How do Smells Travel Underwater?

22 September 2008
Presented by Diana O'Carroll.

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Sharks seem to smell blood underwater from miles away in mere seconds - but how fast do smells really travel underwater? We find out in this Question of the Week, and ask why eyebrow hair gets out of control as we age...

In this episode

00:00 - How do sharks smell blood underwater?

How do smells travel underwater and how can a shark smell a drop of blood in an entire ocean?

How do sharks smell blood underwater?

Thomas Breithaupt, University of Hull

Vince is absolutely right in questioning the scenario in wildlife programmes where sharks apparently are attracted from a distance within a very short time after some smelly substance has been dumped in the ocean. Water molecules in general are carried to the shark by water currents. If there are no water currents then it is molecular diffusion, the random movement of molecules that disperses the odour away from the source. Diffusion is an extremely slow process as Vince experienced in his ink experiment. In general the travel time of odour depends entirely on the local water velocity. Near the water surface water velocities in the ocean can range between a few centimetres per second on a very calm day and several metres per second in a strong current. In summary, odour can theoretically be detected by a shark in several miles from the source and I would estimate that in the ocean this may take at least one minute to reach the shark at a distance of 100m. More likely it will need between ten and twenty minutes. Finally the shark still needs to get to the source and that would take another 10-100 seconds depending on the swim speed of the shark. If smelly things are dumped into the ocean don't expect a shark to be attracted from a distance in less than a few minutes.

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