Menhaden, including the Atlantic menhaden (Brevoortia tyrannus) and the Gulf menhaden (Brevoortia patronus), are small, schooling forage fish that form an immense biomass along the North American coasts. These species occupy an important position within estuarine and coastal food webs. Understanding their diet is necessary to define their ecological function.
Defining the Menhaden Diet
Menhaden are not strictly classified as herbivores because their diet is not limited to plant matter. They are filter feeders that consume a wide array of organisms from the water column, placing them in the category of planktivores. The plankton they ingest includes both plant-like primary producers, known as phytoplankton, and small aquatic animals, called zooplankton. Because menhaden consume both groups, they are sometimes loosely classified as omnivores, though planktivore is the more precise term for their feeding strategy.
The Mechanism of Filter Feeding
Menhaden consume small, suspended particles using a passive method called ram feeding. The fish swims forward with its mouth continuously open, forcing large volumes of water over its gills. The water then passes through a complex filtering structure located on the gill arches, known as gill rakers.
These gill rakers are long, thin, and closely spaced, featuring microscopic secondary structures called branchiospinules that form a fine sieve. The spacing between these filtering elements is not fixed but changes as the fish grows, demonstrating an adaptation to available prey size. Juvenile menhaden possess very small apertures (around 12 micrometers) to efficiently capture minute plankton. As the fish matures, the spacing increases (around 37 micrometers in large adults), enabling the filtration of slightly larger organisms.
Ecological Role as Trophic Link
Menhaden’s diet and feeding mechanism establish them as a trophic link in the marine environment. By consuming vast quantities of plankton, they transfer energy from the base of the food chain to higher trophic levels. This role as an intermediary makes them a forage fish, providing essential nutrients, including oil and vitamins, to a wide range of consumers.
Numerous predator species rely heavily on menhaden, including commercially important fish like striped bass, bluefish, and weakfish, as well as marine mammals, seabirds, and sharks. Menhaden are often referred to as a “wasp-waist” species because their abundance heavily influences the populations of species both above and below them in the food web. Furthermore, by filtering the water and consuming phytoplankton, menhaden can contribute to local water quality management by reducing the biomass that can lead to algal blooms, though the overall effect on large bodies of water is often debated.