Many different aquatic plants can be found in, on and around fish culture ponds. These plants range from microscopic organisms known as plankton algae which drift suspended in the water, to larger plants rooted in the pond bottom.
Certain types of aquatic plants are essential for fish production. However, aquatic plants that interfere with commercial fish production are considered to be weeds. Intensive fish production often involves adding large amounts of commercial feeds and inorganic fertilizers to ponds.
Nutrients introduced into the water through feeds and fertilizers often create an ideal habitat for aquatic weed growth. Submersed aquatic weeds are particularly undesirable because fish harvesting seines will ride up over the weeds and allow fish to escape.
Ponds with dense weed infestations can be impossible to harvest since the weight of the weeds accumulating in the seine can become too great to be pulled. Additionally, separating fish from weeds is a slow process and can severely stress the fish.
Aquatic weeds:
Aquatic plants that cause weed problems may be placed into four groups: algae, floating weeds, emersed weeds (foliage above water) and submersed weeds (majority of foliage below water) (Figure 1).
Algae are the most common group of weeds in aquaculture ponds. Shape and size vary from microscopic single- or multiple-celled plants to branched plants that resemble submersed aquatic weeds. Unlike other aquatic plants, algae do not produce flowers or seeds.
Algae are divided into three groups: plankton algae, filamentous algae (pond moss) and the stoneworts (Charaspp. and Nitellaspp.). Plankton algae produce the majority of dissolved oxygen in the pond and are essential to fish survival.
In the presence of sunlight, green plants release oxygen as a by product of photosynthesis At night, plants and other pond organisms consume oxygen. Because of this diurnal cycle, oxygen concentrations are the lowest at dawn and highest in the mid-afternoon.
Cycle imbalances can lead to oxygen depletion and subsequent fish death. In commercial fish ponds, excessive plankton algae may result from the high feeding rates necessary to produce large fish yields. In many cases, fish production rates are limited by the amount of feed that can be applied without plankton algae blooms becoming so dense that dissolved oxygenproblems cannot be managed.
The complexity of this cycle makes attempts to treat ponds with algicides to “thin out” excess algae growth very risky. Although spot treatments of plankton algae scums are effective, problems with low dissolved oxygen concentrations following algicide applications limit their use in fish culture primarily for the control of filamentous algae and stoneworts.
Certain types of algae produce compounds which cause a musty flavour or odour in fish flesh. These compounds are absorbed by the fish and can cause a highly offensive taste known as “off-flavour.” This condition can be corrected within 3 to 10 days if fish are moved to water that does not contain these “off-flavour”compounds.
There is no definitive evidence that thinning the plankton algae bloom with algicides reduces the incidence of “off-flavour.” Floating weeds float in or on the surface of the water and obtain their nutrients from water rather than soil.
Duckweed (Lemnaminorand Spirodela polyrhiza) and watermeal (Wolffiaspp.) are examples of common floating weeds. Emersed weeds are rooted to the bottom, but have stems, leaves and flowers which extend above the water surface.
They primarily occur on the shoreline and in shallow water up to 10feet deep. Common emersed weeds are waterlily (Nymphaeaspp.) and alligatorweed (Alternanthera philoxeroides).
Submersed aquatic weeds grow under and up to the water surface. Most submersed weeds have flowers and seedheads that extend above the surface of the water. Examples of common submersed weeds include hydrilla (Hydrillaverticillata) and Brazilian elodea (Egeriadensa).
Authors:
JamesL. Shelton and Tim R. Murphy
