I googled your expression, and the closest I came to finding an equal English expression was "Save some for the fish." I don't know if that's a great equivalent, though.
Some other translations courtesy of Wikiquote at
http://en.wikiquote.org/wiki/Spanish_proverbs were:
"If you don't need something, leave it for others to use."
"Don't take that which is not rightly yours."
One source I frequently use online is
www.wordreference.com as a quick Spanish-English (and other languages) dictionary. It also has a Languages Forum component in which you can find translations for expressions, slang, nicknames, etc.
Hope that helps a little! I'm sure a native speaker might be able to give you some better translations for the expression.