American gizzard shad

What it is

This fish belongs to the herring family. Adults have deep bodies that are green-silver on top and plain silver underneath. A faint spot with purple shine may appear behind the gill cover. The skin often feels slimy. Its mouth sits low on the head and lacks teeth in adults. A key feature is its muscular gizzard, a stomach chamber filled with sand or rocks that helps grind food. The dorsal fin begins behind the pelvic fins, and the last ray is notably long. The anal fin has many soft rays. A branched lateral line system runs along the head and front body.

Where it lives

Native to large areas of North America, this species ranges from North Dakota south to New Mexico and east to Florida. It rarely goes north of New York Harbor. While absent from many Great Lakes historically, it likely entered Lake Erie after the last ice age. It thrives in freshwater lakes, reservoirs, rivers, and streams. It also tolerates brackish water along the Atlantic coast. The fish prefers shallow lakes with muddy bottoms and cloudy water. Survival rates drop in clear water or areas with heavy plant growth.

How it lives

Young larvae eat phytoplankton and tiny animals like rotifers. As they grow, diets shift. In many reservoirs, adults consume sediment detritus, while those in natural lakes may keep eating zooplankton if available. They ingest sand to help digestion in their gizzard. Feeding happens mostly during the day. High numbers can deplete zooplankton communities, affecting other fish. They redistribute nutrients by eating deep sediment and excreting it in forms other organisms can use. This activity can increase algal production. Predators include birds like herons and mammals like raccoons.

Breeding and life cycle

Spawning starts in mid-May or early June when water warms. Fish gather in shallow water during evening or night hours. They do not pair off. Females lay clumps of eggs that stick to underwater plants. No parents guard the young. A two-year-old female carries about 12,500 eggs, rising to 380,000 for a four-year-old. Fry hatch at 3.3 mm long and start feeding within days. Growth is rapid, allowing populations to expand quickly. They can hybridize with the threadfin shad.

Fish and people

Fisheries managers introduced this shad into many waters as food for game fish like bass and walleye. However, they grow fast and often become too large for predators to eat. High densities can outcompete other fish, leading some to label them nuisances. They are used in toxicology testing as indicators of water quality. In culture, they pose risks to infrastructure. A large bloom once blocked cooling intakes at a Canadian nuclear plant, forcing a shutdown. Their high thiaminase levels can cause vitamin B1 deficiency in predators like alligators.

Relatives and look-alikes

It is closely related to the threadfin shad. The two species can hybridize. Both share similar body shapes and habits, though the threadfin is generally smaller.

Questions people ask about American gizzard shad

Good reading and sources

• American gizzard shad on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data