What Do Sardines Eat?

Sorry if I put this in the wrong place, but I didn't know where else it should go. lol My cat, Cher, eats a can of sardines each morning for breakfast. She loves them! But, I've always wondered...what do sardines eat? Do they eat like algae and kelp or do they eat other fish or plankton? I've always heard that you should only feed herbivore (plan eating) animals to carnivores (meat eating) animals. I don't know if that is true, but I'm kind of curious what they eat.