Because they are saltwater fish, alewives are not well adapted to life in fresh water. In fresh water, the salt concentration in a fish's body is higher than in the surrounding water. This causes water to leak into the cells of the fish. Fish that are well adapted to fresh water have larger kidneys to remove the excess water, but alewives lack this adaptation. Alewives often die in great numbers in early summer.
Alewives live for about six or seven years and usually begin to reproduce at about two years of age.