[thedoc (OP)]

Hazel Barraclough  asked the Naked Scientists:
   
Hi there,

Can you tell me why it is that, when lightening strikes the sea (or other large bodies of water), it doesn't kill the fish.

Many thanks

Hazel  

 
What do you think?

[Geezer]

For much the same reason that lightning striking an aircraft does not kill all the passengers!

Water is a good conductor, and a lake or the ocean has a lot of it, so the current density flowing through any chunk of water is not very great. It's "easier" for the current to flow around the fish than it is to flow through the fish. Also, the current density is likely to be greatest near the surface of the water, so the deeper the fish are, the less current they will experience.

On the other hand, humans are much better conductors than air, so it's "easier" for the current to flow through than around a human standing in air.

[syhprum]

I should think that a few fish are in fact killed by lightning but not enough to have any noticable effect on their numbers.
 

[Bored chemist]

Any fish near "ground zero" way well get killed.
Fish and the lake may well be comparably good conductors (especially in fresh water).
Also, even if the water is a ten fold better conductor that would mean the fish got an eleventh of a thunderbolt. It's not likely to survive that.
There's also the possibility of blast damage.

[Geezer]

Also, even if the water is a ten fold better conductor that would mean the fish got an eleventh of a thunderbolt.

At the point where the lightning strikes that's probably true, but the current density varies with the inverse of a power between two and three from the distance from the strike, so you don't have to go far before the current is harmless.

Also, as there are a lot of harmonics in the current, there is probably a significant skin effect that prevents the current penetrating too far into the water at the higher frequencies.