Lakeside residents, bassclubbers, scientists and lake managers have discovered that exotic plants such as hydrilla and water hyacinth may produce some short-term benefits in a fishery. But over time, the non-native species tend to overrun indigenous species, dominating a reservoir, to the detriment of the entire ecosystem - including bass and other gamefish.
State agencies virtually coast-to-coast are seeking preventative measures, rather than relying on traditional mechanical or chemical treatments that must be repeated. Field experiments with native species have sparked hopes of learning how to fill a biological niche that intrusive exotic plants are threatening to occupy.
Early on, researchers are finding that what works in public lakes can be equally effective in private ponds. Studies and test plots are under way in a number of states, including Tennessee, Oklahoma, Kansas, Missouri, West Virginia and Illinois.
In Texas, the Corps of Engineers and the Parks and Wildlife are monitoring test plots in seven major public reservoirs - Cooper, Choke Canyon, Grapevine, Conroe, Jacksonville, Waco and Coleman - that are popular with anglers.
"It looks like that snowball is about to turn into an avalanche," said Dr. Michael Smart, research ecologist with the Corps and team leader of the Texas study.
"We've been pushing this concept since the early 1990s, but it has been slow to build. Now we've got a full head of steam."
In some test plots, researchers are planting 20-25 species, protecting them from hungry grass carp and turtles, and conducting what Smart called "a systematic review of what works best."
Pond managers already know that some species are more useful in a small closed system than others, but the research is helping to separate the "good plants" from the "bad" ones, Smart said.
"Bottom line, native plants are good, exotic plants are bad," Smart said. "We need to recognize the differences between the two, and manage our lakes to promote the benefits of native plants, while inhibiting the harm of exotic plants."
Ultimately, the goal is not merely to negate the use of weed-chopping boats and aquatic herbicides, Smart says. Instead, researchers such Dr. Gary Owen Dick at the University of North Texas in Denton hope to refine another effective implement to the lake manager's tool kit.
Here are excerpts from a paper Smart and Dick compiled for the American Fisheries Society:
Plants generally provide multiple benefits, serving as the base of the food chain and providing the structure of an ecosystem. They also provide habitat and food for fish and other aquatic wildlife, improve water quality, and stabilize shallow-water sediments.
Scientists say aquatic plants aid reproduction and survival of some fish species, most notably popular gamefish such as largemouth (Micropterus salmoides) and sunfish (Lepomis spp.). The plants serve as a substrate for epiphytic algae, which in turn serves as food for grazing invertebrates. These grazers serve as food for predators, including other invertebrates, and are critical as forage for many species of fish.
Plants also serve as cover for small fishes, increasing the survival rates of the fish and improving population recruitment. Larger fish tend to congregate near plants while searching for prey.
Numerous studies show the importance of aquatic plants, especially in the absence of submersed structure such as logs, rocks and drop-offs.
In general, aquatic vegetation is desirable until it grows to great excess, or completely covers the water surface over large expanses. Resulting surface mats interfere with human use of the water resource, and can cause severe environmental problems.
The bad plants, including problematic exotic species such as hydrilla (Hydrilia verticillata), Eurasian watermilfoil (Myriophyllum spicatum), waterhyacinth (Eichhornia crassipes) and giant salvinia (Salvinia molesta), tend to grow to the surface in dense concentrations.
Fisheries biologists say a coverage of greater than 20-25 percent may disrupt the natural order of a pond or take. When the coverage reaches 40 percent, it may deter growth-rates in fish.
This much we know for sure: Good plants, including most native species, do not grow in thick surface mats or in dense canopies that may lead to poor water quality. Instead, their growth proves beneficial to fisheries and other aquatic wildlife.
Many plants remove nutrients from the water column and from sediments, which reduces levels of nitrogen, phosphorus and potassium. These plants tend to compete with filamentous and planktonic algae, but a healthy aquatic plant community can reduce the frequency and severity of nuisance algal blooms.
