Sport and tournament bass anglers generally have one thing in mind—consistent harvest of bass larger than "average-size." Bass size and growth is generally limited by the abundance of preferred food. Highly productive, nutrient-rich waters tend to support fast-growing bass that feed on abundant prey fish, the most common of which are the gizzard and threadfin shad. The interaction of shad and largemouth bass is one that is sometimes misunderstood by anglers.
Gizzard and threadfin shad are native to Alabama and are common in nearly every large impoundment and river system in our state. These fish, though generally similar in appearance, are different in both adult size and life history. Threadfin shad or "yellow-tails" rarely exceed seven inches in length, while gizzard shad are deep-bodied fish and may attain lengths greater than twelve inches. Quick to mature, both shad species are prolific spawners, producing thousands of young from late March through August. However, both species seek somewhat different habitats to live and reproduce. The abundance of young shad provides an excellent food source for bass and many other sport fish.
In most impoundments where they are found, shad numbers and pounds per acre greatly exceed that of all sport fish combined. In fact, shad alone can comprise several hundred pounds of fish per acre in a fertile body of water. It is the abundance of shad that may cause problems for other fishes during a portion of their early life. Because they are such prolific spawners, both species of shad may produce so many young that juvenile sport fishes are overwhelmed by competition for food. Shad are efficient at feeding on zooplankton (microscopic animals) and may reduce the availability of these organisms that are also necessary food items for juvenile sport fishes. In addition, unlike most juvenile fishes, shad of both species can switch to alternate food sources after stripping a body of water of available zooplankton. Therefore, in part, shad owe their great numbers to adaptability. They are able to switch to alternate food sources that other fish species cannot utilize.
Conversely, a low abundance of shad is also detrimental to bass and other sport fish. Largemouth bass are often dependent on consistent reproduction of shad as a food supply. When shad abundance is unusually low or high in a reservoir, the consistency of shad availability to bass as a food source is a problem. Consistency of shad reproduction is sometimes cyclic in nature and may lead to changes in bass populations.
Cyclic abundance of gizzard shad populations has been well understood as a problem in Lake Eufaula, a popular reservoir in southeast Alabama. In this reservoir, gizzard and threadfin shad typically have several years of abundant spawns followed by poor spawns. Largemouth bass populations generally exhibit low first-year survival, high mortality, and reduced abundance following years with poor shad spawns. An upswing in the shad population only begins after a die-off of large gizzard shad. This may occur when they become stressed, diseased, or environmental conditions are unfavorable. Improved shad spawning activity usually follows die-offs of large shad. As a result of the increased shad spawning activity, bass populations usually improve for several years. Though little can be done to prevent the cyclic nature of shad populations, largemouth bass populations can be managed under these circumstances by setting length limits.
In 1992, a 16-inch minimum length limit on bass was established in Lake Eufaula to improve the first-year survival of bass. Bass minimum length limits may inhibit extreme swings in gizzard shad populations in these situations by allowing bass to consume larger adults. Since the minimum length llimit was imposed at Lake Eufaula, gizzard shad population cycles hav not been as extreme and the bass population structure has improved. During this same period, anglers increasingly returned more bass, allowing for a buildup of larger fish. This increase in the bass population resulted in slower growth rates and lower average body weight. On November 1, 2000, the minimum size-limit on bass in Lake Eufaula was reduced to 14-inches to allow a greater harvest of these fish and improve fish growth.
Another potentially negative impact of shad on sport fish is the decline of bluegill populations. Just as with bass, direct competition with shad for food can result in reduction of bluegill populations. For example, in balanced farm ponds managed specifically for bream and bass, bluegill and redear sunfish comprise the primary food of largemouth bass. Unfortunately, shad are sometimes introduced in ponds; and they may reduce bream survival, abundance, and angler catch rates. As shad populations increase, the reduced abundance of bream may actually result in negative effects on bass growth and survival. This is especially true when gizzard shad are stocked and the pond or lake in time becomes "overstocked" with shad too large for bass to consume.
Unlike gizzard shad, threadfin shad are susceptible to bass predation throughout their life because of their smaller adult size. Yet, despite their small size, threadfin shad have also been blamed for reduction of bass, bream, and crappie (white perch) populations in reservoirs throughout the Midwest and Southeast.
Despite the potential for negative shad and bass relationships, gizzard and threadfin shad are the preferred food of adult largemouth bass. Juvenile shad provide a substantial source of food in the summer when young bass switch from a diet of zooplankton and insects to fish. Good survival and growth of young bass is dependent on a consistent and abundant food supply.
In Alabama, recent research links reservoirs with above average water fertility to fish populations with higher densities of shad and, consequently, higher survival of young bass. The explanation for this relation in Alabama reservoirs is many-fold, but consistent availability of shad as food through summer and fall months is one very critical factor. Small bass beginning to feed on fish must grow quickly, gaining adequate fat reserves that will ensure over-winter survival. On the other hand, shad must grow quickly to avoid being eaten by bass. Lake Jordan, Weiss Lake, and Millers Ferry Reservoir in Alabama are typical of impoundments with consistent numbers of both species of shad. These reservoirs have food-rich waters, little water level fluctuation, and drain large, fertile watersheds. Good survival and well-structured bass populations are reflected by the relatively consistent, above average catches of bass by anglers fishing these reservoirs.
Stockings of both gizzard and threadfin shad have varied results that may or may not benefit the overall fish population. The uncertainty of bass and shad relationships often prevents fish managers from stocking shad in ponds and small lakes where they do not occur or where they may cause harm to the overall fish population. Most large public reservoirs in Alabama currently have native shad populations of one or both species. Before stocking shad, both the potential positive and negative changes to the entire fish population should be considered as well as what management goals and costs may be necessary to maintain the fish population and overall angling enjoyment. If you have questions, contact the District Office.