Do you have built the dam? (level 2 of engineering) (Highlight for spoiler) and the scarecrow? (bring an adept farmer to the twisted pieces of wood, near the thorns, twice)
If yes, the only thing to do is to put all your villagers on research and all your children on mushrooms. And wait for new crops (play in 2x speed, it'll come earlier). Take care of their health, can you make a stew that restore their health? (level 2 of exploration)
You don't need master farmer, a master scientist is sufficient for the algae-eating fish
Good luck!