Fish Hatchery Management - fisheries & aquaculture

HATCHERY OPERATIONS 103selected. Control methods may be mechanical, biological, or chemical,depending upon the situation.Mechanical control consists of removal of weeds by cutting, uprooting, orsimilar means. While specialized machines have been developed for mowingweeds, they are expensive and not very practical except in special circumstances.In small ponds, hand tools can be employed for plant removal.Even in larger bodies of water, mechanical removal of weeds may be feasibleprovided that work is begun when the weeds first appear.Biological weed control is based on natural processes. Exclusion of lightfrom the pond bottom by adequate water depth and turbidity resultingfrom phytoplankton is one method. Production of filamentous aleae thatsmother submerged rooted types of weeds is another.The most inexpensive form of weed control for many ponds is control orprevention through the use of fertilizers. When an 8-tl-2 grade fertilizer isapplied at a rate of 100 pounds per acre, every 2 to 4 weeks during thewarm months of the year, microscopic plants are produced that shade thebottom and prevent the establishment of weeds. Although 8 to l4 applicationsare needed each season, fish production is increased along with theweed control achieved. Generally, most aquatic weeds may be controlledby fertilization in properly constructed ponds. However, such a program offertilization will be effective in controlling rooted weeds only if the secchidisk reading already is l8 inches or less.winter fertilization is a specialized form of biological control effective onsubmerged rooted vegetation if the ponds cannot be drained. An 8-8-2grade fertilizer or equivalent is applied at a rate of 100 pounds per acreevery 2 weeks until a dense growth of filamentous algae covers the submergedweed beds. Once the algae appears, an application of fertilizer ismade at 3- to 4-week intervals until masses of algae and rooted weeds beginto break loose and float. All fertilization is then stopped until theplants have broken free and decomposed. This will start in the late springand generaliy takes from four to six weeks. Phytoplankton normally replacethe filamentous algae and rooted weeds and should be mantained byinorganic fertilization with 100 pounds of U-8,2 per acre applied every 3to 4 weeks.Lowering the water level of the pond in the late fall has been helpful inachieving temporary control of watershield. This practice also aids in thechemical control of alligator weed, water primrose, southern water grass,needlerush, knotgrass, and other resistant weeds that grow partially submergedand have an extensive root system.Plant-eating fish that convert vegetation to protein have been consideredin biological control. Among these are grass carp, Israeli carp (a race ofcommon carp), and tilapia. Experiments have indicated that the numbersof Israeli carp and tilapia required to control plants effectively are so large