Shad: The forage that fuels sport fish

Threadfin shad are great forage for predators during their entire life cycle because they don’t grow to large sizes.

Shad are the predominant forage for all fish-eating game fish in Mississippi waters. The shad spawn will start this month, signaling the new crop of this essential forage fish.

And 2014 will be an interesting year for shad dynamics after the extremely cold winter that undoubtedly took a heavy toll on the populations.

Shad biology

Mississippi waters are home to two shad species: gizzard shad and threadfin shad. Both fish have a dark back, silver sides and a sharply keeled belly.

Both primarily live in the open-water areas of lakes and slow waters in rivers, occasionally swimming among sparse aquatic vegetation or along the edges of dense vegetation.

The two species can be easily distinguished in several ways, but the simplest way is to look at tail fin color and eye color. Gizzard shad have grey tails and yellow eyes; threadfin shad have yellow tails and white or gray eyes.

Although sharing the same habitat, the two shad differ in several important ways. The young of both species feed on zooplankton.

Threadfin shad continue to feed on zooplankton throughout their lives. Although important forage fish, their appetite for zooplankton also makes them competitors of the young of many sportfish.

Juvenile and adult gizzard shad feed on zooplankton, phytoplankton and energy-rich organic sediments. The gizzard shad’s bottom-feeding habitats contribute to the high densities of gizzard shad that sometimes are problematic in high-fertility waters.

The gizzard shad is a cosmopolitan species extending coast to coast and from tropical Florida to the Canadian border.

Winter is tough on gizzard shad, but they survive in ice-covered lakes.

Threadfin shad are not tolerant of cold water; massive die-offs occur when the water temperatures drop into the 40s; 41 degrees is considered the lower lethal temperature. Water temperatures this past winter plunged into the mid-30s at least as far south as Ross Barnett; I expect very few threadfin shad survived in the northern two-thirds of Mississippi.

The two shad also differ in maximum size.

Both fish grow quickly, but threadfin shad only grow to 6 or 7 inches long and provide useful forage throughout their life cycle.

Gizzard shad, on the other hand, can grow to 8 to 12 inches in their first year and to 14 to 16 inches the next year, sizes too large to be useful forage by all but the largest black bass, striped bass, and blue and flathead catfish.

In the eyes of a predator fish, shad are the ideal forage. Their elongate body allows piscivores to consume relatively large shad if they are available. Largemouth bass and flathead catfish, for example, can consume shad up to one-third their own body length. And the high oil content of these easily swallowed prey makes for a high-energy meal.

Shad management

Because shad are the dominant forage for most game fish throughout the southern United States, it makes sense to manage them. Biologists have invested heavily in understanding shad population dynamics.

Unfortunately, what affects shad is largely beyond fisheries managers’ control.

The ideal forage fish is abundant and remains so year after year. Shad feed low in the food web, and their abundance is related to water fertility — nutrient-rich waters can support several hundred pounds or more of shad per acre. Obviously, some waters will support more pounds of shad than others.

Fertilizing reservoirs to boost shad production is tremendously expensive, and several attempts have been unsuccessful. Fortunately, Mississippi’s reservoirs, with few exceptions, are moderately to highly fertile.

Stable abundance requires successful production of young every year. In Missouri, gradual rises in water level and warm spring temperatures are conducive to strong gizzard shad year classes. In Lake Texoma on the Texas-Oklahoma border, strong gizzard shad year classes are associated with high and stable water levels.

In river-run reservoirs, such as those on the Tennessee or the Tombigbee rivers, high flows during late spring and early summer can prevent the development of dense zooplankton needed to nourish abundant young shad, so too much water can have a negative effect.

The ideal forage is vulnerable to predators throughout most of its life. Threadfin shad, which only grow to 6 to 8 inches certainly qualify as ideal. Further, threadfin shad can mature in their first year of life and spawn in late summer or fall. Thus, small threadfin shad are available throughout much of the growing season of warm-water game fish.

Gizzard shad, on the other hand, can grow quickly to sizes too large for most predators. The growth rate of shad is related to water fertility. The same nutrient-rich conditions that produce abundant shad might also result in fast-growing shad that have limited value as forage. Compounding the problem of fast growth of gizzard shad is that high-density populations of large shad tend to have very low production of young shad.

The bottom line: Some lakes can have tons of shad but little forage for sport fish.

Again, Mississippi is fortunate. The Magnolia State’s waters are productive enough to produce abundant shad, but the populations remain productive. At this time, the best shad management strategy is “if it ain’t broke, don’t try to fix it.”

Shad in 2014

As noted earlier, this will be an interesting year for shad. Gizzard shad populations should be low after the cold winter.

Conventional fishery thinking suggests that the presumed low-density shad populations should produce a strong year class.

Gizzard shad have high fecundity — usually more than 100,000 eggs per female — so it won’t take many spawning adults to reestablish the population.

Threadfin shad, on the other hand, will remain a mystery until late summer or fall. No efforts are made to monitor shad in early spring, so nobody knows how many threadfin shad survived the long and cold winter.

And the fecundity of the smaller threadfin shad is only about one-tenth that of gizzard shad. It may be several years before threadfin rebound.

Several fisheries biologists have suggested that threadfin shad might suppress gizzard shad, and long-term population trends of gizzard and threadfin shad stocks in Tennessee’s Normandy Reservoir support that speculation.

Might the abundant threadfin shad in Mississippi’s reservoirs explain why gizzard shad populations remain slow growing yet productive? This year, and maybe next, m be the natural experiment to test that hypothesis.

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About Hal Schramm 163 Articles
Hal Schramm is an avid angler and veteran fisheries biologist.

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