It is viewed as desirable by many fishermen, particularly bass anglers, but it is a feared nuisance by lake managers who spend millions of dollars annually attempting to control this invasive aquatic plant.
Hydrilla is native to Asia. Its arrival in North America is attributed to the aquarium trade. It is understandable that this lush, bright-green plant with vigorous growth would be a desirable aquarium plant. Unfortunately, plants and animals that start out in pet and tropical fish stores often find their way to our lakes and rivers, and such is the case with hydrilla.
Hydrilla now grows in most states in temperate and subtropical North America.
Hydrilla is very similar in appearance to elodea, a native plant. But the two plants are easily distinguished by looking at the underside of the leaves: Hydrilla has small spines growing from the leaf midrib, while elodea lacks the spines.
But rather than strain your eyes, rely on your sense of touch. If you pull a strand of elodea between your finger and thumb it feels smooth, even slippery. Pull a strand of hydrilla the same way, and the spines make the plant feel rough, raspy and dry.
While established in several states since at least the 1960s, Mississippi is one of the more recent states to be invaded. Hydrilla grows in Ross Barnett, where it is closely monitored, and lake managers spend more than $100,000 annually to keep it in check.
It is in Lake Monroe, in Pickwick, and throughout the Tenn-Tom Waterway. Last year, in Bay Springs Lake Â the northern-most impoundment of the Tenn-Tom Â it topped out in water 16-feet deep.
Ken Langeland at the University of Florida Center for Aquatic and Invasive Plants has studied hydrilla for more than 25 years. He describes hydrilla as the "perfect aquatic plant."
The physiology of the plant allows it to grow to depths of 30 feet, and it can flourish with less light than native submergent plants. New plants can grow from stem fragments, reproductive structures call turions that grow on the plant, and tubers on the roots.
It can root in bottom types ranging from soft sediment to gravel.
Add it all up and it is easy to see how this plant can rapidly spread, outcompete native plants, and grow to nuisance levels, even in fairly deep lakes.
Hydrilla as habitat
There's no doubt about it: A few acres of hydrilla here, a few acres there benefits fish and fishing. The leafy plant provides extensive surface area for algae to grow, which in turn provides food for abundant invertebrates like insect larvae, crayfish and grass shrimp. These invertebrates are food for fish like sunfish and a variety of minnows that in turn are forage for other sportfish.
It is easy to understand why many fish concentrate around hydrilla.
Besides concentrating fish, particularly bass and sunfish, hydrilla also makes for easy fishing. Hydrilla usually holds fish in shallow water where anglers are more effective, and the rather stiff stems allow anglers to fish a variety of lures and live bait at the edge, over and even in hydrilla.
Fishing is usually better in clear water, and hydrilla can clear the water. By slowing water movement, suspended sediments settle to the bottom. By taking up plant nutrients like nitrogen and phosphorus that cause algae blooms, hydrilla also can suppress algae.
The problems with hydrilla
While 5 percent, 10 percent or even 20 percent of a lake covered with hydrilla can benefit fisheries, particularly bass fisheries, shallow-water lakes and ponds can develop excessive coverage of hydrilla and the fish-concentrating effect of aquatic vegetation is lost.
The habitat is still good, but fish can disperse through vast acres of the good habitat.
Habitat quality may deteriorate with excessive growth. Dense, expansive stands of hydrilla produce abundant oxygen during the day but use it at night. Fish-killing oxygen depletions are an imminent threat of heavy hydrilla infestations.
Unlike native plants that grow only a few feet tall or grow to the surface and stop growing, hydrilla often continues to grow, forming mats of intertwined hydrilla stems at the surface - a condition often referred to as "topped out."
After the hydrilla tops out, the plant stems below the mat lose their leaves, and that means the plants no longer support the algae and invertebrates that fuel the food web. Bass may still be there and easily caught, but the overall benefit to the food web is lost.
Hydrilla may be the perfect aquatic plant, but it is not well behaved - hydrilla grows where conditions are suitable for the plant, not for people. Most of us fish in multi-use, public waterways. While 20 percent coverage of hydrilla may be good for fishing, it may or may not be a problem for other users of the water. It depends where it grows. Even 5 percent of a lake's surface area with dense hydrilla can be a problem if it grows in marinas or around private docks.
Hydrilla can be controlled with approved chemicals, but the cost can easily exceed $1,000 per acre per year, and chemicals usually need to be applied two or more times each year. The herbicides have a noticeable effect in less than one month, and managers can determine where to remove hydrilla.
Hydrilla can be more cheaply controlled with triploid (non-reproducing) grass carp for about $100 to 200 per acre, and the control lasts several years. Hydrilla reduction will be slow, and only by careful stocking of the triploid grass carp can the hydrilla be reduced without being eliminated.
Whether using chemicals or triploid grass carp, hydrilla control puts a drain on limited fiscal resources.
Hydrilla can be an asset to a fishery, but this is rare. Pickwick, however, may be one of those rare cases.
In most Mississippi waters, hydrilla may initially benefit fisheries but then become a significant problem that will require substantial funds to remediate. Native aquatic plants provide all the benefits of hydrilla without the problems that result from excessive growth. A far better use of money spent controlling hydrilla is to invest in establishing native aquatic plants.