Florida study shows how Rodman Reservoir drawdowns stoke its trophy bass fishery

In Florida, Rodman Reservoir is well known for its trophy bass and periodic drawdowns. A new study gives biologists a better handle on how the occasional changes in water level help stoke the lake’s fishery.

Managers have drawn down Rodman Reservoir regularly since it was impounded on the Ocklawaha River in the late 1960s to control zealous growth of aquatic plants like hydrilla and water lettuce.

“That’s the primary tool they’re using to control the vegetation out there,” said Eric Nagid, a biological scientist with the Florida Fish and Wildlife Conservation Commission. “It’s been really successful.”

It’s also had the benefit of producing some strong year classes of largemouth bass, which has made the lake popular for anglers looking for trophy fish. A 14-pound bass caught there in January was good enough for the top-tier “Hall of Fame” designation under the state’s TrophyCatch program, the second such fish from the lake  in two years.

Drawdowns and fluctuating water levels have long been associated with stronger fisheries, but the reason why isn’t always clear. Nagid led a study that showed drawdowns on Rodman Reservoir in the early 2000s were linked to strong years for bass reproduction, but another look later in the decade didn’t show the same pattern.

In a new study recently published in the North American Journal of Fisheries Management, Nagid took one more look to figure out what was going on. He and his co-authors assessed the Rodman Reservoir bass population from 1992 to 2012, this time weighing the strength of each year class with trends in hydrilla coverage.

The changes in hydrilla coverage, rather than the drawdown itself, turned out to be the key. When drawdowns were followed by expanding hydrilla coverage, a strong bass year class generally came next. Drawdown years that were followed by a loss or no change in hydrilla were weaker for bass production.

This 14-pound, 1-ounce largemouth from Rodman Reservoir earned "Hall of Fame" status on Florida's TrophyCatch program (Courtesy Eric Nagid)

This 14-pound, 1-ounce largemouth from Rodman Reservoir earned “Hall of Fame” status on Florida’s TrophyCatch program (Courtesy Eric Nagid)

People have generally assumed that drawdowns bring good bass years by flooding dewatered habitat around the time of the spawn, creating a sudden abundance of habitat for the recently hatched juvenile fish. Studies have also backed the idea of the “trophic upsurge,” in which the drawdown stimulates the exposed sediments to, once flooded, release nutrients that fuel the plants and insects at the bottom of the bass food chain. Between the freshly habitat and wealth of food sources, more bass appear to survive to adulthood and then continue to grow quickly — sometimes reaching 8 pounds by year four or five, Nagid said.

But this study shows it can be more complicated than that.

“There are more strings attached than just the ups and downs of water,” Nagid said. “There are so many other factors, biotic and abiotic that can influence what is going on.”

There are always going to be strong or weak year classes that are dependent on forces that “we just don’t have a handle on,” Nagid said. Luckily for anglers on Rodman Reservoir, he could dive into the data and find that one of the factors was hydrilla expansion. That could give the reservoir and fishery managers there a tool to send a strong pulse of young bass into the population every few years by planning drawdowns tailored to encourage hydrilla growth. They could manage water releases to keep water clear during the hydrilla growing season to let plenty of sunlight through, while patches that aren’t affected by drawdowns could be culled with pesticides.

“Even though these management strategies are specific to Rodman Reservoir, conceptually it still should anywhere that you have a reservoir and dense hydrilla that you can knock back,” he said.

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