TAMU NEWS The Fallacy of Aquatic Vegetation

Article by TODD SINK, Associate Professor and Aquaculture Extension Specialist, Texas A&M AgriLife Extension Service and BRITTANY CHESSER, Aquatic Vegetation Management Program Specialist, Texas A&M AgriLife Extension Service

Photos courtesy of TEXAS A&M AGRILIFE EXTENSION SERVICE

Time and time again, landowners construct a new pond and as soon as it begins to fill, they contact our offices and ask what aquatic vegetation they should plant to support their fish.

We provide them with a simple answer—none.

Don’t worry, aquatic vegetation will come on its own eventually, whether you want it or not. It may be one year or five years, but it will eventually be introduced and established. We suggest you enjoy those first few years of a weed-free pond while you can.

Approximately three years following pond construction, more than 90% of these inquiries are landowners seeking control methods because aquatic vegetation has overgrown and become a nuisance. Most often this leaves pond owners with the task of long-term management that can often be quite expensive.

Think of our popular warm-water fish species commonly stocked into farm ponds, such as largemouth bass, bluegill, redear sunfish, channel or blue catfish, hybrid striped bass, or fathead minnows. Do they really need aquatic vegetation for any reason? Do any of these fish eat aquatic vegetation?

No. If they did, we would not need to import non-native species such as grass carp from Asia or tilapia from Africa to manage aquatic vegetation. Rooted or vascular aquatic vegetation only dies back annually during the fall and has thick cell walls that break down very slowly through bacterial decomposition.

For these reasons, it is not a great source of detritus to feed detritivores in your pond, compared to phytoplankton, the microscopic, single-celled algae that gives pond water a light green tint. There may be up to 10,000 + phytoplankton cells per milliliter of water, living an average of 14 days, producing a constant rain of detritus, unlike rooted or vascular vegetation, being produced for food.

Some inaccurately believe rooted or vascular vegetation is needed for oxygen production. This is not true and most rooted or vascular vegetation species are net oxygen consumers—consum- ing more oxygen underwater than they produce. They do produce oxygen during daylight hours, but they are still living tissue and must respire or consume oxygen to live during periods of darkness. There can be 8-14 tons of rooted vegetation per acre, and all that living tissue consumes large quantities of oxygen at night.

For this reason, low dissolved-oxygen fish kills caused by excessive aquatic vegetation is the number one cause of fish kills in farm ponds in Texas, occurring at night or just before dawn. Conversely, phytoplankton produces up to 80% of a pond’s usable excess oxygen with the remainder coming from the atmosphere through diffusion and agitation from wind and wave action.

Some may argue that fish need rooted vegetation for habitat or cover, which is not correct. Yes, there are thousands of oxbow, playa, resacas, and coastal marsh “lakes” in Texas. Playa lakes go dry most years and don’t have fish. Oxbows are frequently muddy like the rivers they were formed from and tend to have little vegetation. And resacas, which are not full oxbows, are still connected to the rivers from which they formed and are again often muddy with little vegetation. Coastal marsh lakes are not freshwater systems where we find fish species common in ponds.

In short, most of our “natural lakes” in Texas are muddy, turbid, and have little aquatic vegetation, yet the fish species we stock in our ponds have been here for millennia. Where did these fish species evolve? Primarily in our rivers and streams that tend to be turbid and muddy with little aquatic vegetation.

So, if pond fish species evolved in systems in Texas without significant aquatic vegetation, what do they use as habitat? Small fish use shallow water as habitat to avoid predation by larger fish. How do fish avoid aerial or overhead predators? They move deeper. Depth changes and bank contours serve as the primary habitat used by our common pond fish species.

There are urban centers in Texas with thriving fish populations including bluegill, bass, and catfish that are nothing more than concrete bowls formed by storm water ditches, irrigation projects, and flood relief infrastructure. These habitats have no rooted vegetation at all.

Too much aquatic vegetation can lead to a phenomenon called prey avoidance, in which large predatory fish, such as largemouth bass, cannot obtain sufficient food because their prey are hidden in thick aquatic vegetation where they cannot successfully maneuver and hunt. This can lead to poor growth, reduced fishing quality, and eventually stunting as predatory fish cannot satisfy energy growth requirements.

While there is a time and place for aquatic vegetation, for instance shoreline plants for nutrient uptake, erosion control and other specific uses, rooted aquatic vegetation is NOT needed to support a successful fishery. If in doubt or worried about fish habitat, there are other alternatives such as adding wood (brush piles, trees), rock (rock piles, boulders, gravel spawning flats), or synthetic structures to your pond that are better habitat and easier to manage than aquatic vegetation.