kashrut kosher - Fins and Scales

I once heard that if a fish has fins it's enough - and you don't need to find out if it had scales.

1. Is this true?


2. What is the source?


3. Why did the Torah write fins and scales, if fins is enough of an identifying mark?

Answer

I believe you have the cases reversed. If you can tell that the fish has scales then you do not need the fins because all scaled fish have fins as well and you know the fish is kosher. The converse is not true and if you find a piece of fish which you can identify as having fins you may not eat it until you ascertain it also has scales. (Shulchan Aruch Yoreh Deah 83:3)