Menhaden, commonly known as bunker or pogy, are small, oily forage fish that travel in immense schools along the Atlantic and Gulf coastal waters of North America. These fish are often referred to as “the most important fish in the sea” due to their sheer biomass and foundational position in the marine food web. The menhaden species, belonging to the genus Brevoortia, are integral to the health of their ecosystems. Understanding what bunker eat relies on understanding their continuous, non-selective filtering of the surrounding water column.
The Primary Diet: Microscopic Plankton
Menhaden are non-selective filter feeders, meaning their diet is composed almost entirely of microscopic organisms suspended in the water column, collectively known as plankton. This diet is broadly separated into two categories: phytoplankton and zooplankton. Phytoplankton, which are microscopic plants or algae, form the primary food source and provide the bulk of the energy menhaden require.
Zooplankton, including tiny crustaceans and larval forms of marine life, serve as a secondary source of nutrition, especially for protein. The composition of the menhaden’s diet changes as the fish matures; smaller, juvenile menhaden tend to consume a higher proportion of phytoplankton. As the fish grow, their dietary preference shifts to include more zooplankton.
Unique Feeding Mechanism
The ability of menhaden to consume these microscopic particles relies on a specialized anatomical feature: the gill rakers. These fish are ram feeders, a mechanism where the fish swims forward with its mouth open, funneling large volumes of water through its mouth and over its gills. The water then exits the gill openings, but the food particles are retained.
The gill rakers are long, fine, and numerous, forming a dense, net-like sieve that strains the plankton from the water. Juvenile menhaden possess incredibly fine gill raker spacing, averaging around 12 micrometers in some populations, which allows them to efficiently capture extremely small plankton. This filtration is so effective that an adult menhaden can process thousands of gallons of water in a single day, continuously separating food from the surrounding water.
Ecological Impact of Their Diet
The vast quantity of plankton consumed by menhaden significantly impacts the coastal environment. By consuming massive amounts of phytoplankton, menhaden act as a biological control on primary producers in the water column. This feeding behavior contributes to regulating water quality, as the removal of excess phytoplankton can help mitigate the intensity of harmful algal blooms. Algal blooms, fueled by nutrient runoff, can deplete dissolved oxygen when they decompose, creating hypoxic “dead zones.”
Menhaden’s consumption effectively cycles nutrients and energy through the ecosystem. A single large school can significantly reduce the biomass of phytoplankton in a localized area, a process known as grazing. As they digest and excrete waste, menhaden regenerate nutrients like nitrogen back into the water. This dual action maintains the balance and health of estuarine and coastal marine ecosystems.