American and Hickory Shad are anadromous fish, spending their adult lives foraging in saltwater before migrating to freshwater rivers for spawning. This life cycle requires a radical shift in their nutritional needs, directly reflected in the type and size of food they consume at different life stages and locations. The diversity of their feeding habits, from river nurseries to the open ocean, ensures they can support the energetic demands of both extensive ocean migrations and reproduction.
Feeding Habits of Juvenile Shad
The diet of young shad is concentrated on the readily available, small organisms found in the freshwater rivers and estuaries where they hatch. As larvae turn into juveniles, they begin to feed on a range of microcrustaceans, a process that fuels their rapid growth before they venture out to sea. Specific prey items include zooplankton, particularly species like Daphnia, as well as copepods and amphipods commonly found suspended in the water column.
Juveniles also consume small aquatic invertebrates and insect larvae, such as dipterans like midges and mosquitoes, which they encounter closer to the river bottom or surface. This intense period of feeding in the river nursery is important for building the energy reserves necessary to survive the downstream migration toward the ocean. In some systems, young shad may remain in these freshwater areas for several months, continuing to feed and grow until the late summer or early fall when they begin their seaward journey.
The Adult Shad Diet
Once American and Hickory Shad leave their natal rivers and enter the ocean, their diet shifts entirely to marine-based prey, which makes up the bulk of their feeding life. Adults are primarily pelagic feeders, meaning they forage in the open water column, often forming large schools. Their main food source is zooplankton, with larger copepods and mysid shrimp forming a significant part of their diet.
The shad’s ocean feeding is most active during the summer and fall months as they migrate to northern feeding grounds, such as the Gulf of Maine, to build energy reserves. While primarily consuming plankton and small crustaceans, they will occasionally consume other items. These additional food sources can include small fish eggs or larval fish, though these are a minor item in their overall diet.
Upon beginning their migration back into freshwater rivers to spawn, adult shad typically cease active feeding for the duration of the upstream journey. However, some studies indicate opportunistic feeding on copepods and mysid shrimp may occur in the lower estuarine portions of the river during migration. This suggests feeding cessation is more a function of food availability than a physiological inability to eat.
The Mechanics of Shad Feeding
Regardless of their life stage or location, shad employ a specialized biological apparatus to capture their food, functioning as efficient filter feeders or strainers. This mechanism centers on the gill rakers, which are bony, comb-like projections extending from the gill arches. These structures are tightly packed and act as a sieve, trapping tiny food particles while allowing water to pass through and exit the fish.
Shad use a technique known as ram suspension-feeding, where they swim forward with their mouths open to continuously filter large volumes of water. The closely spaced nature of the gill rakers is an adaptation that allows them to efficiently strain the microscopic plankton and small crustaceans that constitute their primary diet. This physical structure determines the size of the food particles they can retain, providing the necessary context for their plankton-heavy diet.