Fish have to be kosher

 Fish baked in coconut milk

Photo + Recipes: Mimi's Israeli Kitchen

B”H 

In Sefer Vayikra (Leviticus), G - d is giving the Kashrut (kosher) laws to the Jewish people because He wants His people to eat kosher food. For instance, the Jewish Kabbalah explains that kosher food influences our souls in a positive way. 

Sefer Vayikra (Leviticus) 11:9 teaches that kosher fish need to have fins and scales. Vayikra 11:10 teaches that everything that does not have fins and scales in the water, seas and streams, may not be eaten. 

What if a fish only has scales or fins ? Is the fish still kosher ?

The Mishna in Talmud Chullin 66 defines that kosher fish must have fins AND scales. Furthermore, variousTalmudic commentators discuss the meaning of scales. Example: Scales should be removed with a knife and in case the knife cannot remove those scales, there may be a doubt whether such scales can be considered scales. Some scales may be attached or even part of the skin and then there could be a doubt if such a fish is kosher. However, most fish we buy on markets have scales which can be easily removed by a knife.  

Fish or creatures from the sea which are not kosher: Shrimps, shells, crabs, eel or crayfish. 

Links:

Jonah and the Fish - A Kabbalistic Insight

Where are the skeletons of Noach's Generation dying in the Flood ?