Misconceptions and Fallacies

[Midnight Star]

myth: If I cut my skin or barnacles or oyster shells, saltwater is the best thing for them.

truth: Wait too long to clean the wounds, and your name may become Bob.

 

[rockjock3]

Any blood, not just human blood. Sharks detect it at 1 part per million and smell it from a distance of approximately 1/4 mile. Not quite "miles" away.

Dave

I understand the science behind it. They can pick up any smell from that distance. Here is the rub though. Humans are not on their biological food menu and therefore do nothing for them smell wise.

If you are walking down the block and smell steaks cooking (if you enjoy good steaks) then you will notice and it means food to you. If you are walking down the street however and smell somebody cooking old tires, you might notice but you don't really care and definitely don't get hungry.

Yes they can smell any blood or substance that emits a smell in the water. They don't however percieve human blood as blood from a food source or even as blood for that matter they simply smell it as an odor that they are not familiar with. Yes they might investigate it on that point just as they might investigate it if you were to pour a bottle of perfume into the water. They aren't going to percieve you as food though once they arrive on scene, unless you give them some reason to by acting as food in some manner (struggling, flapping around, fleeing and so on).

 

[Sideband]

Nobody can hear you scream at 200ft. This is not true....once at 200ft on air I called my dive buddy up from 240ft, also on air....and I got her really close and I took my reg out of my mouth and screamed into her ear....1 minute later at the surface she told me she heard it.

What did you yell, "You are ascending waaaay too fast!!!!!"? After a 200'/minute ascent I hope you didn't die.

Joe

 

[roakey]

myth: You can't measure the seriousness of a sting or contact from a marine animal by how much it initially feels or whether it burns or hurts.

truth: So that was a blue skinned octop ... (falls over).

You mean blue ringed perhaps?

Roak

 

[Midnight Star]

You mean blue ringed perhaps?

I sure did. Thanks for correcting that.

-----

Mike.

 

[Codyjp]

I understand the science behind it. They can pick up any smell from that distance. Here is the rub though. Humans are not on their biological food menu and therefore do nothing for them smell wise.

If you are walking down the block and smell steaks cooking (if you enjoy good steaks) then you will notice and it means food to you. If you are walking down the street however and smell somebody cooking old tires, you might notice but you don't really care and definitely don't get hungry.

Yes they can smell any blood or substance that emits a smell in the water. They don't however percieve human blood as blood from a food source or even as blood for that matter they simply smell it as an odor that they are not familiar with. Yes they might investigate it on that point just as they might investigate it if you were to pour a bottle of perfume into the water. They aren't going to percieve you as food though once they arrive on scene, unless you give them some reason to by acting as food in some manner (struggling, flapping around, fleeing and so on).

wouldn't it be more like me smelling a kangaroo BBQ as opposed to rubber tires? you make a valid point though. I once cut my finger diving and had a funny feeling the rest of the dive.

I have always wondered if it would be possible to creat a shark mace (pepper oil or something else undesireable...) I could market it to surfers because divers are too smart!

 

[Charlie99]

Let's not get carried away. 1ppm is actually quite a lot of blood.
Let's take a cylinder of water radius 0.25 miles and say 60ft depth.
Unless my arithmetic is wrong that's 328 million cubic ft.
So to fill that entire cylinder with 1ppm would need 328 cu ft of blood. That's hardly what I'd call a scratch.
Now you may just be unlucky in that the blood drifts down to the shark leaving a fairly concentrated trail...

Various sources give the sharks smelling sensitivity as either 1ppm or 1 part per 25 million (40pp billion).

Even assuming 1ppm, the fallacy of your calculations is that it assumes even distribution. What really happens is that even a slight current will generate a scent "trail" that leads quite a ways downcurrent but isn't very wide, so the dilution required to reach out 1/4 mile isn't nearly as large as your calculation. I'll bet that the same sort of uneven mixing will happen in the vertical dimension also, so even when in deep ocean the scent trail will go mostly downcurrent and won't change much in depth.

I think what happens in real life is that sharks swim across a scent trail, and then follow it back upcurrent to the source.

 

[Midnight Star]

I have always wondered if it would be possible to creat a shark mace (pepper oil or something else undesireable...)

They found out from local fishermen, that sharks hate the "scent" of decaying shark meat. So much so, that they pretty much fled from the general area of it. In the lab, they "extracted" essence of rotted shark meat, and tried it out on what looked like (I didnt get a good look, but something like small whitetips or reef sharks) ... when that stuff started spreading in the water, they took off! Literally. I can only imagine what it smelled like though.

-----

Mike.

 

[roakey]

Now who is in 'the laboratory'?

Anything much beyond simple depth versus pressure formulas is completely insignificant when compared to the physiological and psychological effects of the diver in the diving environment.

Please note all I was using was simple depth versus pressure calculations. Exactly what I made reference to as being useful.

All of that amounts to nothing?

I never said it amounted to nothing, I said it was insignificant.

With an experienced diver, all those factors probably amount to maybe ending your dive a couple minutes early at 60feet. That's insignificant. On the other hand, overweighting an inexperienced diver can have a profound effect on the dive time (read: air consumption). But not for the reasons you mention. Which means, of course, that for an inexperienced diver, the reasons you gave become even more insignificant!

Stop looking at the little stuff, you're missing the big picture. Go back to my original note and re-read what really causes increased air consumption on an overweighed, inexperienced diver. Stop championing the stuff that falls into the "noise" region of the discussion...

Roak

 

[roakey]

I sure did. Thanks for correcting that.

You're wel<gack>!

Thud.