How Do Gamefish Respond to Floods?

In January 2019, Mississippi had a flood event that affected much of the state along the Mississippi River. The Mississippi Department of Wildlife, Fisheries and Parks closed hunting seasons on a number of WMAs and set out supplemental feeding stations for wildlife affected by the rising water.

Such flooding events are part of the normal rainfall pattern throughout the Southeast, and they certainly affect wildlife populations. Under the surface, they affect gamefish populations, too, but in much different ways.

Matt Marshall is a fisheries biologist with the Alabama Department of Conservation and Natural Resources. He says the short answer to how flooding affects gamefish is, "It depends."

"It's a complex issue with both positives and negatives," he says. "Fish are adapted to flood responses; it's a natural process. It depends on the time of year when flooding occurs. For example, wetter winters produce better spawning and recruitment of crappie."

Other factors that affect the degree of impact include the species of fish involved, the length of the flooding event and the type of aquatic habitat.

Since Alabama's most important gamefish is the largemouth bass, the influence of floods on bass is an important issue for anglers. Marshall says time of year determines whether a flood is beneficial or deleterious to bass.

"If you have a flood event during the spawn, you can have a decrease in recruitment," he says. "The bass can build their nests at the normal water level, and then if it goes up six or seven feet, and you have turbidity issues, you could have fewer eggs hatch. The bass could compensate for that by increased survival of fry later in the spring and summer, so you might not really see the effects. Or you might see it several years down the road, with a slightly reduced catch rate of bass."

At other times of the year, flooding may benefit bass.

"In some riparian areas, you're adding nutrients back into the water, and you have increased forage," Marshall says. "There may be more invertebrates and more forage fish that move into bass cover."

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In Florida, fisheries biologist Ryan Hamm agrees that it’s a complex topic.

"The effect on crappie depends on the timing of flooding during the year," he says. "It also depends on the conditions before the flooding. We've seen crappie in drought-stricken lakes during the fall, winter and spring months; then when we have a high-water event, we've seen crappie gravid and full of eggs in June."

The supposition biologists make, he says, is that the fish are taking advantage of good conditions for spawning because conditions were so bad when they normally would spawn.

"One thing that's different in Florida is that we have broad, shallow lakes, where a lot of other reservoirs in the Southeast have a lot deeper areas," Hamm says. "In many of these lakes—such as the Harris Chain of lakes, which is a popular crappie fishery—even when we have hurricanes, because of the water control structures there, we don’t see a six- or seven-foot increase in water levels. So we don’t often have those impacts or conditions."

In Lake Okeechobee, Hamm says, there are large expanses of marshes.

"There, if you have extended periods of high water that impact emergent vegetation, that will impact black crappie populations," he says.

If an extended period of low water allows emergent vegetation to grow up and then the water level rises and cover it, he says, it will create very productive conditions for black crappie.

"That's not exactly a flood event, but it is a change in water level," Hamm says.

OPENING THE FLOODGATES

Dennis Riecke, a fisheries biologist with the Mississippi Department of Wildlife, Fisheries and Parks, says fish move and utilize floodwaters to go wherever they want to go.

"Flooding will allow them to access bodies of water that are close to them and not normally connected, but with the floodwater they are connected," he says. "It certainly will allow them to use floodwater (areas) that previously were terrestrial."

High water also may allow fish species to move into and colonize bodies of water they have not had access to in the past. For the fish, this means access to more resources and more food items than they usually have. Riecke spoke particularly of a food study done on catfish after a flood.

"The biologists found mice in the stomachs of catfish," he says. "Floodwaters will wash all kinds of organisms in. Animals that haven't had time to escape will be washed into the water, and some of them will be preyed upon."

Floodwaters also wash all kinds of organic matter in the form of leaves, sticks, dead grass and other plant matter into bodies of water, which can be problematic.

"All of that starts to decompose," Riecke says. "That decomposition process consumes oxygen, and if there's a lot of organic matter that is being decomposed, there may not be enough oxygen left for the fish. You could have a fish kill, which we often see that after hurricanes. It's the result of natural processes, not because of contaminants or toxins."

If the high water occurs during a fish's spawning season, Riecke says, those fish may spread out through a larger area than they usually do to spawn.

"They may now have a lot more suitable habitat," he says. "Some fish that lay their eggs on vegetation need that spring rise to access spawning grounds on the floodplain."

The bottom line here is that high-water events are not as hard on any fish species as they are on wildlife. If anything, floods enable fish species to proliferate, as high water also can allow populations—both and non-native—to expand and enter new bodies of water. If the fish species moving into new water is an exotic, however, that can present a whole new set of problems for fisheries biologists to manage.