Fish farmers face a difficult challenge in maintaining a healthy balance among the water, fish, and microscopic flora and fauna in their pond systems. Algae are an important component of the natural nutrient cycles in ponds, but in high numbers, algae can create problems. On the positive side, important chemical changes and metabolic activities in the water are made possible through the release of oxygen and removal of carbon dioxide by algae during photosynthesis.
Algae in Farm Dugouts
Early in the summer, algae growth may go unnoticed, but as temperatures rise, algae multiply quickly and create "blooms" at or near the water surface. As the number of algae increases, clumps form and plants die, using up large quantities of oxygen. A large algae bloom die-off depletes oxygen in the water, and can cause major fish mortalities.
The principal type of algae that cause "summer kills" are planktonic algae. These single cell or chain-like groups of cells are free floating and green, blue-green, or brown. They commonly appear as small specks or "grass clippings" in the water.
If the pond water color changes from dark green to a pale green or brown, it generally means the oxygen level is low, likely due to significant algae die-off. To detect changes dissolved oxygen (DO) levels. The DO should be monitored and checked every night during the spring, summer, and fall.
Prevent Algae Problems
It is better to anticipate and prevent problems caused by algae than to delay until the situation becomes serious. Algae problems can be alleviated in the ways outlined below.
Aeration during hot, sunny periods will oxygenate, circulate, and cool the pond water, decreasing the opportunity for blooms to occur. Aeration systems tend to inhibit algae growth by preventing the recycling of nutrients from bottom sediment.
Keeping algae populations in ponds under control depends largely on managing the adjoining lands. Fertilizer and barnyard run-off contribute additional nutrients to pond waters. These additional nutrients, such as nitrogen and phosphorous, can be responsible for many bloom outbreaks.
Solutions to this run-off problem include excluding cattle from having direct access to the pond, maintaining a 30 meter vegetation buffer around the pond shoreline to filter runoff nutrients, and stabilizing banks further with rocks, gravel, or other materials.
Barley straw offers an environmentally acceptable and cost-effective method of algae control. When barley straw is put into water, it starts to rot. During this process, a chemical is released that inhibits the growth of algae. The chemical released by the straw does not kill algae cells already present, but it prevents the growth of new cells.
In still waters, such as lakes, ponds, and reservoirs, the minimum quantity of straw needed to control algae is 10g straw per square meter of water surface. It is better to apply the straw in a loose form retained in some type of netting or cage, anchored in the desired location.
Straw is best applied in the autumn, winter or very early spring, when the water temperature is low. Barley straw can be effective in controlling algae when used on its own or in combination with various chemicals.
Chemical control of algae works best when algae is in the early stages of development. At this time, seeds or other reproductive bodies are absent, and the potential for re-establishment is minimized.
If algae concentrations are not massive, the risk of eliminating the water's oxygen supply is lower. The risk is that during decay, the algae can remove large amounts of oxygen.
Depending upon the situation, the fish farmer has to select the appropriate control method to achieve management goals. Two algicides commonly used are Cutrine Plus and Copper Sulphate (CuS04/bluestone).
Cutrine is a chelated (chemically locked-in) copper compound. This concentrated product provides a long contact, or killing time, contains no sulfates, prevents the build-up of toxic copper carbonate precipitates, and causes little harm to non-target aquatic organisms. Cutrine-Plus has worked effectively against several noxious weed species when combined with other chemicals or when used by itself.
Cutrine is compatible in a wide range of water qualities (fresh, brackish and salt water). Application rates vary from 0.6 to 1.2 gallons per acre foot of water treated. For optimum results, apply cutrine to warm waters, with temperatures above 60 degrees Fahrenheit.
Copper sulphate is the most widely used, and most economical algae control available. Copper sulphate is applied to the pond's surface and is most effective in controlling weeds during the mid-growth stage (filaments of approximately 1 cm long), when algae most resemble grass clippings.
Copper sulphate is most effective in water temperatures of 60 degrees Fahrenheit or higher. However use caution with this control method because high concentrations are toxic to fish and food organisms.
The chemical can be dissolved and applied to the water surface as a spray, or it may be put in a fine mesh bag and dragged through the water until dissolved. In small dugouts, a bag can be hung at the center of a rope and walked through the length of the pond by two people, one on each side of the dugout. The recommended amounts of copper sulphate required for varying water depths when fish are present.
Spot treatments of one-quarter of the pond over three to four-day intervals may lower algae densities without consuming all the dissolved oxygen. Treatments can be combined with spray or bubble aeration to alleviate low oxygen problems.
When properly applied, these chemicals can be very effective. The recommendation is to treat less than half of a water body at a time to help reduce the potential for fish kills. Treat the remainder of the pond at three to four-day intervals.
To purchase chemical products, contact local farm supply dealers. In water bodies where there is an outlet to a stream or other water body, a permit may be required before any chemical application can be made to the water.