Fish Hatchery Management - fisheries & aquaculture

HA'|CHERY OPERAI'IONS9IScreensVarious materials have been used to construct pond or raceway screens.Door screening and galvanized hardware cloth can be used, but clog easily.Wire screening fatigues and breaks after much brushing and must be replacedperiodically. Perforated sheet aluminum screens are used commonlyin many fish hatcheries today. They can be mounted on wood or metal angleframes. Redwood frames are easier than metal ones to fit to irregularconcrete slots in raceway walls.Perforated aluminum sheets generally can be obtained from any sheetmetal company. Some suggested sheet thicknesses are l6 gauge for largescreens (ponds, raceways: 30 x 96 inches) and ltl-20 gauge for smallscreens (troughs: 7 X l3 inches). Round holes and oblong slots are availablein a number of sizes (Figure 36). Horizontal oblong slots are preferred bysome fish culturists who feel they are easier to clean and do not clog asreadily as round holes. They can be used with the following fish sizes:Slot sizeFish size--X-- xln.1Ixlfry up to 1,000/lb1,000-200/lb200-30/tb- X'2'30/lb and largerPerforated aluminum center screens can also be used in circular rearingtanks, but only the bottom 2-3 inches of the cylinder should be perforated.These provide some self-cleaning action for the tank and prevent shortcircuitingof water flows by drawing waste water off the bottom of thetank.Pond ManagementPRESI,ASON PREPARATIONProper management of earthen ponds begins before water is introducedinto them. During the winter it is advisable to dry and disk ponds to promoteaerobic breakdown of the nutrient-rich sediments. Although some nutrientsare desirable for fingerling culture, because they promote algalgrowth on which zooplankton graze, an overabundance tends to producemore undesirable blue-green and filamentous algae. Relatively new pondswith little buildup of organic material, or those with sandy, permeable bottomsthat allow nutrients to escape to the groundwater, are less likely thanolder or more impermeable ponds to require drying and disking. They may