Introduction
The American shad ( Alosa sapidissima ), whose scientific name means "most delicious of herrings," is the largest member of the herring family and an anadromous fish, meaning it migrates from saltwater to freshwater to spawn. This species lives in coastal areas of the Atlantic Ocean and travels upriver to spawn in brackish or freshwater rivers. Humans introduced American shad to the Pacific Ocean in the late 1800s, where they are now found in many habitats. This article delves into the diet and feeding habits of the American shad throughout its life cycle, from larvae to adult, and explores how these habits are influenced by habitat and environmental factors.
American Shad: Physical Characteristics and Distribution
American shad can be found in coastal waters at depths ranging from 0 to 250 meters, and sometimes up to 375 meters. They are typically gray to bluish-green on their back when alive, with silver sides and a white belly. American shad have one or more black spots in a row on their shoulder, and rarely, some may have two rows of black spots. Their colors darken when they prepare to spawn. They commonly range from 45 to 76.2 cm in length and weigh between 0.9 to 5.4 kg.
The American shad is native to the Atlantic coast of North America, spanning from New Brunswick, Canada, to central Florida. They have also been introduced to the West Coast and are found in river systems such as the San Francisco Bay/Sacramento River system in California since the 1800s. In Connecticut, spring runs of shad occur in all major rivers, including the Connecticut, Housatonic, Pawcatuck, and Thames rivers, with the largest run occurring in the Connecticut River.
Habitat and Behavior
American shad spend their adult lives at sea and usually return to the river of their birth to spawn. Shad will swim farther upstream than any other herring. In the Connecticut River, shad have migrated as far north as Bellows Falls, Vermont, navigating dams via fishways. Some adult shad migrate back out to sea after spawning, but spawning stress is great, and dead shad are a common sight during late spring. Juveniles spend the summer and fall in the rivers and migrate to the ocean in the late fall.
American shad are social animals that swim in schools. In the ocean, they avoid larger species of fish, which may prey on them. American shad have the ability to detect ultrasonic signals up to 180 kHz, often enabling them to escape predators that use ultrasonic signals for communication. During the spring, summer, and fall, thousands of American Shad can be seen at the surface, but they tend to go deeper during the winter before the spawning season.
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Spawning Habits
Adult American shad migrate from the Atlantic Ocean to the east coast rivers of the United States and Canada to spawn. Depending on the temperature, they spawn in Florida’s rivers as early as November or as late as June in northern waters. Males travel upstream first as the water temperature reaches an optimal 50 degrees Fahrenheit, followed by the females. The peak temperature for spawning is 65.3 degrees Fahrenheit.
American shad are broadcast spawners and spawn multiple times. Adult American shad spawn in rivers during late winter. Spawning occurs overnight, starting at sundown and continuing after midnight. The males may also nudge the female's belly until she releases her eggs in open water, which the males then fertilize. Females release 200,000 to 250,000 eggs each year, and may take rest periods of 1 to 3 days between spawning cycles. The eggs travel downstream for several kilometers before they hatch in about 10 days.
Newly-hatched larvae average 10 mm long. The young shad spend a year downstream in brackish estuaries before relocating to the sea. Juveniles make their way from rivers into the ocean, sometimes traveling from the United States, Canada, and Mexico to spawn. Shad in the North are iteroparous, meaning they will continue this cycle and spawn multiple times throughout their lives. However, the shads native to the south are semelparity, meaning they only participate in a single reproductive season before death.
Diet and Feeding Habits Throughout Life Stages
Larval Stage
Shad larvae feed on their yolk sac for 4 to 7 days after hatching before actively seeking food.
Juvenile Stage
Young shad feed on insect larvae and zooplankton during their time in freshwater. Once they have left spawning areas, juveniles eat more substantially, consuming zooplankton, small shrimp, worms, and occasionally fish eggs. Studies in the John Day Reservoir have shown that juvenile American shad generally feed on numerically abundant prey, even if those prey are less preferred than larger-bodied zooplankton. A decrease in cladoceran abundance and size in August, coupled with large percentages of Daphnia in juvenile American shad stomachs, indicates heavy planktivory. Smaller juvenile American shad primarily feed on Daphnia in August but switch to more evasive copepods as the mean size of fish increases and Daphnia abundance declines.
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Adult Stage
Adult American shad primarily feed on plankton, similar to other herrings. They will also eat small shrimp, fish eggs, worms, and the occasional small fish. American shad eat very little food, if any, during their spawning migrations. They can retain digested food during their migration to survive. In freshwater, the adults eat little and only occasionally feed on small prey. They may filter feed but generally eat very little, if at all, during their journey. Adult American shad eat a wide variety of zooplankton, shrimp, and small fishes.
Zooplankton Composition in Diet
Zooplankton constitutes a significant portion of the American shad's diet, particularly for juveniles. The five major taxa collected in zooplankton tows include:
- Bosmina longirostris
- Daphnia
- Cyclopoid copepods
- Rotifers
- Calanoid copepods
Differences in zooplankton abundance are largely due to differences in water temperature and flow.
Predators and Defense Mechanisms
In the ocean, American shad avoid larger species of fish, which may prey on them. When threatened, American shad hide or flee to blend into their environment. Additionally, bears and some birds eat American shad, although these predators are not dependent on this species.
Human Interactions and Conservation
American shad have been harvested for meat and eggs for hundreds of years, and are a sport fish and are also bred in hatcheries. The meat and eggs are consumed. Efforts are underway to restore rivers with shed populations and stricter fishing limits have been imposed in a number of states.
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However, like other anadromous fish, American shad populations have been reduced by multiple factors such as overfishing, inadequate fish passage at dams, predation, pollution, water withdrawals, channelization of rivers, changing ocean conditions, and climate change. The Atlantic States Marine Fisheries Commissions conducts a coast-wide population assessment of American shad, and the last assessment found shad populations to be depleted from historical levels (ASMFC 2020).
Climate Change Impacts
Climate change may pose a problem for American shad going forward, as diadromous fish are amongst the functional groups with the highest overall vulnerability to climate change based on a recent marine and migratory fish vulnerability assessment (Hare et al. 2016). The Northeast Shelf and surrounding Northwest Atlantic waters have warmed faster than the global average (Pershing et al. 2015). New projections also suggest that this region will warm two to three times faster than the global average from a predicted northward shift in the Gulf Stream (Saba et al. 2016). Freshwater habitats will be affected by a high rate of sea-level rise, as well as increases in annual precipitation and river flow, magnitude of extreme precipitation events, and magnitude and frequency of floods (Hare et al.
Conservation Efforts in Massachusetts
The Massachusetts Division of Marine Fisheries (DMF) assists cooperative efforts to monitor shad spawning runs in the Merrimack River and conducts an annual backpack electrofishing survey of shad in the South River and Indian Head River and a beach seine survey for juvenile shad in the Taunton River. Additionally, the DMF has partnered with the U.S. Fish and Wildlife Service to stock juvenile American shad in the Taunton River (Chase et al. 2022; Mattocks et al. 2022). Massachusetts ended the commercial harvest of shad in 1987. Recreational harvest was closed for all rivers except the Connecticut, Merrimack, and their tributaries in 2014 and the creel limit was reduced from 6 to 3 at that time.