Question of the Week

How do sharks smell blood underwater?

Sun, 21st Sep 2008

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Vince asked:

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?


Thomas Breithaupt, University of Hull

Oceanic Whitetip SharkVince 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 donít expect a shark to be attracted from a distance in less than a few minutes.


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I used to do shark fishing. We dumped "smelly stuff" into the water as an attractant. We rarely saw sharks in less than about 10 minutes. DoctorBeaver, Fri, 26th Sep 2008

As a student in High School in Germany I asked my biology teacher that same question. She looked at me as if I was stupid and said something similar to: "just like in air, you dummy". She did not say dummy, she just thought that I should not be asking her questions she cannot answer well in class.

So, I still don't know. But it sure seems to be not plausible that the smell is traveling faster than through air.

Karsten Karsten, Sun, 4th Jan 2009

if that is so is that why the shark attacks its prey knowing exactly where it is. jennie, Thu, 10th Dec 2009

So i wouldn't want to bleed in the ocean? -jenni 10th dec. 09 jenni, Thu, 10th Dec 2009

Hey Jen. I'd say no. Don't bleed in the water in areas sharks frequent. You are not their natural prey so they'll think twice before attacking but then again A MEAL IS A MEAL and humans swimming breaststroke is in some ways similar to the way a wounded seal swims. Sharks are not known for their good eyesight and there's a reason shark attacks happen. You shouldn't be afraid of sharks though, if you keep away from "hot spots" and use other methods of swimming than breaststroke you'll be fine. If by "bleed" you meant menstruation then you shouldn't swim in the oceans for other reasons! I'm not 100% sure but I believe women are extremely sensitive to infections during menstruation and sea water is not exactly sterile... note that I'm not 100% sure of anything in this post so you might want to doublecheck it. but I'd say .. 90% sure re: jennifer, Wed, 30th Dec 2009

I went on this web site to know how sharks can smell blood in think sharks sharks are very cleaver and can sence anathing under water SEAN, Mon, 4th Jan 2010

i dont buy the idea that sharks can smell blood from sofar away, smell in water must move very slow as the water is so much more dense than air, and in air there is wind to carry the smells, there is no such thing in water.. Im sure sharks have another sense that we humans are not aware of and cannot comprehend, it could be somethig as simple as sharks being able to sense acidity levels, or feel electric currents in ewater or something of that nature. The idea that sharks can smell blood is nonsense to me. Karim, Mon, 18th Jan 2010

The sensitivity of sharks presents two puzzles (1) the concentration of blood reaching the shark is reputed tobe about one molecule in x-litres of sea-water ? How could the shark possibly detection the direction of the Source from a single molecule? (2) The time required for the return trip source-to-shark plus shark-to-source is too short? My Gordon Setter dogs could always locate the correct stone under sea-water which cannot be explained by our Physics ? Ken Green, Sat, 14th Aug 2010

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