Expert: Walter Hintz - 6/4/2008

Question
Hi Walter,
sorry I know this isn't exactly your area, but I couldn't see any other "experts" who might answer my question.
You often hear it said that sharks can "smell" a drop of blood in the water from a long way off - perhaps even miles away! What exactly does this mean? Surely the blood would have to mix with the water and then travel to the shark (in a very diluted state). This surely would take a long time. Even if the shark actually detected this blood how could it then know where it is coming from? Is this just an Urbavn Legend?
Thanks, Ray

Answer
Hi Ray;  We have a tendency to compare our senses to those of other animals.  That doesn't work  The sense of smell in fishes is not as sensitive as land animals.  In fact a shark can sense a change in concentration of blood in water as low as 1 part per million; a very low dilution. Obviously there is a limit as to how far away it can be detected.  It seems that the limit is about 1 mile.  The shark could detect the source because as it approached the concentration would increase.  Even with this in mind it is highly unlikely that if I were to get a nose bleed in the water all sharks within a 1 mile radius would all head my way.