Dear drowser: a slight correction, because I did not make it clear. An alewife has a fat tummy, which to some indelicate colonist looked like a pregnant woman's abdomen. It is not a true herring, apparently.
A bit more trivia. Gov. Bradford in his history told of the Indians teaching the colonists to plant maize in hills, in which were buried one of more alewives to provide nutrients. He also instructed them not to plant the maize seed until the leaves on the white oak trees were the size of a mouse's ear, which coincided with desirable soil temperatures.