The Question

Storm clouds of torrential rain and flashes of white-hot light suddenly appear on the horizon, just as when the sailboat was about to hoist anchor. The captain, traumatized by his decades of experiences with tempests, runs off the boat. You follow, but only because you hear him scream, "Thunder! AAAHHHHH!"

Lightning storm

Seemingly failing to gain laughs from that attempt to write drama into this post, I shall now go straight to the point of this post.

This next question comes from the Fun Stuff to Do website.

When lightning strikes the ocean why don't all the fish die?

The Short Answer

Water is a good conductor of electricity. and electricity cannot kill any living thing that it does not pass through.

The Long Answer

First, let's get this concept out of the way: Pure water is a very poor conductor of electricity. Water becomes a good conductor because it can dissolve stuff in it which will allow it to conduct electricity. A good example of this is the sweat coming out of your body, which is very rich in mineral salts that allow electricity to travel through it, and your body. But what is electricity?

The term "electricity" has so many meaning in everyday conversations that it cannot be defined clearly in a scientific manner. Instead, most physicists and engineers use the term as an overall idea, with more precise terms used to convey the exact effect being discussed. In this post, we will take a look at the idea of electric currents, electrical resistance, and potential difference (AKA voltage).

Let's define these important terms before anything else. An electric current is, quite simply, the flow of particles that carry with them an electric charge. Electrical resistance is the property of an object that allows it to remove energy from these moving charges. And finally, potential difference is a measure of how much energy one unit charge loses as it passes from one point to another.

Normally, these quantities are studied in classrooms, lecture halls, and science laboratories in terms of a circuit, where there is a source of energy, such as a battery, which drives the electric current (what we commonly call as electricity) through some wires and an electrical device. Although this is quite interesting in its own right, that does not quite explain how the fish and all other water organisms stay alive after a lightning strike.

This leads us back to lake or ocean water. Although ocean water is much saltier than lake water, both types contain enough salt and dissolved substances to conduct electric current. Electric current is created when electrically charged particles move, and this is the reason fish usually do not die during a lightning strike

There are three factors at play here. The simplest idea is that even though lightning can carry up to 100,000 amperes of electric current (where 0.2 amperes is considered enough to kill a person),  that current is distributed through a very large volume of water, so the electric charges that comprise the current are spread out. So although the total current is the same, the current per unit volume of water is minuscule. In effect, a fish receives a current that is too small to do anything.

Look at how the charges (in yellow) spread out as they enter the water. Any fish that experiences this charge flow will have very few of the charges passing through it.
Look at how the charges (in yellow) spread out as they enter the water. Any fish that experiences this charge flow will have very few of the charges passing through it.

The second concept is that water still has electrical resistance, and this further reduces the current flowing through the water.

Charge moving through air and water
Current is the rate of charge flow in a conductor. The slower the charge travels, the less current.

The most difficult concept to wrap your head around is the idea that when a conductor is electrically charged, the interior will not carry any charge at all. All the charges will instead build up on the surface of the conductor.

Charges in a conductor
Like charges repel. A bunch of like charges in a conductor can move, and will repel each other until they are more or less evenly distributed.

This is the reason why it is advisable to stay away from the edge of the water in a thunderstorm; any electrical charges transferred to the water will travel mostly on the water's surface.

The electric charges cannot get to the fish.
The electric charges cannot get to the fish.

Although some unlucky fish near the surface of the water will still die, lightning just cannot enter the water and electrocute all the water life.


Let us take a look at the three concepts, and their applications.

  1. Small currents can kill - the reasons for this has been used for novel purposes. However, higher amounts of currents than what can cause death have been used to attempt to bring people back to life. Furthermore, knowing that the currents have to pass through the body to do lasting damage has led to the development of suits that help protect workers from the effects of electricity.
  2. Electrical resistance - Used everyday to design circuits as simple as a flashlight circuit to as complex as supercomputers.
  3. Conductors can only build up surface charges - This is the main idea behind the Van de Graaf generator.

Final Thoughts

I hope that the warnings given to you against swimming in a thunder now make total sense. And I seriously hope you like that attempt at fiction at the beginning of this post. 🙂