Fish need water

Spring floods are good for riverine systems

As I write this in late March, the Mississippi River is at flood stage, 12 feet above normal stage for this date, and rising. Hopefully this will not be a repeat of 2011, when the river reached a record high stage at Vicksburg and threatened to top or blow out levees.

Barring social and economic catastrophe, this is good news for fish in the Mississippi River, in Mississippi’s flood control reservoirs and probably other Mississippi waters.

A floodplain is the land lateral to the river that is seasonally flooded when the river rises during the annual spring flood. Many smaller rivers also have floodplains that might be flooded repeatedly throughout the year following heavy rainfall events.

For the Mississippi River, much of the historic floodplain has been separated from the river by tall, continuous levees. Despite the levees, the lower Mississippi River — from the confluence of the Ohio River in Kentucky to the mouth of the Mississippi downstream of New Orleans — still has 1.6 million acres of “active” floodplain that receives some amount of flooding in most years. Some refer to the active floodplain as the “batture.”

This seasonal flood is critical to fish production in the river.

In lakes, the primary production — the first level of the food web that determines how many pounds of fish per acre a lake supports — results from algae suspended in the water. Primary production occurs within the lake.

In rivers, especially turbid rivers like the Mississippi River, algae provide little primary production. Rather, terrestrial plants that grow on the floodplain provide much of the primary production that fuels the food web.

The plants and plant material like leaves and brush accumulate on dry land most of the year and are unavailable to the river. It is the flood that makes the energy in plant material available to the river’s food web.

Few native fish eat plants. But when the rising waters turn the floodplain into an aquatic habitat, the plant material provides the food resource for aquatic invertebrates that pass up the aquatic food web.

Some fish, like blue and channel catfish, follow the flood waters onto the floodplain and eat these invertebrates, as well as terrestrial insects, amphibians like frogs and salamanders, reptiles such as skinks and snakes, and even small mammals.

The slowly flowing or standing water on the food-rich floodplain also provide ideal habitats for spawning for many fishes and food-rich conditions for their young.

Smallmouth buffalo, for example, wait for the flood to quickly move onto the floodplain to spawn. Crappie, sunfish, and white bass spawn on the floodplain.

When the flood waters recede, they carry the primary production from the floodplain, as well as the invertebrates and the young of many fishes spawned on the floodplain, into the river to provide vital energy sources for the remainder of the year.

Thus, the fish do not need to move onto the floodplain to benefit from the production there.

Without floods, the Mississippi River would produce far fewer fish and production of young by many fish would be limited — or spawning might even fail.

This same process applies to oxbow and lakes inside the levee like Albemarle, Chotard, Lee and Whittington.

Floods affect fishes in Mississippi’s flood-control reservoirs — Arkabutla, Enid, Grenada and Sardis — in a similar way.

The “rule curve” — the prescribed seasonal water levels for these lakes — keeps reservoir water levels low through the winter. But when the Mississippi River is high, water is held in the reservoirs to prevent flooding in the Yazoo River backwaters.

If rainfall is heavy, as it has been this year, the reservoirs fill early.

The high water results in flooded vegetation on floodplains at the upper ends of the reservoirs, as well as flooded vegetation along much of the shore. That means crappie, sunfish and bass will have abundant spawning habitat.

The flooded vegetation will nourish abundant invertebrates that provide essential food for the young fish, while the vegetation also provides good cover that will benefit their survival.

Anglers who fish the flood-control reservoir can look forward to lots of fish-holding cover this year, and strong year classes of crappie and bass entering the fishery in a couple years.