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 which 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 awhile for a shark a mile away to taste it?
– – David, Belgium
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.
As you probably know already, sharks are pretty well-suited to this whole predation thing. They have excellent hearing in the low frequencies and can pick out the sound of something thrashing around in the water well over 1,000 feet away. They're also outfitted with sensory cavities called the ampullae of Lorenzini, which register the faint electrical fields generated by living bodies. And then there's the shark's sense of smell, which though not quite as phenomenal as was once believed (you used to see a stat claiming that 70 percent of a shark's brain was devoted to olfaction, which seems to have been a real overstatement) is still plenty acute: Sharks can detect some chemicals at concentrations of around one part per 25 million, and experts claim they've seen sharks go nuts over a single drop of blood in a 2,000-gallon tank. (Catfish, for the record, can detect chemical concentrations measured in parts per billion, making it a good thing they're not prone to bite your leg off.)
A chemical (and for our purposes, blood) disperses in water via diffusion and mixing. Diffusion is a random and relatively slow process by which a concentrated group of molecules drifts apart, and in still water that's the main way an odor would spread. But the ocean isn't still, and so the primary determinants for how scents travel through seawater are the churning of waves and the flow of currents. For this reason, a shark's standard response upon smelling something yummy is to swim into the prevailing current, as this will likely lead to the source; often it'll home in by swimming in a series of decreasing spirals. The upshot is that while a shark would likely have to catch some breaks dispersal-wise to identify the scent of blood a mile away from ground zero, at a distance of a quarter mile it's got a decent shot at picking up some dinner.
OK, so: You and your pals are out in the ocean, bleeding away; the prevailing tidal current is somewhere around one meter per second, or 2.24 miles per hour. If a shark is upcurrent from you, it might take awhile for the pertinent molecules to drift into its range. But if the shark happens to be fortuitously positioned a quarter mile downcurrent, the scent of blood could find its nostrils in a little under seven minutes. Midsize sharks have been clocked swimming at 24.5 mph, so conceivably if our specimen got a particularly good read it might take only a minute or so to locate you, giving us roughly an eight-minute gap between blood entering the water and shark cruising up with a bib on. It'd take an admirable commitment to the principles of cinema verite to spend that kind of screen time waiting for the shark to happen along.
Let's say, though, just for fun, that you and the shark happened to be hanging around the Skookumchuck Narrows on the coast of British Columbia, where tidal currents can reach 16 mph. (We'll ignore that the raging waters themselves would have a fine chance of doing you in.) Here a shark situated a quarter mile downcurrent might catch a whiff a mere minute after you started bleeding; if it kept the spiraling business to a minimum and swam on something like a beeline back to the source (fighting the current all the way, remember) it might arrive on the scene in under two minutes, for a turnaround time of less than three minutes total. While that's not quite lightning fast by shark-movie standards, a) it's getting close, and, b) I'll hazard a guess that three minutes goes by pretty quickly when you're bleeding and you suspect a shark might be on its way.
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