Too Much Of A Good Thing Is Bad For Hydrilla

Published 5:00 pm Saturday, May 30, 2020

Sometimes Mother Nature gives. Sometimes she takes away.

Take, for example, hydrilla. A native of India that first found its way to the U.S. as vegetation for aquariums in the 1950s, hydrilla was abundant in East Texas lakes 20 years ago.

It was a recreational nightmare around boat docks and swim areas where it surfaces and forms thick mats. On the other hand, edges and holes were bass magnets making the plant revered by fishermen. As hydrilla expanded through East Texas, it created some of the best fishing habitat on aging lakes where native vegetation had disappeared.

A tug-of-war developed between fishermen, recreational boaters and property owners over whether the non-native grass should come or go. Local water authorities, and sometimes property owners, had the final say resulting in a variety of approaches using chemical treatments and the release of grass carp. In some cases, it was a 100 percent effort to eliminate the hydrilla. At the urging of Texas Parks and Wildlife Department, plans were made to treat around boathouses, boat lanes and more open water, leaving the fish cover where possible.

In recent years, however, something strange happened. Hydrilla began to disappear completely, but not as the result of treatments going awry. It has simply been Mother Nature taking it away.

“There are several reasons for the decrease in hydrilla in many of Texas’ lakes. Increased water level is having an impact. Since 2016, heavy spring rains have led to increased water levels throughout much of East Texas. Lakes Sam Rayburn and Toledo Bend are prime examples of where high spring water levels have led to a decrease in hydrilla since 2016,” explained John Findeisen, Texas Parks and Wildlife Department Aquatic Habitat Enhancement team leader.

The decline is not consistent statewide. Findeisen said a number of lakes like Caddo, Nacogdoches, Fairfield and others still have ample amounts of hydrilla.

Caddo Lake is an interesting example for those wanting to solely blame chemical treatments for the lack of hydrilla. Because of giant salvinia, there is no lake in Texas that has been chemically sprayed more, yet 15 to 35 percent of the Texas side of the reservoir has had hydrilla in recent years.

“Nearly 9,000 acres of giant salvinia are treated at Caddo Lake annually and hydrilla coverage has increased with the decrease of giant salvinia,” Findeisen said.

He noted that when treating other plants like giant salvinia, the herbicide is applied to the floating part of the plant. For hydrilla, the chemicals are injected into the water column in greater volume.

“Hydrilla can be an important habitat component in many Texas reservoirs. Unquestionably, it provides desirable and beneficial fish habitat if total lake coverage is less than 40 percent or so. It’s been our experience that you really need vegetative coverage of at least 10 percent to significantly affect recruitment dynamics at the fish population level. In most of our reservoirs, native plants simply don’t provide this level of coverage due to inherent reservoir conditions, primarily water level fluctuations,” said Tim Bister, TPWD Inland Fisheries district biologist from Marshall.

Bister said the vegetation can increase recruitment of a year-class by providing cover for young bass as well as creating habitat for adult fish.

“This is why many bass anglers are passionate about hydrilla management and the vast majority of them like to fish lakes with hydrilla,” he said.

However, the plant is still non-native and remains prohibited. There is also concern about over-expansion. Bister noted more than 40 percent coverage can be biologically negative. Also, at more urban reservoirs, like Lake Tyler, it is a social issue for a portion of the lake users.

“Ultimately, hydrilla’s classification and management status is lake-by-lake based on biological and social factors. For example, 20 percent coverage of hydrilla is beneficial fish habitat at both Sam Rayburn and Lake Conroe. At Sam Rayburn, it is also socially desirable and accepted. However, at Lake Conroe a high proportion of the lake constituents simply aren’t going to tolerate any plant coverage. Contrast this to 50 percent or greater coverage, which becomes problematic biologically and socially at any lake,” Bister said.

He added that at more than 50 percent coverage rates, studies have shown that while bass may be plentiful, growth rates are down because it is hard for them to find forage.

“We have seen it appear and reappear in several lakes for a year or two, only to slowly thin out and disappear again,” said TPWD district biologist Jake Norman of Tyler. “Fork has been the most famous for this in recent years. Hydrilla has yet to return to the pre-drought abundance, but remains in a few creeks on the lake. We have seen it pulse up and down since 2014, and grow out to 15 feet for a summer, only to completely disappear the next.”

Norman believes the lack of nutrients in reservoirs is another issue. He said hydrilla had previously been found on lakes Tyler, Jacksonville, Tyler State Park, Purtis Creek State Park, Athens and Hawkins in his district, but Fairfield is the only place it can still be found in abundance.

TPWD biologist Todd Driscoll of Jasper said hydrilla exists in his district on Rayburn, Kurth, Naconiche, Nacogdoches and Pinkston at an acceptable rate of 15 to 20 percent.

“Hydrilla is 100 percent beneficial from a largemouth bass population dynamics perspective in these lakes, and we promote the positives of hydrilla at these lakes with anglers and controlling authorities,” Driscoll said.

Those lakes are in contrast to what has happened at Toledo Bend.

“Similarly, hydrilla has been 100 percent beneficial at Toledo Bend. However, since 2015, hydrilla coverage has been way down. No question high winter and spring inflows in 2015 knocked hydrilla down to only trace amounts, and similar spring inflows since then have discouraged any significant regrowth. We would sure like to see hydrilla coverage return again. Anecdotally, it does appear the bass fishing success has decreased due to lack of hydrilla, and our electrofishing surveys have reflected a population decline during this period,” Driscoll said.

TPWD’s Bister said Brandy Branch, Gilmer and Welsh have also maintained about 20 to 35 percent coverage in recent years without negative issues, while hydrilla has disappeared on lakes Bob Sandlin and Murvaul. He believes low water levels during drought periods in the last 20 years had an impact. In the case of Sandlin, native plants have replaced some of the hydrilla and bass catch rates during electrofishing surveys are back to what they were when hydrilla existed.

“We did not see much of a negative impact to the bass population or to bass fishing at Murvaul because hydrilla was not located throughout the reservoir and there was an abundance of emergent aquatic plants to provide fish habitat. However, submersed aquatic plants are definitely an important component of quality fish habitat. During 2019, we started a native aquatic plant establishment project at Murvaul to improve fisheries habitat throughout the reservoir,” Bister said.