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Low Down on Lake Toho Draw Down
Part 2: Hydrilla
By Paul Crawford

February 11, 2004

One of the major issues pertaining to the draw down of West Lake Toho is an attempt to address a common problem facing fisheries management, a massive hydrilla infestation. While not the sole, or even most pressing issue surrounding the draw down, it is arguably the most controversial and least understood aspect of the activities.

For those unfamiliar with this noxious plant and what it can do to a lake, consider a few facts. West Lake is a relatively small place of just 19,000 acres at normal pool. It is currently estimated to contain 14,000 acres of hydrilla most of which stretches from the bottom of the lake all of the way to surface. Add another couple of thousand acres of natural emerging plants around the lake, and you start to get the idea, there just isn't much open water anywhere. There are miles of the lake where a wind that should be causing white caps can't even raise a ripple due to the hydrilla mats. Boaters can't run around the mats, they are forced to run right through them. Find a path through the mat where previous boats have sheared off the top a couple of inches under the surface and it qualifies as open water. It is both fascinating and terrifying what a plant that grows over an inch a day can do.

Hydrilla has been controversial ever since its release into the wild. Traditional lore holds that an aquarium dealer released a few sprigs into a fresh water canal near Tampa. From whatever source, hydrilla spread with awe inspiring speed and quickly overwhelmed native vegetation. At first, all lake users universally condemned the matting nuisance. But one needs to keep in mind there was a reason it was imported for aquariums to start with, fish just love it. Beneath the shady mats, hydrilla provides comfort and oxygen. Cooler in the summer and warmer in the winter, it quickly became a favorite habitat for forage and predator alike, something fishermen figured out soon after its introduction. From villain to hero label attached to hydrilla, misguided fishermen intentionally furthered an already alarming spread of the plant in the process earning the wrath of lake managers, homeowners, and pleasure boaters alike.

Fast forward a few years and the critics have been mostly proven right. Hydrilla has spread throughout the Eastern United States and become more of a problem than even the most cynical of the critics could have thought. Still, Florida remains the front line of the fight since if offers virtually ideal growing conditions. Official policy assumptions hold that Hydrilla is present in 100% of the navigable fresh water in Florida and control measures are becoming more futile each year.

If Florida is the main battlefield, then the Commanding General of the Hydrilla War is Florida Department of Environmental Protection's Jeff Schardt. Jeff has led the assault on Hydrilla for over 10 years, been the author of numerous research articles on hydrilla control, was a key member of the committee that investigated the Harris Chain debacle, and is responsible for State weed control on, among other places, the Kissimmee Chain of lakes.

We contacted Jeff for a response to some critical web sites that were expressing outrage about the DEP completely eradicating hydrilla during the draw down. To say we were surprised by the response is an understatement, he simply laughed. "We'd probably kill it if we could, but we just don't know how." Thus began an education on a surprising plant.

Hydrilla has been called "The Perfect Weed." If you roam around the University of Florida's botanical archives, you can find over 3,500 articles on hydrilla and its control. With all of this effort and study, one would assume we know a lot about this plant, what it can do, what it can't do, and why. One would be wrong. We do know a lot about the plant, but it continues to surprise scientists with what it can do and how quickly it can do it.

The obvious key to controlling any spreading plant is to control its reproduction. That might be easier if hydrilla reproduced in some predictable way. In fact, hydrilla has at least 4 mechanisms for reproduction and several variations on the theme. Hydrilla, like many plants does flower. There are actually two main types of hydrilla in the U.S., monoecious or dioecious, which refers to whether or not both the male and female buds occur on the same plant. OK, there are several plants that self-pollinate, so what's the problem? Well, hydrilla also produces "Turions" which are very compact dormant buds that fall from the plant when they mature. These little fellows can lay dormant for years, then form new plants. They can survive several days out of the water, and while dormant resist herbicides. They also have the property that they can be swallowed and passed unscathed through a duck's digestive tract. Therefore anywhere a duck can go, you can potentially have a new outbreak of hydrilla. Add to that one the fact that a single sprig of hydrilla can break off from the plant, float around, root itself on the bottom, and clone a new plant. And if that wasn't enough, hydrilla also produces "Tubers" which are kind of like tiny potatoes on the plant roots which can be found a foot deep in sediment. It doesn't produce just a few tubers, but around 6,000 per square yard. These tubers are the ones that actually cause the most trouble because we don't know of a single viable way of controlling them. You can kill the plant with herbicide and next season the tubers will replace it 10 times over. How are you going to stop something like that?

