Aquatic Pest Control

The presence of plant life is desirable in aquatic habitats. Algae and aquatic macrophytes are the basis of the food chain. They provide food for small animals, which in turn become the food for successively larger animals. Plants also provide shelter and breeding habitat for a wide variety of animals including fish, waterfowl, and aquatic mammals. Consequently, the maintenance of a good balance between vegetation and other aquatic life is important to the productivity of aquatic habitats.

Aquatic plants do not threaten the viability of aquatic systems in most circumstances. Only when their growth begins to have a negative impact on human activities is it considered to be weedy. The point at which plants become weeds depends largely upon the location in which they are growing and the intended use of that body of water. Hence the concept of "weed" is largely one of human perception. The primary reason for weedy growth is the combination of large areas of clear, shallow water with the presence of adequate nutrients (primarily nitrogen and phosphorus) in the system. Shallow, clear water allows the penetration of sunlight to the bottom, where the growth of algae and macrophytic vegetation begins. Aquatic plants are composed largely of water and consequently, relatively low levels of nutrients allow for rapid growth in terms of fresh weight. A major reason for the existence of weed problems is the introduction of exotic species from other countries. These plants quickly replace native vegetation and may dominate large acreages. This group includes the most troublesome weeds such as water hyacinths, alligatorweed, Eurasian watermilfoil, and hydrilla.

A number of factors must be considered in making environmentally-sound and cost effective management decisions. These include the type of plant, the use of the body of water, physical constraints, water quality, and the potential impact on fish and wildlife populations present in the target area. The combination of these factors determine what management options, if any, can be used for the weed problem in any given location.

The type of plant (i.e., species) must be the first factor considered in the process of making any management decision. Strategies effective on one species may be ineffective even on very similar species. For example, most filamentous algae are controlled readily with copper; a few species (e.g., Pithophora and Spirogyra) do not respond well to copper. Brazilian elodea is closely related to hydrilla, looks very similar, and occupies essentially the same habitat. Drawdown is very effective on Brazilian elodea but not on hydrilla. Often more than one tactic may be required when several species are present. Consequently, correct identification of the target species is critical. Identification is discussed in the final chapter of this manual.

Another important factor to consider in a management decision is the use of the waterbody, because all weed management practices can be expected to have an impact on the environment. The use of herbicides may be very limited if the target area is in a public water supply or if the water will be used for livestock or irrigation. All aquatic herbicides may have some restrictions associated with water use following application. Different restrictions may apply to water intended for irrigation, livestock watering, public water supplies, fishing and swimming. Water use restrictions are based on the persistence of the herbicide, its toxicity to non-target vegetation (i.e. crops) and aquatic organisms, mammalian toxicity, and human health considerations and vary widely (from a few days to one year) with the herbicide and the specific formulation. Certain herbicide formulations may be toxic to fish at normal field application rates and obviously would not be appropriate for use in aquacultural ponds. Recognition of the potential problems associated with water use following treatment, determining whether or not to use a specific situation, and correct application techniques are all major issues which face the aquatic plant manager. Ultimately, the management strategy selected for a particular weed problem will be determined not solely by the nature of the weed itself, but also by the intended use of that water. The selection of appropriate management strategies, herbicide use, and water use restrictions are discussed in detail in the following chapters.

Several other factors also influence management decisions. The physical constraints of the system may prevent the use of specific technology. Often it is impossible to operate equipment because of shallow water, the presence of obstacles (stumps, trees, logs, etc.), the lack of roads into the area, or the lack of suitable launching facilities. Such circumstances may prevent spraying or mechanical harvesting. Mechanical harvesting sometimes is not feasible because of the lack of a place to dispose of the harvested vegetation. The presence of desirable vegetation nearby (agricultural fields, residential property, etc.) may limit access or prevent spray operations. In other cases, the use of biological agents may not be practical. For example, stocking of triploid grass carp may not be feasible, because there may be no way to contain the fish in the target area. Releases of biocontrol insects (e.g., flea beetles on alligatorweed) may be ineffective because of mosquito control or agricultural spraying operations in the vicinity, which interrupt the life cycle of the insects.

The impact of weed control operations on water quality as well as the influence of water quality on herbicide efficacy both impact management decisions. The presence of decaying vegetation following herbicide application, the release of nutrients form the dead vegetation and ensuing algal blooms, and the general deterioration of water quality in the vicinity of the treated area may not be acceptable to the public. Concern about the presence of pesticides in the water also may limit management options. High turbidity due to suspended soil particles or dense phytoplankton blooms, high alkalinity, and presence of high concentrations of metallic ions, such as iron, all may reduce herbicide efficacy. Dense populations of filamentous or planktonic algae also may reduce efficacy by absorbing herbicides applied for macrophyte control. The ultimate impact on fisheries and wildlife populations in the target areas also are important. Management decisions must consider the possibility of fish kills as well as habitat destruction. The impact of management operations on endangered species in the target area also must be considered.

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