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.
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.