Half a croissant, on a plate, with a sign in front of it saying '50c'
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Shark security

Keep beaches safe with sonar surveillance
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A network of buoys deployed about a kilometre offshore around particularly popular beaches, equipped with sonar to watch out for sharks. They would be tuned to recognise the size and speed of an approaching shark, and if a dangerously-sized one was detected approaching the beach for the smorgasbord, a series of deterents could be released. These could take the form of pulses of sound focused on the shark from a network of powerful transmitters, or various noxious chemicals released into the water to put the toothy fella off his lunch. If these fail to deter the hungry fish, alarms could scream out from the beach informing the bathers that a particularly stubborn gourmand was approaching, and to get to the shore as fast as their tempting limbs could propel them. It's more eco-friendly than a big net anyway.
Nadir, Oct 10 2001

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       Do you think such a system could distinguish between carnivorous sharks, their more placid cousins and dolphins?
Aristotle, Oct 10 2001
  

       //various noxious chemicals released into the water// They do this in the Irish Sea and there isn't a shark in sight.
stupop, Oct 10 2001
  

       Different sizes. If we're talking about great whites, tigers, bulls and the like I think they're all fairly large, larger than a harmless Caribbean reef shark or a dolphin, and much smaller than a basking or whale shark. Speeds are most likely different too. It could be engineered to tell the difference I'm sure.
Nadir, Oct 10 2001
  

       The speed and movement of a large shark would be quite different to a diver or even a school of fish. The system could be tested in known shark feeding waters, maybe training it up using a kind of nervous net system to allow it to develop its own intelligence.
Nadir, Oct 10 2001
  

       I thought this was going to be security sharks like security blankets. Well, Calvin had a security *tiger*.
pottedstu, Oct 10 2001
  

       IIRC, nervous nets can't train (unless you consider the original construction the training), they don't have any memory, you can't create nonlinear decision boundaries, and they scale horribly. i.e. they're not really good for anything except Tilden's PR. Instead use a multilayer perceptron, radial basis function or a support vector machine. Maybe a hidden markov model or a Boltzmann Zipper if you want to incorporate time series info like swimming patterns. Not as sexy as nervous nets, but at least it'll work better than chance.   

       whee!
prometheus, Oct 10 2001
  

       prometheus, if you read 'neural net' for 'nervous net' I think you'll get Nadir's intentiion. Unless something has happened in the years since I studied them, a multilayer perceptron is basically the same as a neural net. I agree Tilden's nets aren't really applicable.
wiml, Oct 11 2001
  

       I read something some years ago about soldiers who went swimming in the ocean off of South Africa. Every so often, they'd throw a couple grenades far out into the surf to scare the sharks away. I don't know if it really worked or not, but they never mentioned anyone getting bitten.
Guncrazy, Oct 14 2001
  

       That could easily have the opposite effect to the one intended, [Guncrazy], as the grenades are likely to blow up other marine life, the smell of which would attract sharks.
Lemon, Oct 15 2001
  

       Gee, I was planning a system very similar to this one. So it's definitely a + :-).   

       Something that could perhaps help in distinguishing sharks from dolphins is the fact that sharks have no bones, only cartilage. I guess sonar reflection would be somewhat different.   

       As a deterrent I was told that in those islands where they have "dive with sharks" underwater excursions they sometimes hit sharks in the nose when they get more frisky (I guess these are not realy big sharks). So, for bigger sharks we could have a minisub equiped with a shootable boxing glove that would hit sharks in the nose until they went away. When the shark turned away from the beach the minisub would shout "and don't come back!!!"... hmm, maybe I should post this as an idea. Can I?
PauloSargaco, Oct 22 2003
  

       Another way to distinguish sharks from other sea life is based on the shape. Dangerous sharks usually have very pointy noses and the fins in the chest area have very sharp angles in relation to a dolphin's, for instance. Same applies for dorsal fins.   

       // they'd throw a couple grenades far out into the surf to scare the sharks away //   

       I once read a story of a diver who used to scream to scare sharks away. He was surprised when tried it on a different country and didn't work. The reason was preciselly that in that area they used dinamite for the same effect.
PauloSargaco, Oct 23 2003
  
      
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