--------------------------------------
Editor's note: This article originally appeared in the January-February 2006 issue of FLW Outdoors Magazine. Learn more about FLW Outdoors Magazine and how to subscribe by clicking here.
--------------------------------------
Shad are the dominant forage for a variety of sport fish – principally black bass, but also walleyes, striped bass, white bass and even catfish – in many U.S. waters. Maximizing sport-fish production often depends on shad populations.
The shad of concern are gizzard shad and threadfin shad. Both species feed low on the food chain and are usually abundant in fertile waters. But not all waters have the high nutrient levels needed to produce the rich plankton that, in turn, supports large shad populations. Fertilization, which works well in farm ponds, has not produced the desired results in infertile reservoirs.
River-run reservoirs with short retention times often do not support many shad. Even though the water may be rich in nutrients, the quick passage of water through these systems prevents the development of dense plankton populations needed to support abundant shad. Increasing the retention time may increase shad numbers but may not be compatible with the purposes for which the reservoir was built. For example, high demands for electricity in the summer result in short retention time in hydropower reservoirs.
High and stable water, when shad spawn, usually at water temperatures of 65 to 75 degrees, often benefits shad recruitment. This time period coincides with the spawning of black bass and crappie, and reservoir operators are often willing to provide desirable water conditions if possible. But, again, the primary purposes for which the reservoir was built take precedence over fish needs.
Large populations of shad do not necessarily mean a lot of forage for sport fish. Shad must be small enough to be consumed by predators. Gizzard shad grow quickly in most waters, often reaching 10 inches in two years, and are only vulnerable to the largest predators. In highly eutrophic (fertile) waters, gizzard shad have the nasty habit of achieving a high biomass of large, old fish that produce few young – lots of shad, but no useable forage.
In a research project in small Texas impoundments, this cycle was reversed with high-density striped bass populations. After several years of striped bass predation, the adult shad population crashed, and the few remaining shad produced abundant young that benefited largemouth bass. In some Midwestern states, managers routinely thin dense, old shad populations with fish toxicants. The beneficial results usually last only a couple years.
Threadfin shad grow to a maximum of 6 inches and offer a solution to the “too-big-to-eat” syndrome common with gizzard shad. Where established, threadfin shad also tend to suppress gizzard shad. To cash in on these desirable attributes, managers have stocked threadfin shad in many southern waters.
But there is a dark side to threadfin shad. They eat large zooplankton and, therefore, compete with a variety of fish, including the young of all sport fish. But a bigger problem is their intolerance of cold water. Threadfin shad start dying at about 50 degrees, and a sharp temperature drop or prolonged temperatures below 45 degrees will wipe out a population. A good forage population is not only abundant, but also stable. Reservoirs at the northern edge of the threadfin range often require restocking; something you obviously don’t want to do frequently.
Watershed factors, like nutrient inputs and reservoir operations, are dominant influences on shad populations. Managers have a few tricks for producing good shad populations, like stocking threadfin shad or controlling water levels, but these strategies don’t work everywhere. Because good fishing depends on shad populations in many waters, biologists continually look for effective ways to manage shad.
