HATCHERY OPERATIONS 103 



selected. Control methods may be mechanical, biological, or chemical, 

 depending upon the situation. 



Mechanical control consists of removal of weeds by cutting, uprooting, or 

 similar means. While specialized machines have been developed for mow- 

 ing weeds, they are expensive and not very practical except in special cir- 

 cumstances. In small ponds, hand tools can be employed for plant removal. 

 Even in larger bodies of water, mechanical removal of weeds may be feasi- 

 ble provided that work is begun when the weeds first appear. 



Biological weed control is based on natural processes. Exclusion of light 

 from the pond bottom by adequate water depth and turbidity resulting 

 from phytoplankton is one method. Production of filamentous algae that 

 smother submerged rooted types of weeds is another. 



The most inexpensive form of weed control for many ponds is control or 

 prevention through the use of fertilizers. When an 8-8-2 grade fertilizer is 

 applied at a rate of 100 pounds per acre, every 2 to 4 weeks during the 

 warm months of the year, microscopic plants are produced that shade the 

 bottom and prevent the establishment of weeds. Although 8 to 14 applica- 

 tions are needed each season, fish production is increased along with the 

 weed control achieved. Generally, most aquatic weeds may be controlled 

 by fertilization in properly constructed ponds. However, such a program of 

 fertilization will be effective in controlling rooted weeds only if the secchi 

 disk reading already is 18 inches or less. 



Winter fertilization is a specialized form of biological control effective on 

 submerged rooted vegetation if the ponds cannot be drained. An 8-8-2 

 grade fertilizer or equivalent is applied at a rate of 100 pounds per acre 

 every 2 weeks until a dense growth of filamentous algae covers the sub- 

 merged weed beds. Once the algae appears, an application of fertilizer is 

 made at 3- to 4-week intervals until masses of algae and rooted weeds be- 

 gin to break loose and float. All fertilization is then stopped until the 

 plants have broken free and decomposed. This will start in the late spring 

 and generally takes from four to six weeks. Phytoplankton normally re- 

 place the filamentous algae and rooted weeds and should be mantained by 

 inorganic fertilization with 100 pounds of 8-8-2 per acre applied every 3 

 to 4 weeks. 



Lowering the water level of the pond in the late fall has been helpful in 

 achieving temporary control of watershield. This practice also aids in the 

 chemical control of alligator weed, water primrose, southern water grass, 

 needlerush, knotgrass, and other resistant weeds that grow partially sub- 

 merged and have an extensive root system. 



Plant-eating fish that convert vegetation to protein have been considered 

 in biological control. Among these are grass carp, Israeli carp (a race of 

 common carp), and tilapia. Experiments have indicated that the numbers 

 of Israeli carp and tilapia required to control plants effectively are so large 



