# Shark, You're what's for dinner: How quickly can sharks pick up on blood in the water

The calculations in the example assume that the bleeder is stationary with the water current washing the blood downstream.
Most shipwrecked and immersed victims will drift with the stream. This brings you back to diffusion and - to a small extent, in my opinion, wave action.
My guess is that the swimming, thrashing bleeder will attract the shark with the sound he/she makes. Sound travels exceedingly fast under water so propagation time can be disregarded leaving the swimming speed and hunger of the shark and to be the only variables determining how fast they show up.

Cheers,
/RS

Here is a link to the article.

Yes, there’s a distinct lack of relativity built into Cecil’s answer. His Skookumchuck Narrows example only works if you are swimming upstream at 16mph, or hanging onto something grounded, and the shark likewise.

In Cecil’s current column, he spends a fair bit of space discussing blood transport via water currents, and talks of currents going at various speeds. But speeds relative to what? Surely, if a shark is just chilling, waiting to sniff out a meal, it’s at rest relative to the water, not relative to any land masses. Likewise, anything producing blood (at least, that a shark would be interested in) would also be floating/swimming in the water, and therefore also roughly at rest relative to the water. If the shark is at one position relative to the water, and the bleeding victim is in another position relative to the water, what difference does it make if that whole mass of water is moving at a uniform speed?

Now, one could certainly have mixing effects from velocity shears in the current, if, for instance, the surface of the water is moving relative to deeper water. But Cecil doesn’t seem to spend any space on this possibility. Was that in a longer version of the column, which an editor cut out to fit the space? If so, that editor should be chastised, since what’s left is oversimplified to the point of being wrong.

See other thread on same topic

Yeah, I saw it after I posted (it was buried when I read the column earlier today, and got bumped while I was posting). I’ve already reported my thread for merging.

bibliophage
moderator CCC

Not necessarily. Movement of an object by currents is not always at the same speed. Mass, surface area in the water, and bouyancy are factors, for instance. I don’t know all the factors, much less the equations, but I have seen divers on a drift dive move down-current faster than the boat they left from, even when the boat driver isn’t using the engine. I think it’s entirely possible that one’s blood might move along on a current faster than one’s body.

In my experience, (as a PADI DM and diving physician) the big difference is between objects that are deeply submerged (such as a diver/surface swimmer) and things that aren’t submerged a lot (such as your average dive boat) presenting a large area to the wind.
Yes, boats and people can have significantly different speeds in the same water but people and blood (and sharks) drift at about the same speed in the same body of water.

It is once equilibrium is reached. If you start off moving relative to the current, then how long it takes you to come to rest relative to it will depend on your mass, surface area, etc., but once you do come to rest, you’re doing the same thing as everything else at rest relative to the current.

Some years ago, I had a scary encounter with a large tiger shark in the ocean off the tip of the Yucatan peninsula. My SCUBA buddies (who saw the whole thing and actually fled, figuring: why add to the feeding frenzy?) estimated this shark to be over 10 feet long. It honed in on my flippers and followed me closely for a few minutes (or hours, I couldn’t tell). We were at about 60 feet deep. It finally lost interest and swam away.

I had washed up on some fire coral earlier that day, so there were some open sores on my legs. Also realized later that I had started my period. You hear about bears and mestruating females, but this made me wonder later whether that shark had picked up on it, too.

In the book “Jaws”, as I remember, the initial attack on a woman was based on her mensrual flow attracting the Great White shark. I was freaked out by this, and my marine biologist shark specialist Dad told me not to worry about that, the odds were slim on the shoreline exposure. Here’s a marine biologist’s opinion, pretty much in line of what my Dad said. Gotta say though, those “Jaws” years for my Dad were juicy for his speaking engagements, and our 70’s station wagon was full up with dead shark aroma from the bolster of demand.

I was scared after thae “Jaws” hubbub, but, after going on many longline shark catches, and Mexican shark excursions, dives and observation, sharks are not as an explosive issue as seemed. Though, if one gets you, it’s pretty horrid.

I appreciate Cecil’s noting that sharks have very keen hearing. In 1946 my father was stationed on Midway Island. He learned that when the garbage scow went out and began dumping that sharks appeared quickly. Flying around the island, he often spotted a group of sharks, sometimes a few miles from the scow, but noticed that when the began to dump garbage, the sharks would immediately turn and head for dinner!
It would seem that sharks would at least be aware of and alerted, if not attracted, to swimmers thrashing in the water.