Differences between threadfin and gizzard varieties
For a lot of freshwater fishermen, a shad is a shad is a shad.
But not to biologists.
They know that the fresh waters of the southern United States hold two species of shad — gizzard shad and threadfin shad.
And biologists definitely like threadfin shad.
Unlike their bigger cousin the gizzard shad, which rapidly grows too big for all but the very largest of game fish to eat, the threadfin stays conveniently “bite-sized.” Most are 3 to 5 inches long, and only the largest will grow to their maximum size of 7 inches.
And unlike gizzard shad, which thrive in cold water, threadfin shad are subtropical. In the U.S., their native range was confined to the Deep South. Only the states of Louisiana and Mississippi lay entirely within their range. But they are found through eastern Mexico and Guatemala, into Belize (British Honduras).
Because they stay small and predator fish love to eat them, they have become the darling of fisheries biologists, who have stocked them throughout the country, including Hawaii.
Their one drawback is they are cold-sensitive. Massive winter kills of threadfin shad outside their native range (and sometimes even within it) are documented. At water temperatures of 50º F, they quit feeding. At 47º, they become inactive — and at 39º they die.
In some northern reservoirs, they are able to overwinter by relying on warm-water discharges from power plants. In others, their populations just boom and bust. But large threadfin shad kills are uncommon in Louisiana, if they occur at all.
While superficially at least, threadfin and gizzard shad look alike, there are some easy-to-recognize differences besides size.
The mouths of threadfin shad are located at the tip of the snout, while the mouths of gizzard shad are located under the snout — giving the fish an overhanging upper-jaw and snub-nosed appearance.
Color is another indicator, with threadfin shad having varying amounts of yellow mixed with their generally silver color. The yellow is especially noticeable in their anal (belly) and caudal (tail) fins.
Juvenile menhaden (pogies), which will also occur in fresh waters near the coast, also have yellow highlights in their fins, but they have entirely different trap door-shaped mouths. While menhaden and both shad species will have a large black spot on each side behind their heads, menhaden most often have approximately eight faint dark spots set in two rows behind the large spot.
Usually, a broad golden stripe occurs in the light area between the back and the sides of threadfin shad. Yellow coloration is especially noticeable in breeding males.
The shad spawn
Threadfin shad begin spawning later in the spring than gizzard shad, at water temperatures of 70º F and above. Spawning takes place early in the morning and is usually done near some structure, such as brush, logs or water plants.
A single female will be flanked by several males, as the small group swims erratically near the surface. When they near submerged structure, they discharge eggs and sperm. The fertilized eggs sink quickly and become slightly sticky so that they can adhere to the structure.
Both sexes will spawn several times during a season, with females producing 2,000 to 24,000 eggs per spawn. The eggs hatch in three days and the young threadfin shad form dense schools that travel near the surface — and may have 1,000 young fish per square yard.
From the time of hatching until death two or three years later, a threadfin shad’s diets remain the same: about half phytoplankton (microscopic algae) and half zooplankton (microscopic animals). Unlike their bigger cousin, the gizzard shad, they do not forage in bottom detritus for food.
The maximum reported size for threadfin shad, which was recorded in the Bogue Falaya River here in Louisiana, is 8 inches long. Females grow larger and live longer than males, with an occasional female living to age 4.
Threadfin shad are not only a favored food item for largemouth bass, but in many reservoirs crappie living in open water feed heavily on them. They have no food value for humans, however. Besides their small size, they are very oily and have many tiny bones in their flesh.
Threadfin shad are too small to have commercial value, as gizzard shad do for crawfish bait. But they are excellent trotline bait, and are used by both recreational and commercial trotliners.
Most used as trotline bait are caught with a cast net directly by the fisherman who plans to use them — they are seldom available for purchase.
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