Shad have a distinct advantage over other fish species because of their ability to filter large amounts of water through long, closely set gill rakers. As the water passes through the gill rakers, free-floating plants and animals are filtered out as food. This enables shad to compete much more effectively for food with other fish that rely on the same diet, such as sunfish and recently hatched bass and crappie. Shad also graze for algae and small insects over logs and other underwater objects, but their main way of feeding is simply swimming and pumping water through their mouths and out their gills.
Gizzard shad are especially prone to cause problems in very fertile bodies of water due to their ability to quickly grow so large that most predators cannot eat them. It is not uncommon for adult gizzard shad (eight inches long and larger) to comprise 60 to 80 percent of the total fish in fertile impoundments. When this happens the shad often out-compete sunfish and other young-of-year sport fish for food, and will even become so overcrowded that their body condition will decline to the point that the shad cannot produce many offspring. Since these shad will be too large for most sport fish to eat, sport fish such as bass, bream, and crappie grow and reproduce very little until most of the large shad die and the remaining shad spawn again.
Threadfin shad can also produce large numbers of offspring which will out-compete sport fish species for food. While threadfin do not grow too big for predators to eat like gizzard shad, they can still make up most of the fish biomass in a body of water due to their filter feeding ability and high reproductive rates. High densities of shad only occur in slow moving rivers, reservoirs or small impoundments with relatively high fertility rates. Water bodies with low fertility rates or which stay muddy are not conducive to filter feeders and will not support large numbers of shad.
Since shad can become overcrowded in small impoundments, they are not generally recommended for stocking. In certain situations, fisheries managers have had some success with periodically applying small concentrations of rotenone to small impoundments with excessive numbers of adult shad. By conducting a “selective treatment,” the adult shad do not become overcrowded and small young-of-year are produced consistently.
Some private pond owners have actually gone back to stocking shad as forage, although now they are more careful to only stock threadfin. Even when only threadfin shad are present, well-fertilized ponds often tend to become shad crowded within two to four years after shad are established. Ponds in this situation generally support mostly small bluegill, with a bass population lower in number than ponds with no shad present. However, once bass grow to a size they can prey on the shad, bass growth rates are extremely high.
Pond owners considering stocking threadfin shad to manage for large bass should certainly consider the negative consequences. There will be fewer bass and bream available for harvest, the bream growth rates will be adversely impacted, and the shad population will probably need to be partially poisoned when the adult shad become overcrowded and stop spawning. Shad are also extremely sensitive to water quality and temperature changes, so sudden shad die-offs are not uncommon.
There has been very little large-scale research conducted on establishing threadfin shad in small impoundments, so reliable stocking rate recommendations are not available at this time. Last but not least, any introduction of threadfin shad has the potential to also introduce gizzard shad. While gizzard and threadfin shad are certainly important as forage fish in many of Alabama’s rivers and reservoirs, the potential problems shad can cause should be carefully considered before the are stocked in any body of water.
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