaped objects traveling through water. He wanted to understand the phenomenon known as 'drafting' whereby dolphin calves position themselves close to their mothers' side in an apparent attempt to reduce the forces required for swimming.

His analyses showed that the movement of water around the two dolphins generates two effects, both of which help the younger dolphin stay with its mother.

The first is similar to the slipstream effect that is well known to racing cyclists, while the second (the Bernoulli effect, which also causes aircraft to stay aloft) tends to pull the calf sideways, in towards its mother's flank. If the calf and the mother are side-by-side and almost touching, the movement of these two effects means that the calf can almost get an energetically free ride, relying on its mother's swimming efforts to carry it along.

In the ideal position the mother can provide close to 90% of the thrust needed for the young cetacean to move at around 8.5 km per hour. The author writes: "The maximum thrust is provided when the calf's center of mass is approximately at 2/3 of the mother's length. This position does not change

Contact: Gemma Bradley
BioMed Central

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