In terms of general recreation, economic value, and public health, Hydrilla and other aquatic invasive species are bad news….. but what about for a fishery, especially that of one of the most sought after species in the United States; The Largemouth Bass? The opinions are as different as the plants themselves when it comes to bass fishing, especially in the South-eastern United States. How can ANYONE claim that hydrilla isn’t good for bass when people routinely drag 10+ pounders out of “grass” choked waters, year after year? It almost seems counter intuitive to make a claim that hydrilla and other invasive species can actually harm a bass fishery rather than be its saving grace. Well, the answer too many of these questions is all a matter of perspective. Given that in the following weeks, the water temperature will be rising to that “perfect” temperature and many of us will be out seeking spawning giants, this week we will talk about the good and bad side of having hydrilla in your favorite bass fishing waters.
Every angler out there knows that aquatic vegetation, especially hydrilla, can hold bass. Fish use aquatic plants for shelter and refuge from predators, as an indirect source of food (plant->Invertebrate->fish->Piscivorous fish), and as spawning habitat. Aquatic plants are an essential component to maintain healthy fish stocks in a natural waterbody. More importantly, aquatic plant diversity and density can make or break an important fishery like that of the Largemouth Bass. Many fisheries scientists agree that plant densities around 25-30% are the magic number for maintaining a healthy fishery with the “upper cap” of about 60% before a fishery is adversely affected. Furthermore, a waterbody with diverse assemblages of aquatic macrophytes will yield more diversity further up the food chain. In other words, the more species of aquatic plants that are present the more species of prey and predatory fish that can be supported in that system. Maintaining diversity and appropriate plant densities is where the problems really begin with hydrilla and other invasive plant species.
Aquatic invasive species like hydrilla do not bother themselves with remaining at “beneficial” density levels. Given that hydrilla has no native predators and characteristics which make it more competitive than native plant species, hydrilla can quickly consume an entire water body reaching the uppermost limits of density. Many of you might be asking, “Why then do we see such success in catching fish around hydrilla?”
There are a number of factors which make hydrilla waterbodies very productive, especially early on it its establishment. First off, in the southeast, the majority of our waterbodies are man-made reservoirs. Despite their “natural nomenclature”, Lake Gaston and Kerr Lake are both man-made reservoirs. Large native plant populations did not typically exist in the rivers that were present before the creation of such reservoirs, thus reservoirs often have large areas of shallow shoreline with little to no competition for space. Once hydrilla is introduced into a system like this, it can and will spread rapidly! Initially, the transition of the water body from no plants, to some plants, can be very beneficial to a fishery, even if those plants are invasive species. This new cover allows juvenile bass to forage without being eaten as often by other predatory fish, including other bass. Hydrilla also provides new opportunity for bass to ambush other prey species like bluegill. This is why we often see bass clustered around hydrilla and hydrilla mats. These fish orient themselves to the new structure just as they would have traditionally to brush, rocks and other structure prior to hydrilla introduction.
As hydrilla expands and provides habitat for a fishery, anglers will often see an increase in fishing success, leading many to believe that hydrilla is GREAT for the waters in which they fish. This is the dangerous assumption that many fisherman make, because as hydrilla expands in a waterbody, it grows well beyond the threshold of being “beneficial” and monopolizes the entire lake to itself out competing what little native vegetation may exist. Once that balance is tipped, the “benefits” of hydrilla quickly diminish. Hydrilla begins to fill every inch of the water column, potentially limiting the growth of fish to trophy sizes. The increased cover can lead to an increase in the number of individuals present to consume precious food resources, therefore many infested lakes yield high numbers of stunted bass. There is also speculation that the increased cover of hydrilla and decrease in overall size of the fish will limit them to staying within the protective cover, thus not allowing them to venture out and consume larger prey species which help these fish grow to their coveted sizes. Increased hydrilla biomass can also decrease dissolved oxygen, increase pH and temperature and decrease the availability of more pelagic prey species such as shad.
There a number of variables that can determine how hydrilla will expand in a system. The waterbody itself can determine how hyrdrilla will impact a fishery. Natural lakes with shallow maximum depths and optimal sediment types are often destroyed by hydrilla infestations as the plant is able to grow in virtually every inch of the lake. On the other hand, reservoirs may limit the overall occupation by hydrilla in the fact that they often have depths greater than that at which hydrilla can grow and sediment types that may not be as conducive to hydrilla growth. This heterogeneity in habitat type can be the only thing that saves a fishery in these waterbodies whereas, in native lakes, it can decimate the entire dynamic of the food web.
So is Hydrilla good or bad for a bass fishery? The answer in short is, well, it depends….Early on, the “new” cover provided by hydrilla provides fish with a number of beneficial aspects as well as providing fisherman with a target to catch these fish. This can be especially true in our reservoirs, however these benefits can be very short lived. In the long run it can yield stunted populations, monopolize habitats, alter important seasonal food webs and complicate an angler’s ability to fish. In most cases, the cost/benefit ratio of having hydrilla in a system just isn’t worth the risk. There are a number of beneficial native plant species that can provide the same quality habitat without causing the same long term effects. Tune in next week for more information on your lake environment.
For more information on fish and their interactions with aquatic vegetation, please see “Interactions between fish and aquatic macrophytes in inland waters” in the link provided under “Web Links for Additional Information”.
Web Links For Additional Information:
Interactions between fish and aquatic macrophytes
AERF Best Mangement Practices Handbook
NCSU Hydrilla Information
If you have questions please contact your Aquatic Extension Associate, Brett M. Hartis, at (919)-515-5648 or email at bmhartis@ncsu.edu.
