Weed Removal, Bass population Removal?
March 04, 2014
In 2012, Greg Matzke, a fishery biologist for the Wisconsin Department of Natural Resources (DNR), made a startling discovery on Florence County's Lake Ellwood. During a comprehensive fish survey which included spring, summer and fall netting and electrofishing, he discovered that all the lake's largemouth bass were older than age-5. Approximately 91 percent of them were at least seven years old. The absence of younger fish indicated steady recruitment failures. Such losses of bass recruitment over multiple years are unprecedented in the natural lakes of Wisconsin.
"The current largemouth bass population is in serious trouble," Matzke reported. "With no natural reproduction since 2007, bass abundance will drop rapidly, with potential for the complete loss of this species unless the current situation changes."
Matzke next began looking at the lake's panfish populations. What he found was stunning. Panfish abundance had fallen about 75 percent over the last 10 years, with bluegill and rock bass abundance down an estimated 65 percent and 89 percent respectively. Intense sampling throughout 2012 found only a single black crappie under six years of age, showing another alarming recruitment failure. Analysis of Lake Ellwood's northern pike population was even more disappointing: There were no pike under age-8!
The Wisconsin DNR had never encountered such dramatic recruitment failures of so many species simultaneously. He shared his data with other biologists and all were nonplussed. Matzke and his team scrambled to collect more data and tried to find a cause for this collapse. Surveys in 2002 had shown normal abundance, size structure, growth, and recruitment in all of these species. What had happened in the last 10 years to prevent reproduction?
The only thriving species of gamefish in the lake were smallmouth bass. Their abundance and size structure had increased in the last decade and recruitment was high. Because Lake Ellwood's smallmouths were doing so well while the other species were collapsing, the focus turned to the lake's historically sparse but important aquatic plant community. The fish species showing recruitment failure are dependent on aquatic vegetation for spawning as well as cover and food for their young. Matzke noted that smallmouth bass were not nearly so dependent on vegetation.
The Smoking Gun
Eurasian milfoil was discovered in Lake Ellwood in 2002. Herbicide treatments began in 2003 and increased each year. By 2007, recruitment of northern pike, largemouth bass, and black crappie had ceased. "When I analyzed the data it was obvious that recruitment problems seemed linked to herbicide treatments," Matzke said. When he graphed fish abundance by year-class over the last decade and overlaid a chart of herbicide treatments, he found what he believed was a critical connection. Fish numbers fell as the amount of herbicide increased. In the year following a relatively low application of herbicide (2010), young bluegill and black crappie to a lesser extent reappeared, but their continued health seemed in jeopardy.
"We still wonder what stage of reproduction failed in these species," Matzke said. "Aquatic vegetation plays a major role in spawning site selection and in the survival of eggs and fry. Plants are also the source of primary production providing food and habitat for young fish and prey items, including invertebrates and minnows. It seems likely that one or all of these important phases of reproduction have dwindled in Lake Ellwood."
In April 2013, Matzke met with the Lake Ellwood Association to reveal his data and conclusions. He told the group that the main cause for failed northern pike, largemouth bass, and black crappie recruitment appears to be the loss of aquatic vegetation. "The 2,4-D herbicide used on Eurasian milfoil had been successful in reducing the abundance of this invasive species significantly and other native plants were also harmed by the treatments," he told them. He added that he had no reason to believe the chemicals directly caused reproductive failures, but that their use indirectly caused recruitment failure by eliminating too much aquatic vegetation young fish need to survive. Matzke called for a change in the way the Lake Ellwood Association has been managing the lake's aquatic plants and recommended that chemical treatments for milfoil be stopped.
"We need to promote and strengthen aquatic vegetation in Lake Ellwood," he told the group. "The loss of plants, including the invasive milfoil, has almost certainly wiped out a great deal of forage for young fish."
Lake Ellwood contains a dwindling and rapidly aging population of largemouth bass, black crappie, northern pike, and bluegill. The DNR began a five-year study in 2013. The team, headed by Dr. Greg Sass of Wisconsin's Fisheries and Aquatic Sciences Research Station, is conducting intensive plant and fish monitoring, along with fish diet studies and plankton evaluations.
Last fall's sampling showed promise. Since chemical treatments have ceased, fish and plant communities have responded. The researchers found strong year-classes of bluegills and largemouth bass, along with more native aquatic vegetation. Matzke hopes that trend continues.
Could chemical herbicide treatments for milfoil and other plants such as curlyleaf pondweed be reducing recruitment in other lakes? No other lakes that have been chemically treated have been surveyed so thoroughly. Recruitment problems due to loss of plant cover could be taking place throughout the region where the invasive plant is being battled. There is no way to know if this is happening, and until now there's been no reason to find out. Biologists in other regions are only now learning of Matzke's findings.
"We want to examine the limnological details behind the crash, so that we can prevent similar situations," he says. Lake Ellwood continues to contain some Eurasian milfoil and likely always will. What's become clear is that its treatment likely caused problems far worse than it presence.
*Eric Engbretson, Florence, Wisconsin, is a professional underwater photographer and his images have appeared in many In-Fisherman publications. This is his first written contribution.