Subject: Infra Red and Water

I suggest using white light. Red light is preferentially absorbed by water; that's why water looks blue. When white light goes through it, the red light is absorbed out leaving only the bluer wavelengths.

Anything that is warm will emit IR, but since fish are cold blooded they will emit as much as their surroundings, making them impossible to pick out. It's not that they don't emit IR, it's that they blend in with everything else!

[Note (November 30, 2000): It was pointed out to me that I made two errors here. One is that I wasn't distinguishing between heat and infrared emission, which are different. Something warm might emit at certain wavelengths of IR, but not all of them, and there is a broad range of IR light. Also, when using what is essentially a flashlight to illuminate the fish, you don't have to worry about IR emission, you have to worry about IR reflection. Warm or cold, a fish will reflect IR light, so it could be used to illuminate things. However, it's still not a great idea because IR light will be absorbed by the water.]

In the end, you might want to call a photographer. Maybe you can try getting in contact with a nature or underwater magazine; they may have photographers on staff that can help answer your question better.