Fish Hatchery Management - fisheries & aquaculture

30 FISH HATCHERY MANAGEMENTFIGURE 13. Three linear clarifiers located at the Jones Hole National FishHatchery, 114 ftx4l ft and 6 ft deep. Each unit has a sludge scraper system forsludge removal. These are long redwood boards attached by chain to move anddeposit sludge into the sumps at the upper ends of the clarifiers. This system isdesigned to pump sludge to drying chambers. (FWS photo.)A warmwaterfish rearing pond acts as its own settling basin. Exceptwhen the water level is so low that any water movement scours the bottom,draining a pond usually does not cause much waste escapement. However,during seining operations when the bottom is disturbed, levels of suspendedsolids in the effluent can increase several hundred times. Special attentionshould be given to discharges at such times. If water flow through thepond cannot be stopped until solids can resettle, the effluent may have tobe filtered or diverted away from receiving streams. Likewise, pollutantloads from other hatchery operations can increase sharply at times. Periodsof raceway cleaning are examples, and there should be means available tohandle the added waste concentrations. Sometimes, raceways and tanks canbe vacuumed before they are disturbed, although this is labor-intensivework (Figure 15).SOLID WASTE DISPOSALOver half of the total nutrients produced by hatchery operations are in theform of settleable solids. They must be removed frequently from lagoonsand clarifiers, because they rapidly decompose and would otherwise pollutethe receiving waters with dissolved nutrients.The "solid" wastes from settling basins and various filtration unitsaround a hatchery, being g0% water, can accumulate into large volumesthat must be disposed of. Hatchery sludge has considerable value as a fertilizer.In warm climates and seasons, it can be spread directly on the