It’s a forage thing: the right amount of the right size of baitfish
Mississippi’s flood-control reservoirs — Arkabutla, Enid, Grenada and Sardis — are regarded by many anglers as the best in the country for big white crappies.
The 5-pound, 3-ounce, world-record white crappie from below Enid Dam supports that contention. Vehicles with license plates from across the Southeast and Midwest parked at launching ramps lend further support. But crappies swim in waters throughout the United States. What’s the deal with the flood-control reservoirs?
In past months, I have explained that low water levels in spring can curtail successful spawning. Loss of young crappies through the water-control structures (entrainment) during the winter and early spring — when huge volumes of water need to be released to maintain flood storage — can greatly reduce the number of crappies that survive the winter. Spawning success and overwinter survival affect recruitment: the numbers of crappies available to anglers. This time, the subject is what is known and not yet known about why growth rates of flood-control reservoir crappies are among the fastest in the country.
It is an axiom of fisheries management that fast growth requires an abundant food supply, but an abundant food supply is a function of both the amount of prey and the number of crappies sharing the resource. In flood-control reservoirs, the occasional weak year-class means fewer crappies competing for food. The loss of crappies to entrainment also reduces the competition for food.
It is also true that the abundant food has to be the “right” food — readily available, easily captured and large enough to provide a net energetic gain. That means more energy gained from what is eaten than energy expended to capture and process the meal. But the forage needs to be small enough that it can be easily captured and ingested. This might be the missing piece of the puzzle.
Keith Meals, a fisheries biologist for the Mississippi Department of Wildlife, Fisheries and Parks who oversees fisheries management in the flood-control reservoirs, suggests that gizzard shad provide the “right food” for the big crappies. He has sampled 3-pound crappies with 7-inch gizzard shad in their stomachs.
Biologists rarely regard gizzard shad as good crappie forage. In many lakes and reservoirs, gizzard shad grow to 8 inches in one year and 10 inches or larger in two years. These big shad are too large for most crappies, even the giants in the flood-control reservoirs. Throughout reservoirs in the Southeast, threadfin shad are often the dominant forage for crappies, because they do not grow as large as gizzard shad. But gizzard shad in the flood-control reservoirs grow slowly.
“Research has found gizzard shad in Sardis Reservoir grow to 3.4 inches at age 1 and 6.3 inches at age 2, a size easily eaten by a big crappie,” Meals said. “That is slow growth for gizzard shad compared to many waters; and the lengths at ages 2, 3 and 4 are significantly smaller than in Bay Springs Lake on the east side of Mississippi.”
If slow-growing gizzard shad are the right forage and contribute to the fast growth of crappies, then solving the mystery of big, fast-growing crappie requires understanding why gizzard shad grow slowly.
The flood-control reservoirs are less productive than many other Southeastern reservoirs due to the low-fertility soils in their watersheds. Possibly, discharge of most of the water volume of these reservoirs also flushes out nutrients that commonly accumulate in other lakes and reservoirs with more-stable water levels. The low nutrients support minimal amounts of plankton eaten by young gizzard shad, slowing their growth. And the low water levels during fall and winter may also limit the food available to larger gizzard shad that feed on organic matter on the lake bottom. Less food for shad results in slower-growing shad.
But if the lakes have low productivity, conventional wisdom predicts low numbers of shad too, but shad recruitment is generally good in the flood-control reservoirs.
Research on Sardis and Bay Springs was designed to find out why shad recruited well at Sardis but not at Bay Springs. A major difference between the lakes was the size of zooplankton. In Bay Springs, zooplankton in spring were mostly large adults. While these big zooplankton were the right forage for larger shad, they were too big for small shad, so most small shad starved. However, zooplankton in Sardis were small in spring as water levels rose after U.S. Army Corps of Engineers rule curves — provided rainfall was sufficient — and plankton populations expanded. Small zooplankton were the right forage for shad fry and resulted in high shad recruitment. Numerous shad meant more of the right forage for crappies, but the low productivity and low abundance of food resulted in slow growth of shad. Hence, a lot of “right-size” gizzard shad.
Clearly, important questions await in answers, but waiting is not necessary to enjoy the great crappie fishing in Mississippi’s flood-control reservoirs.
