Menhaden are filter feeders that travel in large, slow-moving, and tightly-packed schools with open mouths. Filter feeders typically take into their open mouths "materials in the same proportions as they occur in ambient waters". Menhaden have two main sources of food: phytoplankton and zooplankton. A menhaden's diet varies considerably over the course of its lifetime, and is directly related to its size. The smallest menhaden, typically those under one year old, eat primarily phytoplankton. After that age, adult menhaden gradually shift to a diet comprised almost exclusively of zooplankton.