Question

Apparently a shark can smell blood up to a quarter of a mile away. How does smell travel in water? It would seem strange that if you drop ink in water it takes ages to dissipate so how can the individual particles of a smell travel so far and apparently so fast?

Answer 1

You are 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 you experienced in your 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