Before and After Weed-spraying

Nothing causes heartache among anglers faster than aquatic plant control. Anglers bemoan the impending loss of their fishery. Environmentalists come out waving dog-eared copies of Rachel Carson’s Silent Spring and preaching against “poisoning aquatic habitats.”

Meanwhile, lakeshore property owners and water-skiers rally to defend their property values and recreation against evil plants taking over their lake. Fishery biologists are caught smack in the middle, searching for a scientific answer to the dilemma.

Many people distrust aquatic herbicides. But these same folks may give little thought to spraying their roses for bugs, or to public mosquito-control programs, which also use poison to achieve their goals. Many states also restrict use of herbicides in aquatic ecosystems, fearing contamination of drinking water. Yet biologists today have a better understanding of these herbicides and have found that control can be achieved at low doses, sometimes just a few parts per billion (ppb).

Bass anglers often oppose herbicide treatments, believing herbicides cause fish to leave the area and stop feeding. This attitude is supported by popular articles and statements by professional anglers. But do herbicide treatments actually affect bass? Those observing herbicide treatments could easily come to such a conclusion, since the process is hardly subtle. Noisy airboats make multiple passes over plants, spraying chemicals (usually with a blue or green color), which could cause any bass to evacuate.

Lake Seminole Studies

Until recently, little research has examined the effects of herbicide treatments on bass behavior and movement. But studies done by Auburn University at Lake Seminole, Georgia, have been revealing. From 2000-2003 the U.S. Army Corps of Engineers used the herbicide fluridone to eliminate almost 4,500 acres of dense hydrilla in the Spring Creek arm. Control was achieved at a low dose of 6 ppb. Most native plants tolerate at least 10 ppb, so they prospered as hydrilla dwindled. The aquatic plant community changed from a monoculture of hydrilla to one composed of more than a dozen native species. To examine effects on largemouth bass behavior in Spring Creek, we implanted 35 bass over three pounds with radio tags and tracked them for two years.

These bass didn’t leave the treatment area and showed little change in behavior. After hydrilla was eliminated they moved more, inhabited deeper water, and switched habitat from hydrilla flats to deeper standing timber, abundant in Spring Creek. During daylight hours bass moved more than twice as much as before herbicide treatment. Since water clarity declined substantially after treatment, bass likely felt more comfortable moving during the day, whereas they’d moved primarily during low-light periods previously.

Results from two Bassmaster tournaments back up these findings. While some anglers struggled to catch bass (and blamed their lack of success on the herbicide treatment), others adapted to the new conditions and sacked hefty limits. In the 2002 and 2003 tournaments, several pros placed in the top five by fishing drop-shot rigs in the standing timber of Spring Creek, a technique that would have been nearly impossible with dense hydrilla present. Although both tournaments were won by anglers fishing other areas of the lake, the second-place finishers in both years (Brett Hite in 2002 and Kota Kiriyama in 2003) caught their fish from standing timber in Spring Creek, as the tracking data had predicted. The bass were still there and still catchable, but adjustments were required to maximize fishing success.

Local anglers took note of the pros’ change in tactics and began fishing standing timber, especially in early spring. And despite initial resistance to fluridone applications in Spring Creek, area anglers came to support the program after realizing they could still catch plenty of bass. Local icon Jack Wingate, former owner of Wingate’s Lunker Lodge, said, “Spring Creek was always our favorite place to fish and it remains our favorite.” The spring season following herbicide treatments, sunfish anglers experienced exceptional angling for huge redear that moved up along cattail and cutgrass banks instead of remaining scattered in offshore hydrilla beds, their usual pattern prior to the vegetation treatment. Boat ramps were suddenly packed as anglers flooded into the area seeking giant shellcrackers.

Cove Studies

But perhaps it wasn’t surprising that bass didn’t evacuate Spring Creek to escape the herbicide; it would have been a long swim. Herbicides often are applied on far smaller areas, to create openings in dense stands of vegetation rather than eliminate all vegetation. Another study on Lake Seminole examined the effects of herbicide application on bass behavior in two small coves off the Chattahoochee River arm. The herbicide endothall was applied in transects to simulate typical application procedures. This time, 15 bass over a pound were implanted with radio tags and tracked for six months in each cove.

Results were strikingly similar to the fluridone study in Spring Creek. No bass left either cove over a four-month period after application. Bass movement again increased after elimination of hydrilla. But unlike the Spring Creek study, they moved into shallower waters.

Favored habitat changed from mostly hydrilla to a mix of emergent and floating plants. These differences can be explained by habitat available after hydrilla treatment. In Spring Creek, plenty of standing timber was available in slightly deeper water than the adjacent hydrilla flats bass had been occupying, so they moved deeper into it. Neither Chattahoochee cove had much offshore cover, however, so the fish moved shallower where emergent and floating plants offered the best cover.

For the most part, bass remained in areas where they’d lived before herbicide was applied. We also tracked bass during the application itself. Although fish in both coves occupied water less than 6 feet deep, they didn’t move from the airboats going back and forth overhead.

These studies showed that bass are adaptable creatures, able to withstand major disturbances without missing a beat. As their habitat changed, they appeared to make slight adjustments to adapt to the new environment. The general location of bass did not change, but these subtle shifts in location would require changes in fishing approaches.