star trek - How did Khan and his people survive on a planet with no food for twenty years?

I'm enjoying my new blu-ray of Star Trek II, and a question which has always puzzled me came up again. Khan says something to the effect that only his superior will and intellect allowed them to survive after the planet was laid waste.

So - water, you can recycle, and maybe drill from a well beneath the ship. But what about food? There's only one lifeform left on the planet, and he didn't seem to be keeping many of those (the Ceti eels). What were Khan and his people eating? IIRC, this period of time doesn't have straight-up food replicators.