Bob Maindelle Guide Lines

This photo taken from the FM 3481 bridge over Stillhouse Hollow Lake shows how aggressively the invasive plant species known as hydrilla has spread in 2021. Once confined to the lower two-thirds of the lake, hydrilla now completely blankets all of the lake's shallows and multiple coves, thus impeding or preventing access by shore or boat in many locations.

In recent months I have heard a number of my clients and others with more than a passing interest in our local reservoirs comment about the apparent growth of vegetation in Stillhouse Hollow Lake.

Some have speculated that the growth is algae; others thought what they were witnessing was the lake’s bottom being revealed due to low water levels. Neither of these are true.

The rapidly spreading form of vegetation is an invasive species of plant called hydrilla.

Although there has been an ebb and flow in the amount of hydrilla found in Stillhouse Hollow since it first got established in the 1990s, this year saw its spread and coverage hit an all-time high.

In years past, only the lower two-thirds of the reservoir contained the plant, but this year the hydrilla spread into the upper third of the reservoir, making it into the mouth of the Lampasas River and into the cove formed where Trimmier Creek enters the reservoir.

There are a few pros and many cons associated with the growth of hydrilla.

In the short term, hydrilla provides cover to cover-loving, ambush-feeding species of fish like largemouth bass and sunfish. However, too much of a good thing is possible.

According to a 2016 article published by Cornell University’s Cornell Cooperative Extension, “Although hydrilla can provide habitat for fish, it unbalances the predator-prey relationships of some fish. Some predator fish (like sunfish and bass) attack their prey by ambushing them, and benefit from the additional cover provided by hydrilla. However, in the long term this can lead to an overall decline in the fish population, and eventually even the fish that prefer cover cannot hunt when hydrilla becomes too dense.”

A popular saying among largemouth bass fisherman advises, “Find the grass; you’ll find the bass.”

Unfortunately, so much hydrilla has now grown in Stillhouse that entire coves are now completely inaccessible to boating anglers because the matted vegetation entangles the propellors of both outboard engines and electric trolling motors, thus prohibiting access.

It has also become difficult for bank fishermen to access the lake’s waters because the hydrilla has taken over a band of water from the shoreline out to at least 14 feet in depth in most places, thus preventing casting from the shore without getting baits and lures tangled in the weeds.

Once bustling bank-fishing areas like Cedar Gap Park and the unimproved access under the south end of the FM 3481 bridge, now rarely see any use.

According to that same Cornell Cooperative Extension article, there are several reasons why hydrilla does so well versus other forms of native vegetation. First, it can tolerate lower light conditions than most aquatic plant species, which allows it to begin photosynthesizing earlier in the morning, giving it an advantage. 

Secondly, it spreads efficiently through both tubers and turions. And finally, there are no native predators that eat hydrilla, and so their growth is not checked.

The Cornell article states, “These adaptations allow hydrilla to out-compete other plants for space to grow. In fact, hydrilla is so successful it can double its biomass every two weeks during the summer, and can fill the entire water column up to 20 feet deep, therefore creating a monoculture -- a term used to describe areas dominated by a single species, as opposed to a regular ecosystem that contains many species. Monocultures can be harmful when they limit the ability of animals in the area to find food or habitat and by preventing the growth of native plants, effectively reducing biodiversity. 

That same article points out other negative impacts to fish, stating, “Hydrilla also harms fish because it depletes oxygen levels of the water. Hydrilla, like all plants, gives off CO2 and uses oxygen during the night time (although the opposite is true during the day), which can bring oxygen levels to dangerously low levels for fish. Additionally, an increase of hydrilla can cause an increase in released nutrients from sediments that cause algae blooms, again depleting oxygen levels.”

Birds, too, can be negatively impacted by hydrilla. The Cornell article states, “Bird populations are affected by declines in fish population, and can also be harmed by toxic blue-green algae that grows on hydrilla leaves.”

I have already observed that hydrilla now has severely choked many of the slow-tapering flats in the upper third of Stillhouse Hollow which serve as spawning grounds for the lake’s primary forage fish, the threadfin shad.

Although many approaches to slowing or reversing the spread of hydrilla exist, the three most common approaches include the use of chemical treatments (typically for “spot” treating smaller areas), and/or drawing down the reservoir’s level to below the hydrilla’s extent of growth, and/or the introduction of sterile grass carp, a nickname for the Asian white amur.

According to Texas Parks and Wildlife Department’s Triploid Grass Carp Information Sheet, triploid grass carp should be 10-12 inches long when stocked, as smaller carp are likely to be eaten by other fish. To enhance effectiveness of triploid grass carp, overabundant vegetation should first be reduced by winter die-off, herbicide treatment, or water-level drawdown to promote grazing on re-growth. The recommended stocking rate for triploid grass carp is five per acre if the water body has 50% or less plant coverage, and 10 per acre if plant coverage is greater than 50%.

To be clear, I am not advocating for or against hydrilla control at this point in time, rather, my intent for writing this article is to identify what is growing in Stillhouse Hollow so as to clear up any confusion over that, and to point out that, contrary to common belief, excessive hydrilla growth can negatively impact fish and fishing, even for cover-loving species like largemouth bass.

Those concerned about hydrilla growth on Stillhouse and/or those seeking more information on this topic may contact Texas Parks and Wildlife Department’s Waco regional office by phone at 254-666-5190.

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