Why do fish like and eat human spit?

I know it might sound weird or unbelievable but i went to Beth Shalom today and there is a pond, and me and my friends were bored and i think one of us spat in the pond and saw all the fish rush over and eat it, we all did it, watching them eat the spit. Do they mistake it for food? but we saw the same fish eat the spit more than once, are their vitamins in our spit or something wildly crazy?