Is fresh-water eel kosher? I know that the eel is really a fish, with very small scales and elongated fins, so shouldn't this popular sushi fish be kosher?
Eel are not kosher, though they seemingly have both signs that demarcate kosher fish: fins and scales.
This is because the definition of fins and scales – as they regard to kosher – are clearly designated by Jewish law. Not every "scale" meets this limited definition. According to Torah law, in order to be considered a "kosher sign," a scale must be easily removable without tearing the fish's skin.
Eel's scales are part of their skin and cannot be removed without tearing the skin.
Rabbi Baruch S. Davidson