There are actually very few ways of controlling this monster at all. Several insects have been tested and even introduced in limited trials, none of which has made a significant impact. You can go to the much maligned grass carp, but grass carp aren't too picky about the menu and if you introduce enough to control the hydrilla, you'll likely just end up denuding the entire lake of all plant matter. Endothall, a contact herbicide used for a wide variety of plant control can cause a massive kill of hydrilla, much to the distress of the lake, but still doesn't get the tubers after the damage is done. Mechanical harvesting has been tried but even with massive investment, can't keep up with hydrilla spreading in a lake.

The herbicide manufacturers aren't really too interested in developing new control products because the money simply isn't in it. It takes years of research and a huge investment to produce a new, safe and effective herbicide. If you're in the business and had a choice between a market of a few million dollars in state aquatic weed control programs, or a few billion dollars for treating next year's corn crop, where would you put your investment? Yeah, them too.

That leaves us with the one currently available herbicide specifically developed for hydrilla control, Fluridone or better known by the brand name Sonar. This is actually some pretty good stuff. It is a genetically engineered product specifically targeting the growth genes of hydrilla. It doesn't really kill the plant as much as it keeps it from growing and reproducing. If the plant can't grow, it naturally dies off in 6 to 8 weeks. This rather gentle demise has several benefits. You don't get a massive injection of dead plants all rotting at once, but rather spread the decay over a more natural decay cycle which doesn't disturb the fish or surrounding native plants. The transition between invested and controlled allows the eco-system to adjust over time preserving water quality. It has been working at low concentrations which further reduce negative side effects. It's easy to disperse in a water body and it only targets hydrilla.

For all of the good things about Sonar, it has its limitations. First, you have to keep the treatment in the water for 45 to 60 days to be effective. That means you can't use it in rapidly flowing water and that a big rain storm can flush you out overnight. Keeping the treatment in place limits when you can treat to the times you can maintain your water flow. In the South, that usually means you treat in the winter to avoid the spring rains or topical storms which is when hydrilla is dying anyway. That two edged sword implies you can save some money by teaming up chemical and natural die offs, but you can't do it in late summer when control is the most needed. And, of course, you still haven't gotten to the tubers.

There is one other control problem here, hydrilla is a very adaptive plant. In less than a decade, hydrilla has become much more resistant to Sonar. When you think about it, it had to be expected. The first application or two was wildly successful, often to the horror of local fishermen. Massive beds of hydrilla could be virtually wiped out with as little as 3 parts per billion treatment. The problem came with "virtually". The few surviving plants obviously had slightly more resistance to Sonar and therefore went on to produce more highly resistant generations. Each treatment created an even more resistant strain until today you need 16 parts per billion, or over 5 times as great of concentration. So the fishermen are marginally happier since you can't really wipe out all of the hydrilla, but the taxpayer is now paying 5 times as much for the same or less benefits. We can't forget about all of those older, less resistant tubers still buried down there, so we do still get reasonable control but less so every year.

That was another one of my surprises from Jeff. All of the concern about wiping out hydrilla with high concentrations of Sonar during the draw down prompted an interesting question. What did I think about it last year? It turns out that West Lake was treated with 16 ppb Sonar last March and April, knock the yearly crop of hydrilla coverage down from 14,000 acres to about 6,000 acres after the treatment. To be perfectly honest, I was slaying the fish on West Lake during that particular time and didn't notice they had treated at all. So much for the power of fisherman's observations.

So what can we expect in the future? Frankly, it looks kind of grim. There is still no natural control to turn to and any additional herbicides are still years from production. If you have hydrilla now, you're likely to keep it. If you don't have significant hydrilla, it seems to be heading your way. The "northern" strain has been found in Europe and Russia as far north as 50 degrees, or roughly the U.S. / Canadian border. It's moved north of 40 degrees already here in the states. All pattern indications say that as it adapts, reproduces, and grows, the spread will continue. Hopefully, the continuing control measure research will pay off and effective controls will be found in the coming years. For right now, it's a war of attrition that we are doomed to eventually loose.

Why should we care, after all we are fishermen not water skiers. Loosing the hydrilla wars will eventually loose your lakes. Consider a few facts that have been verified by some of the extensive infestations in Florida.

