104 FISH HATCHERY MANAGEMENT 



that these fish would compete for space and interfere with the production 

 of other, more desirable species. 



Extensive development of herbicides in recent years makes chemical con- 

 trol of weeds quite promising in many instances. When properly applied, 

 herbicides are effective, fast, relatively inexpensive, and require less labor 

 than some of the other control methods. Chemical control, however, is not a 

 simple matter. Often the difference in toxicity to weeds and to fish in the 

 pond is not great. Some chemicals are poisonous to humans or to livestock 

 and they may have an adverse effect on essential food organisms. Decay of 

 large amounts of dead plants can exhaust the oxygen supply in the water, 

 causing death of fish and other aquatic animals. It is essential that discretion 

 regarding treatment be followed if satisfactory results are to be obtained. 



An important aspect of vegetation control is the rate of dilution of ap- 

 plied herbicides and the effect of substances present that may neutralize 

 the toxicity of the chemical used. The rate of water exchange by seepage 

 or outflow and the chemical characteristics of the water and pond bottom 

 also affect the success of chemical control measures. Often the herbicide 

 must reach a high percentage of the plant surface before a kill is obtained; 

 the chemical must be applied carefully if good results are to be achieved. 



The herbicide is applied directly on emergent or floating weeds and to 

 the water where submerged weeds are growing. The first type of treatment 

 is called a local treatment, the second is termed a solution treatment ap- 

 plied either to a plot or to the entire pond. 



Conventional sprayers are used to apply the local treatments and in 

 some instances may be suitable for solution treatments. Chemicals for solu- 

 tion treatment are sometimes diluted with water and poured into the wake 

 of an outboard motor, sprinkled over the surface of the pond, or run by 

 gravity into the water containing the weed beds. Crystalline salts may be 

 placed in a fine woven cotton bag and towed by boat, allowing the herbi- 

 cide to dissolve and mix with the pond water. Some herbicides are 

 prepared in granular form for scattering or broadcasting over the areas to 

 be treated. Generally, the more rapidly the chemical loses its toxicity the 

 more uniformly it must be distributed over the area involved for effective 

 results. Also, if the chemical is at all toxic to fish, it must be uniformly dis- 

 tributed. Emergent or floating vegetation receiving local treatments applied 

 with spray equipment should be uniformly covered with a drenching spray 

 applied as a fine mist. 



A number of precautions should always be taken when herbicides are 

 used. Follow all instructions on the label and store chemicals only in the 

 original labeled container. Avoid inhalation of herbicides and prevent their 

 repeated or prolonged contact with the skin. Wash thoroughly after han- 

 dling herbicides, and always remove contaminated clothing as soon as pos- 

 sible. Prevent livestock from drinking the water during the post- treatment 

 period specified on the label. Do not release treated water to locations that 



