The hydrodynamics of dolphin drafting

Staying close to mother helps baby dolphins swim – but they can’t keep together when fleeing from fishing boats

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 as the calf grows, which is probably very useful as the calf has to learn only one such position.”

Analysis of aerial photographs of eastern spinner dolphins show that calves do indeed tend to adopt the position predicted to be the most energy-efficient for forward travel by Weihs’ model.

The drafting situation in the wild is much more complex than the simplest model suggests, however, so Weihs examined several additional scenarios, such as what happens when the dolphins move apart or propel themselves along by flapping their tails. He found that as a pair separate the positive forces on the baby dolphin decrease rapidly, and as the dolphins swim faster the relative energy savings that the calf can make are reduced.

Weihs suggests, “chases by fishing vessels can easily cause the loss of the mother-calf connection,” as the dolphin pair have to move apart when they accelerate to avoid hurting each other as they increase their body movements.

Weihs’ findings could help explain why the dolphin population of the Eastern tropical Pacific Ocean has not recovered, despite recent restrictions on purse-seine tuna fishing. Fishing that took place prior the restrictions may well have disrupted dolphin schools, causing many younger dolphins to die before they were able to breed.

They should also help in evaluating the proposed relaxation of the ’Dolphin-Safe’ definition. This relaxation would allow fishermen to set their nets around dolphin schools to catch the tuna living alongside, so long as no dolphins were killed or seriously injured. Weihs’s results suggest that relying on fishermen’s observations of injuries to dolphins would underestimate the damage inflicted on dolphin populations.

This press release is based on the following article:

Hydrodynamics of dolphin drafting
Daniel Weihs
Journal of Biology, 2004 3:8
To be published 4 May, 2004