Every angler knows that hydrilla holds bass, but what are the ecological effects of these invasive aquatic macrophytes on our favorite green fish.  First of all, what on earth does this plant due to deserve this attention?

Hydrilla has become a world wide invader of many water bodies.  It has the attention of many ecological communities because of its aggressive nature and hardiness.  Native species are out competed by this aquatic macrophyte and it causes chemical changes in the water including wild fluctuations in pH and dissolved oxygen.  Dissolved oxygen (DO) is an important water quality parameter for fish.  High density vegetation will cause the surrounding waters to be high in DO midday, when photosynthesis is at its peak.  The opposite happens at night when respiration still occurs. This emits carbon dioxide into the water, which also increases pH at this time. Hydrilla’s ability to grow at astounding rates has caused many state and regional authorities to come up with game plans to eliminate or at least control the plant.  We’ve all seen those grass carp munching down on hydrilla.  Chances are, your local authority put them there for a reason.  Take a trip to Lake Conroe and you’ll know what I’m talking about.  20,000 acres of hydrilla now down to 1 acre, you could say the grass carp did their job.

So what does this plant invasion mean for our bass?  There are aspects of hydrilla from a fisheries stand point that are a big deal.  Aquatic macrophytes are an important part of the juvenile stages in many fish species.  It allows cover for them to forage with less predation activity.  This is especially important in Largemouth Bass because of their cannibalistic nature.  Juvenile bass are less likely to be eaten in the protection of dense macrophytes by other larger bass.  Aquatic macrophytes are also important during the spawn.  The faster warming waters that hydrilla induces also offer excellent spawning grounds for bass.  Because Largemouth bass are ambush predators, hydrilla is a great ambush area for them.  Smaller Centrarchids species like Bluegill and Green Sunfish are also drawn toward cover like hydrilla.  This is a primary food source for adult bass living around the hydrilla.  There is speculation as to the effects of hydrilla mats on largemouth bass however.  On one hand, they provide a great spawning ground that juveniles can survive in, but the dense nature of hydrilla may cause bass to never reach their potential in size.  This is due to the restricting factor that bass stay around hydrilla after they are spawned and won’t venture out to other larger food sources.  Also, the comforts of the protection from larger predators allow bass to remain smaller by staying in the hydrilla.  This growth reduction can delay the amount of time it takes to sexually mature, therefore reducing recruitment to a population.

So is hydrilla bad for a bass fishery?  In my opinion, certainly not.  The facts about hydrilla heard in a classroom or read in a text book do not out weight on the water experience.  Anyone that has picked up the big stick and punched grass, or ripped a rattle trap though hydrilla in the early morning knows it can produce some mondo bass.  My theory on the hydrilla problem, if you want to call it that, is that it can decrease largemouth bass growth, but only in certain areas.  Backs of pockets and creeks where bass go to get there multiplication on are the only places being diminished.  These are areas that are normally shallow and allow hydrilla to have constricting affects on water quality and bass.  Main lake hydrilla offers a haven for big bass with excellent cover and forage opportunities.  This is due do deeper water, less canopy construction, and more wind mixing which increases DO.  I think there is a solution to the stunted bass problem and it includes another plant to couple hydrilla.  Vallisenaria, or eel grass, has been shown to actually out compete hydrilla.  It is restricted to shallow water which in this case works perfectly.  This will leave the main lake hydrilla beds for larger bass to use as forage areas.  It will also allow good spawning grounds for adult bass, with moderate cover for juvenile bass.  It could eliminate the stunting problem by being sparse enough that small bass will be forced, as traditionally, into the more open waters to search for larger prey items.  Could work, or maybe not, maybe we’ll see in the future as the hydrilla controversy continues.

Justin Rackley
Texas A&M University
Aquatic Ecology and Conservation