When the mats top out on the surface, you'll loose your wave action that oxygenates the lake. Less oxygen of course leads to less bio-mass, or less fish. The waves simply can't kick up a surface mat until the wind gets real bad. How bad? Well there was a documented case on the St. Johns River in 1992 where the wind did howl enough to roll it up. Hydrilla rolled up, floated downstream until it hit the bridges. They then had to shut down the two major East/West highways through Central Florida until the balls could be removed and the bridges saved from being dislodged by hydrilla mats.

Oxygen that does get into the lake becomes more limited in distribution. Many cases the larger mats have significantly less oxygen and different ph in the middle of the mats than along the edges. While it appears the fish are thriving because we are catching them along the edges, actually they are fleeing the increasingly toxic environment in the middle of the mats as dead hydrilla decays and robs the water of what oxygen it had. You can catch them today, just don't expect fresh fish to swim out from under the mats tomorrow.

Hydrilla in shallow water had the obvious effect of reducing the spawning areas. That's largely what the draw down on West Lake is about. It's not so much the active plants prevent the fish from bedding, but that a few years of growth and decay of the hydrilla silts in the shallows and turns the bottom into soft muck. The rotting hydrilla commonly gets so bad in Florida waters that the methane gas lifts the soft muck off the bottom creating mud tussles, or floating islands of rotting muck. If it sounds like a cesspool, it is.

So if you can't keep the plant from spreading, then why kill it in mass with herbicides and just add to pile? Studies show that controlling hydrilla with Sonar substantially slows the build up and provides a better bottom. Left untreated, hydrilla can silt in a lake at twice the rate of a treated lake and have poorer water quality throughout the year. If you think killing off the hydrilla causes problems, not killing causes more problems quicker. Even if you don't like the Hydrilla Wars, there is little choice but to fight them.

You want a final reason to control the mats? Consider the case of Rodman Reservoir, one of the premier fishing destinations in Florida. Hydrilla was allowed to spread out across the shallows because of a lack of funding for control. The fish seemed to thrive as the hydrilla grew, and both numbers and size of the catches increased. Then we had a tropical wave come over, as it does from time to time. We had 4 days of cloud cover, something relatively rare in Florida. The hydrilla went from producing oxygen to absorbing it, and after 4 days virtually every fish in the lake suffocated. It only took a couple of treatments to get ride of the hydrilla after that, but it was several years before the fishing started to come back.

Critics are quick to point to the other extreme, the Harris Chain of lakes which seemed to killed by a massive weed controlling program which turned the lakes into a dessert. Turns out Jeff Shardt spent two years looking at that particular case, trying to find out what went wrong. The conclusions were rather surprising considering the massive publicity given the problem. The killing of the hydrilla may have been a trigger, but found not to be a cause. One must remember that the Harris Chain is directly connected via flowing canals to Lake Apopka, the poster child for how to kill a lake with pollution. Apopka had long been pumping pollutants into the Harris Chain, and those pollutants were the direct cause of the disaster. The hydrilla and other filtering plants had been holding off the event, but any weather change or just the natural seasonal die backs were certain to lead to the same thing. Water samples, trace analysis, all of the clues point to the culprit. But thankfully, so did the solution. As a direct effect of the lakes turning over, most of the hydrilla was killed, and poor visibility for several seasons even killed out the tubers preventing them from sprouting. The results today is one of the few large bodies of water in Florida which does not have an extensive hydrilla problem. Causal treatments have allowed natural plants to come back, and the fish have returned as well. While still recovering, the Harris chain is today one of the better fisheries in Florida with local tournaments regularly weighing in 20+ pound limits. And as a contrast, we look downstream past the lock to Lake Griffin, the last of the lakes connecting Harris to the river flowing into Rodman. It was largely spared the immediate effects of the rest of the chain do to homeowner concerns. Those same homeowners prevented other management efforts such as draw downs or mechanical dredging. The result? Griffin is now as dead as the Late Great Lake Apopka. The fish is filled with gizzard shad, turtles, and deformed alligators. While the rest of the chain has risen like a phoenix, Griffin has been lost and nothing short of draining the lake and starting over is likely to bring it back. So much for leaving well enough alone.

If you're notified about a treatment program, think about Lake Griffin, West Lake Toho, Rodman, and the rest of the Florida lakes. The people working to keep your waters free of hydrilla aren't trying to kill off your bass population, but are trying to save it. Support your local efforts and educate the public in your area. It's your fishing future. Meanwhile, come on down to Florida and share some of our great fishing created by our lake managers. If you bring your boat, remember to watch the water pressure gauge and shut it down when it gets below 10 psi before you blow up your motor. All you have to do is stop, and clear the hydrilla clogging your intakes.

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