How Quickly Can Sharks Detect Blood In The Water?

From The Straight Dope:

Dear Cecil:

We've all seen it in a movie: A small group of people are swimming in the sea. Someone gets hurt, blood touches water, and instantly sharks appear who then devour the party in a ruthless and very painful way. But how fast does the odor or taste of blood go in water? Am I right to believe that it takes a while for a shark a mile away to taste it?

— David, Belgium

Cecil replies:

I'll confess I haven't seen a lot of Belgian shark movies, David, but virtually any Hollywood studio exec would see a major problem with the treatment you've outlined above. If the shark shows up the second the hemoglobin hits the water, where's the unbearable tension? What we're missing is that excruciating interval of stillness between the close-up of slowly seeping blood and the moment the here-comes-the-shark music kicks in. You're right, though, to suspect that this interval does tend to run a little shorter on the big screen than in real life.

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