Friday, June 27, 2014

Megalodon, Revisited

That megalodon show was on again last night on Discovery.  I didn’t watch much of it, but just the fact that it was on again sort of pissed me off.  I saw a comment on a thread the other day at some news website where someone was referencing this show as if it were an actual thing that actually happened, and was telling everyone that megalodon wasn’t really extinct and that they were still around, so it is fooling at least some people.  I noticed that they did a better job of posting disclaimers this time around, but the disclaimer was still pretty vague, and left it open that there is serious scientific study going on to see if megalodon is still around, when there isn’t. 

At all. 

I’ve already posted on why the idea that an 80 footpredatory shark could still be around, and us not know about it, is a stupid concept that is snuggling right up to impossible, but I wanted to comment on the one thing that I noticed as I was flipping through that channel last night.  I only watched for 5 minutes before the stupid caught up with me and I had to go out to the shop and build something, but the scene that, to me at least, really took the taco, was the scene where they are putting out the chum slick, and their fish finder starts going absolutely ape-shit with contacts, and the camera pans to the finder screen, showing lots of large, shark-shaped sonar returns coming back to the transducer. 

Once again, Discovery Channel made me go “dudeomgwtf!”

The problem with such a shot is this:

Fish finding sonar does not reflect off the animal, itself.  Fish, and most other marine critters, are not remarkably much more dense than the water they swim in, and sonar works by reflecting sound off of differences in density, so sonar is essentially incapable of finding the animal, itself.  Instead, it reflects off of the air in the swim bladders of the fish, which has a marked density difference from the water around it, leaving a sort of half-moon shaped sonar return, like this:

Each of the half-moon looking thingies is a fish.  Top middle is a school
of  fish clustered together.  Bottom middle is likely a big single fish.
Middle right is a school of small fish .  Bigger reflection doesn't necessarily
mean a bigger fish, but it can, and usually does.  The horizontal line across the
middle is probably a downrigger weight; lead is dense enough to send a return.  

The sonar returns in the show were shark-shaped, not half-moon shaped, so that graphic was obviously done by someone who either doesn’t understand how fish finding sonar works, or, alternatively, by someone that understands how it works just fine, but was hoping the general populace wouldn’t catch on to his bullshit.

More astute readers might also note that sharks do not have swim bladders; you cannot see a shark on fish-finding sonar.  No matter how big it is.  Or whales, or seals, or anything else that doesn't carry a bolus of air around inside its body cavity, like fish with swim bladders do.  There are also a bunch of species of fish that you likewise can’t see on sonar, because they don’t have swim bladders, either.  Ling cod, sturgeon, and some types of rockfish, for instance, are all fish that do not have swim bladders.  The only reason shark fishermen have fish finding sonar at all is to use it to mark the location of food fishes that the sharks might be eating.  You only know if you’re on the sharks when you actually start catching them. 

So not only is this show a total work of fiction, they really didn’t even do that great a job of making it scientifically or technically believable.


  1. Ummmm. Errrr. Don't whales and seals have, like, lungs?. And what do they have in those lungs if it isn't an air/CO2 mix?.

    Mike, confused.

    1. Good point, Mike. i probably should have addressed this.

      Marine mammals don't show up on sonar, despite the air in their lungs. I'm told that this is because, unlike fish's air bladders, a mammal's lungs aren't otherwise empty sacs of air, but are rather meaty, fleshy parts with air sort of dispersed inside them.

      Sonar is all about density differences, and as I understand it, even an inflated lung is not markedly different in it's density from water, enough to be of note to commercial fish finding software, anyway.

      I am not 100% certain on the mechanics of all of this, but I do know that marine mammals and sharks (and other fish that don't have air bladders) don't register on sonar.

    2. Roger that, with respect to the meaty with air dispersed in it. Sort of like our own lungs in that respect.

      It can't be as simple as (just) density, though. A submerged submarine at neutral buoyancy has a density of 1.00000 with respect to the density of the surrounding water, yet it is detectable on sonar. Probably even a commercial fish finder.

      My own lungs, meaty though they may be, can change my total body buoyancy from negative to positive (sink or float) depending on how much air I have in them.

      I suspect (speculation) that the detectability,or lack thereof, may be a function of the size of the discontinuity vs the wavelength of the sonar pulse, and the small size of the mammalian air sacs render them invisible at fish-finder wavelengths.


  2. Mike you are confusing density with buoyancy the hull of a submarine is very dense steel-- yet they change buoyancy via adding or subtracting water in the ballast tanks.