Plants, while taking in micro-nutrients, can remove potentially harmful metals and other substances from the water. They clean and clear the water.
In photosynthesis, plants remove carbon from the water column, resulting in an increase in growth forms included in that coverage, or the diversity of the plants. For all fish species and size classes to fully utilize the habitat, a pond or lake should have a variety of plants.
The structure and biology of a particular plant species may greatly influence its value to a fishery. Stem length and leaf densities, along with palatability to grazers, are all key factors.
A stand of white water lily (Nymphaea odorata), for instance, consists of relatively sparse, thick stems and large, floating leaves (one per stem). This growth-type provides valuable shade (temperature reduction and visibility) and cover for larger fish, but small fish are vulnerable to predation when associating with this plant species.
On the other hand, Illinois pondweed (Potamogeton illinoensis) is a submersed species with branching stems and numerous leaves. It provides less shade but offers an abundance of cover for smaller fishes.
A community consisting of various growth forms should be of greater value to the fishery than a single-species stand, regardless of species. A pond owner should strive to develop combinations of floating leaves, subsurface leaves and stems that leave open areas for free access, liberal contact with the atmosphere. This will be most beneficial to a broad range of fish species and size classes.
Maybe you hope to attract waterfowl. Ducks and geese feed heavily on aquatic vegetation when making their migratory journeys during spring and fall.
Waterfowl, like most animals, tend to be opportunistic, but observations by the Corps of Engineers indicate that the birds show a clear preference for native plants such as American pondweed and Potamogeton nodosus, over hydrilla and other exotic species. Some waterfowl may totally shun the exotics.
When plants form dense beds, phytoplankton and periphyton growth may wane. The canopy eliminates most light penetration into the water column, Without sufficient light, most algae cannot survive, and grazer communities dependent upon them will not develop. This results in a poor foundation for the food chain and, ultimately, skinny or even stunted fish.
What does it all mean? Just as we monitor water quality and fawn over the fish populations in our ponds, we must recognize the most beneficial aquatic plant species and promote them.
Stocking your own "good plants"
So you want to introduce some native aquatic vegetation species, the so-called "good plants, to your pond?
First, recognize that in natural lakes, scientists with the Corps of Engineers say that aquatic plant communities may take hundreds or thousands of years to develop. In many artificial reservoirs, it takes time to establish native aquatic plants.
What's more, introducing native plants is labor intensive, requires planning and maintenance. "It's not for everybody," said veteran fisheries biologist Mac McCune. "In urban lakes, the plant has to be pretty. It can't spread. And in the event it does spread, it has to be controllable."
At Lake Management Services, based in suburban Houston, McCune has several favorite natives - iris, pickerel, arrowhead and, occasionally, taro (elephant ears).
Any amateur pond manager can establish these plants.
"It requires wallowing around in shallow water (1-3 feet deep) and stabbing them in the mud," McCune said. "A lot of your success will depend on the design of the pond, and how much wind and wave-action you get. If the water is too rough, the wave-action will uproot your plant."
He likes to scoop out a hole in the mud, drop in a fertilizer tablet, set the plant. Then, the protection phase begins.
"Ducks Io-o-ove arrowhead," McCune said. "And nutria will eat any fresh green plant it can reach."
To prevent predation from these and creatures such as turtles, beavers, muskrats and white amur (grass carp), you may need to fence off the area with an enclosure made of wire.
"Then you need to be careful that the plants you're introducing don't crowd out the other species," McCune said. "There's a compatibility issue involved."
His best advice? Consult the Source Guide for the nearest pond professional who has experience with fish ponds.
"Be careful where you buy your plants," McCune said. "The same species that works in a backyard water garden can be a nightmare if it gets loose in a 10-acre lake."
In Missouri, the Department of Conservation found hydrilla in its test plots, designed for native vegetation. Traced back to its origin, field staff discovered they received contaminated brood stock from sources in Florida.
Reprinted with permission from Pond Boss Magazine