Gizzard Shad

Dorosoma cepedianum

Etymology

This shad is named for its muscular, gizzard-like stomach. It is also known as a sawbelly because of the keeled scales found between the anal fin and gills.

Life History

A Lake Erie study found that shad reach 5.5 inches at the end of their first year and 10 inches by the end of their second year. Most individuals spawn first during their third
year of life. By this time, they have grown large enough to avoid most predators. In Lake Erie, shad can live for up to six years.

Diet

Early in life, the gizzard shad feeds on microscopic zooplankton. Large shad consume detritus (decomposing organic matter) and algae in addition to zooplankton. Shad also ingest sand, which probably serves to grind up detritus and algae in their gizzard-like stomach and aid in digestion. The ability of shad to use low-quality food sources in addition to high-quality zooplankton contributes to its abundance. When high shad abundance leads to a decline in zooplankton, shad switch to eating detritus, bringing nutrients from the bottom sediments back into suspension. This leads to increased algae, and therefore zooplankton, production in open water. In can also lead to muddy water and decreased growth rate of bluegill.

Size & Master Angler Entries

Gizzard shad are not commonly caught due to their preference for microscopic prey. They do occasionally strike small spinners, spoons, flies, or live bait. The Michigan State Record is 4 pounds, 2 ounces. Recordclass shad can exceed 20 inches in length.

Management Issues

Gizzard shad are important forage fish when young, but they grow quickly to a size that many predatory fish cannot handle. Across central and southeastern U.S. reservoirs, gizzard and threadfin shads make up 45% of fish biomass. Fishery managers consider threadfin shad a superior species because it does not reach such a large size. In reservoirs dominated by gizzard shad, managers sometimes intentionally cause winter fish kills to remove excess adult shad.

Gizzard shad have a complex role in aquatic systems, and their effects cannot easily be predicted. Young gizzard shad compete with young bluegill for the best types of zooplankton in Ohio reservoirs, where shad depress bluegill numbers. A study in Oneida Lake, New York, found that shad do not compete with yellow perch in a similar manner because larval perch feed most heavily on zooplankton earlier in the year than shad do. Bass, pike, and walleye prefer eating soft-finned gizzard shad to spinyfinned bluegill. Where predators are abundant, shad may actually protect bluegill from predation and lead to higher bluegill abundance.

In addition to their effects as a competitor and preyfish, gizzard shad can have indirect effects on panfish and gamefish. Shad can decrease water clarity, which can have dramatic effects on the ecosystem and limit the ability of visual predators to feed effectively. The role of shad in large rivers and drowned rivermouth lakes of West Michigan is not fully understood, but this exotic has the potential to negatively or positively influence a variety of native species.

Importance to Fishery

Shad are a common catch in cast nets thrown by anglers collecting baitfish. Cast nets to 8 feet in diameter are legal gear in Great Lakes waters of Michigan, but not in rivers and inland lakes. Shad make excellent cut bait for catfish, and can be used as live bait. Live shad should not be transported to inland waters because they are not native to Michigan waters, with the possible exception of Lake Erie.

Young Asian carp look very similar to small shad. Anglers could very easily mistake young bighead or silver carp for gizzard shad (or other baitfish) and transport them to new waters. See: Resources for identification of Asian carp

Dead gizzard shad are a common sight in the weeks following ice out. Bayous along the Grand River, Lake Macatawa, and Mona Lake experienced large die-offs of adult gizzard shad in 2008 following a hard winter. Although the sight and stench of rotting fish are unpleasant, the shad is not native to the Lake Michigan basin and winterkills of adult shad may actually benefit native gamefish.